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3

Filter Induced Bias in Lyα Emitter Surveys:

A Comparison Between Standard and Tunable Filters.

Gran Telescopio Canarias Preliminary Results

J. A. de Diego1 and M. A. De LeoInstituto de Astronomıa; Universidad Nacional Autonoma de Mexico

Avenida Universidad 3000; Ciudad Universitaria; C.P. 04510; Distrito Federal; Mexico

jdo@astro.unam.mx

J. Cepa and A. BongiovanniInstituto de Astrofısica de Canarias, E-38205 La Laguna, Tenerife, Spain

&Departamento de Astrofısica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain

T. VerdugoCentro de Investigaciones de Astronomıa (CIDA), Apartado Postal 264, Merida 5101-A, Venezuela

M. Sanchez-Portal2

Herschel Science Centre (HSC), European Space Agency Centre (ESAC)/INSA,Villanueva de la Canada, Madrid, Spain

and

J. I. Gonzalez-SerranoInstituto de Fısica de Cantabria (CSIC-Universidad de Cantabria), E-39005 Santander, Spain

ABSTRACT

Lyα emitter (LAE) surveys have successfully used the excess in a narrow-band filter comparedto a nearby broad-band image to find candidates. However, the odd spectral energy distribution(SED) of LAEs combined with the instrumental profile have important effects on the propertiesof the candidate samples extracted from these surveys. We investigate the effect of the bandpasswidth and the transmission profile of the narrow-band filters used for extracting LAE candidatesat redshifts z ≃ 6.5 through Monte Carlo simulations, and we present pilot observations to testthe performance of tunable filters to find LAEs and other emission-line candidates. We comparethe samples obtained using a narrow ideal-rectangular-filter, the Subaru NB921 narrow-bandfilter, and sweeping across a wavelength range using the ultra-narrow-band tunable filters of theinstrument OSIRIS, installed at the 10.4m Gran Telescopio Canarias. We use this instrumentfor extracting LAE candidates from a small set of real observations. Broad-band data from theSubaru, HST and Spitzer databases were used for fiting SEDs to calculate photometric redshiftsand to identify interlopers. Narrow-band surveys are very efficient to find LAEs in large skyareas, but the samples obtained are not evenly distributed in redshift along the filter bandpass,and the number of LAEs with equivalent widths < 60 A can be underestimated. These biasedresults do not appear in samples obtained using ultra-narrow-band tunable filters. However, thefield size of tunable filters is restricted because of the variation of the effective wavelength acrossthe image. Thus narrow-band and ultra-narrow-band surveys are complementary strategies toinvestigate high-redshift LAEs.

Subject headings: galaxies: high-redshift — methods: data analysis — techniques: miscellaneous

1. Introduction

The odd spectral energy distribution of LyαEmitter Galaxies (LAEs) convolved with the in-strumental profile can introduce unsought biaseson the characteristics of the candidate samplesextracted from photometric surveys. Particu-larly, properties such as the the Equivalent Width(EW) and the redshift distribution of the subse-quently spectroscopically confirmed LAEs, maybe easily affected by the photometric survey in-strumentation and methodology used. On theother hand, these surveys yield reliable photomet-ric redshifts for LAEs and Lyman Break Galaxies(LBGs), and thus they are useful cosmologicaltools, as these objects trace dark matter halosand subsequently the evolution of matter distri-bution in the universe. Furthermore, LAEs atz ≥ 6.5 are also important to study the last stagesof the reionization epoch (Malhotra & Rhoads2004; Kashikawa et al. 2006; Shibuya et al. 2012).Therefore, it is important to develop unbiased al-ternative strategies to find new LAE candidatesamples.

LBG candidates are selected using the drop-out technique, which consists in comparing im-ages of the galaxy obtained in several broad-band filters that cover contiguous wavelengthranges to both sides of the Lyman break at 912 A(Steidel et al. 1996). In a similar way, LAE can-didates are usually selected by an excess in anarrow-band filter compared to a nearby broad-band image (Cowie & Hu 1998). The later tech-nique is routinely used to find LAE candidatesin the Subaru Deep Field (Taniguchi et al. 2005;Kashikawa et al. 2006, 2011) and the Subaru/XMM-Newton Deep Survey Field (Ota et al. 2010;Ouchi et al. 2010; Shibuya et al. 2012). Narrow-band filters have bandwidths of Full Width at HalfMaximum FWHM ∼ 100 A, and to avoid atmo-spheric OH emission-lines, narrow-band surveysare confined to a limited number of redshift ranges.An important result of the narrow-band surveys isthat the luminosity function of LAEs remains con-stant between redshifts 3.0 ≤ z ≤ 5.7, but evolvesdramatically between 5.7 ≤ z ≤ 6.5, and maybe

1Visiting Astronomer, Instituto de Astrofısica de Ca-narias.

2Asociacion ASPID, Apartado de Correos 412, La La-guna, Tenerife, Spain

beyond (Pentericci et al. 2011; Hibon et al. 2012;Ota & Iye 2012).

Despite the utility of narrow-band filters to findLAE candidates, some groups have used ultra-narrow-band filters to perform this task. Thus,Tilvi et al. (2010) and Krug et al. (2012) have em-ployed a filter with a FWHM ∼ 9 A, installed onthe 4mMayall Telescope, seeking LAEs at redshiftz = 7.7; they expected to find one or two LAEs,respectively, but instead found four LAE candi-dates, and argued about a possible lack of evolu-tion of the LAE LF for redshifts from 3.1 to 8. Re-cently, Swinbank et al. (2012) searched for LAEsaround two quasars at z ∼ 2.2 and one quasar atz ∼ 4.5 with the Taurus Tunable Filter instrumentinstalled on the 3.9m Anglo-Australian Telescope,using a bandpass of FWHM = 10 A, and found alocal number density an order of magnitude higherthan what might be expected in the field.

The spectra of LAEs are characterized by anasymmetric Lyα line profile with a steep bluecutoff due to absorption by neutral hydrogen, asshown in Figure 3 in Hu & Cowie (2006). At red-shifts z > 6, hydrogen completely suppresses thecontinuum of the galaxy at the blue side of theline (Gunn-Peterson trough), while there is a dimcontinuum at the red side. Narrow band selectedLAEs usually have high line luminosities (Lα >1.5×1042 erg s−1) and large line equivalent widthsat rest (EW0 ≥ 20 A, e.g. Dayal & Ferrara 2012;Mallery et al. 2012), although the actual limitsmay substantially vary because of technical con-straints and selection strategies (cf. Hayes et al.2010; Ouchi et al. 2008, 2010). These propertiesprovide clues which suggest that LAEs representan early stage of a starburst in an interstellarmedium of very low metallicity and almost freeof dust, with a radius of about 2 kpc and star for-mation rates around 6M⊙ yr−1 (Taniguchi et al.2005).

The fraction of Lyα photons that escape froma high-redshift star forming region is still an ac-tive and open topic, as well as an important pa-rameter to account for the observable propertiesof LAEs. Neutral hydrogen resonantly scattersthe Lyα photons, changes their escape paths andhinders the realization of the photons that areabsorbed by dust. Ono et al. (2010) place an up-per limit of 20% escaping photons at z = 6.6,but at lower redshifts Blanc et al. (2011) and

2

Ciardullo et al. (2012) find that the photon escapefraction may be as high as 100%. Hayes et al.(2010) consider the Lyα luminosity density atz = 2 significantly underestimated from its in-trinsic value, due to the fact that only 1 in 20photons, on average, reach the telescope; this willspecially happen at z > 6, where the neutral frac-tion of the intergalactic medium may cause sig-nificant suppression of the Lyα line (Santos et al.2004; Hayes & Ostlin 2006; Dijkstra et al. 2007).On the other hand, Lyα photons may be lessattenuated than non-resonant radiation undersuitable conditions within a multiphase scatter-ing medium (Neufeld 1991; Hansen & Oh 2006;Finkelstein et al. 2008). For Finkelstein et al.(2011), dust geometry shapes the spectral energydistributions, and has a major influence in theobserved EW at the rest frame of the Lyα line(EW (Lyα)) distribution in high-redshift LAEs.Finally, the presence of outflows can increase thefraction of Lyα photons that escape from the starforming region (Kunth et al. 1998; Tapken et al.2007; Verhamme et al. 2006, 2008; Atek et al.2008, 2009).

If the observational aspects of LAEs depend ongeometrical effects (dust spatial distribution), ori-entation, and the presence of outflows, the separa-tion between LBGs and LAEs becomes somewhatdiffuse. Thus, Kashikawa et al. (2007) classifygalaxies with contrasts as low asEW (Lyα) > 10 Aas LAEs, while recently Krug et al. (2012) con-sider even lower limits (EW (Lyα) & 4.8 A)for z ∼ 7.7 LAEs. Besides, other observa-tions indicate that the fraction of low ultra-violet luminosity LBGs (MUV > −20.5) withEW (Lyα) > 50 A is about one half (Stark et al.2010, 2011; Pentericci et al. 2011; Schenker et al.2012; Vanzella et al. 2011; Ono et al. 2012). Thepresence of Lyα lines in some LBGs provides astrong evidence of the connection between theseobjects and LAEs. Eventually, this connectionwill promote an effort to build theoretical modelsto explain the underlying physics of LAEs andLBGs (e.g Shapley et al. 2001; Dayal & Ferrara2012; Forero-Romero et al. 2012).

We are conducting a blind search on selectedgravitationally lensing galaxy cluster sky fieldsto obtain samples of LAEs not subjected topossible biases imposed by the methodology ofnarrow-band filters. Hence, we are obtaining

ultra-narrow-band images using tunable filtersof the instrument OSIRIS (OSIRIS-TF) attachedto the 10.4m Gran Telescopio Canarias (GTC).OSIRIS-TF is optimized for line flux determina-tion and thus it can be called a Star FormationMachine (Cepa et al. 2003). This characteristicmakes OSIRIS-TF a powerful instrument to de-tect faint young galaxies with accurate photo-metric redshift estimates. Therefore, apart fromLAEs, we also expect to obtain other high-redshiftcandidates, such as LBGs, AGNs and line emit-ters.

This paper is organized as follows. Section 2 in-troduces the bias produced by the filter bandwidthon the photometric selection of LAEs. Section 3describes the generation of Monte Carlo data forsimulated LAE spectra. Section 4 presents theLAE recovered samples obtained using narrow-band filters on the simulated data, exemplified byan ideal rectangular filter and the NB921 Subarufilter. Section 5 describes the methodology used tosearch for LAE candidates using OSIRIS-TF, andpresents the sample recovered from the simulateddata. Our methodology is then applied on a pilotobservation of real data obtained at the GTC toselect preliminary candidates at redshifts z ≃ 6.5.In Section 6 we cross-check our OSIRIS-TF pre-liminary candidates with photometric archive dataand models of galaxies at high redshift to studythe Spectral Energy Distribution (SED) and toimprove classification. Section 7 presents the dis-cussion of the results obtained with simulated andreal data. Finally, Section 8 presents our conclu-sions.

Throughout this paper we assume a cosmolog-ical model with H0 = 70 km s−1Mpc−1 (h = 0.7),ΩM = 0.27, and ΩΛ = 0.73.

2. The asymmetric continuum bias

The spectrum of a LAE at z ∼ 6.5 shows asharp decay due to the absorption of Lyα photonsby neutral hydrogen that truncates the blue partof the emission line. From the observational pointof view the resulting line is asymmetric, and thesignal is circumscribed to wavelengths longer thanLyα.

The detection of dim objects is a difficult taskand very often the results may vary depending onsubtle details. In the case of LAEs, the spectral

3

asymmetry of the host galaxy continuum back-ground with respect to the Lyα line may enhancethe detection of objects with the largest contribu-tion of the continuum to the total signal. If weuse a narrow-band filter with a FWHM compara-ble to the EW of the Lyα line, the contribution ofthe continuum to the total recorded intensity maychange significantly depending on the redshift ofthe object. Figure 1 shows an example of thiseffect on an ideal rectangular filter of a width of132 A, similar to the FWHM of the NB921 Subarufilter. The figure shows two simplified LAE spec-tra at slightly different redshifts (6.51 and 6.61).The objects have an observed FWHM of 10 A forthe Lyα line, and an identical continuum level.For the LAE at the lowest redshift, the Lyα linelies on the short-wavelength edge of the filter, andits EW is 2.5 times the wavelength range of thefilter (i.e., 330 A in the observer’s frame or 44 A inthe rest-frame), which corresponds to a differenceof one magnitude between the line and the contin-uum, a criterion often used to select LAE candi-dates in narrow-band surveys (e.g. Ouchi et al.2010; Kashikawa et al. 2011). For the LAE at thehighest redshift, to reach the same signal throughthe rectangular filter, the intensity of the Lyα line(or its EW) should be approximately 34% higherthan the line intensity of the former object.

The bias introduced by the asymmetric contin-uum profile at both sides of the Lyα line is morepronounced as the EW decreases. The observed-frame EW for LAEs have values ∼ 100 A, simi-lar to a narrow-band filter FWHM. Therefore, theasymmetric continuum may affect the shape of theLF of LAEs inferred from narrow-band surveys.As objects are less luminous, the volume actuallystudied depends on the amount of continuum inthe range of the filter.

3. Sample simulation

Shimasaku et al. (2006) and Ouchi et al. (2008)have analyzed the redshift distribution of LAEs inSubaru’s narrow-band filters using Monte Carlomock samples of LAEs. For the purpose of com-paring the characteristics of samples obtained withthe OSIRIS-TF and other instruments, we havealso built a simulated sample of LAEs. This sam-ple was made by modeling the LAE spectra witha superposition of an asymmetric triangular pro-

9100 9200 93000

1

Wavelength (Å)

Rel

ativ

e In

tens

ityFig. 1.— An example of the effect of the asym-metric continuum in the detection of LAEs. Theamount of the continuum inside the ideal rectan-gular filter (dotted line) depends on the positionof the Lyα break. In the extreme cases shown inthis diagram, the continuum expands on the wholewavelength range of the filter for objects at thelowest redshift (solid line) or barely in the regioncovered by the Lyα line for objects at the high-est redshift (dashed line). Both objects share thesame continuum level and the observed FWHM ofthe Lyα lines is 10 A. For the object at the lowerredshift, the contribution of the Lyα line is 2.5that of the continuum in the range of the filter.For detecting the highest redshift object with thesame total signal, the Lyα line must be approxi-mately 34% more intense than the line of the lowerredshift object.

file for the Lyα line plus a continuum, which is asimplified version of the profile model by Hu et al.(2004). The Lyα line profile consists of a sawtoothwith the steeper inclination at the line’s blue side.At wavelengths shorter than the Lyα line, the con-tinuum is zero, and it has some constant non-zerovalue at wavelengths equal or larger than the Lyαline. Altogether, we have simulated the spectra of5000 LAEs.

We have used the Schechter function to modelthe LF of LAEs at z ≃ 6.5:

φ = φ∗

(L

L∗

)α

exp

(−L

L∗

)dL

L∗

, (1)

with the parameters given by Kashikawa et al.

4

(2011): α = −1.5, log(L∗/h−2 erg s−1) = 42.76

and log(φ∗/h3Mpc−3) = −3.28.

The simulated LF sample was computed throughthe inversion method of the cumulative Schechterfunction, integrating between 1041 and 1044

erg s−1, a luminosity range that broadly includesthe observed LAEs at redshifts z ≈ 6.5 (e.g.Kashikawa et al. 2011).

A random FWHM sample for the Lyα line hasbeen computed according to the distribution pa-rameters inferred from Table 2 in Kashikawa et al.(2011). These authors have measured the ob-served FWHM of 28 z ≈ 6.5 LAEs with valuesbetween 5.04 A and 25.2 A, with mean of 13 A(428 km s−1) and a standard deviation of 5 A. TheFWHM sample has been shifted to the rest-frameof the LAEs to build the profile of the emitted Lyαline.

The random EW sample at rest-frame (EW0)has also been computed like the luminosity, us-ing the inversion method on the cumulative func-tion extracted from Figure 11 in Kashikawa et al.(2011). We tried to fit the EW distribution us-ing exponential cumulative functions, but we ob-tained a better fit using the lognormal cumulativefunction. Lognormal EW distributions also fit the[O II]λ3727 equivalent widths in the local (z < 0.2)universe (Blanton & Lin 2000), and has been em-ployed by Shimasaku et al. (2006) to characterizethe EW of LAEs at redshift (z ≃ 5.7). However,the Lyα EW distribution in LBG at z ∼ 3 doesnot show lognormal profile (Shapley et al. 2003),possibly because in LBGs Lyα is observed bothin emission and in absorption, depending on theobject. In any case, accurate EW measurementsare difficult mainly because of the low level of thegalaxy continuum, and thus the actual shape ofthe Lyα EW distribution is still an open subject.The EW0 in our simulations has a median valueof 68 A, slightly below the value of 74 A reportedby Kashikawa et al. (2011). By construction, thelower values of the simulation were restricted toEW > 0. Extreme values are useful to test theinstrumental response and to set constraints tothe distribution of real objects. If the simulationsshow that the instrument reaches a certain param-eter or combination of parameter values, but thecorresponding objects have not been observed, itis evidence that there are few if any of them. Onthe other hand, if the simulations show that the in-

strument cannot detect objects with certain com-binations of parameter values, it remains an openquestion whether these objects exist or not. In anycase, because the simulation is based on empiricalparameter distributions, extreme values are veryimprobable, and they cannot significantly alter thestatistical results of this study. For example, inour simulation we find 69 objects with EW0 < 8 A,and 77 with EW0 > 600 A from 2708 OSIRIS-TFrecovered LAEs; of course these numbers dependon the EW distribution adopted, which may becritical for the low EW regime.

To set thresholds for the EWs of LAEs is nota simple task. In some cases the continuum maybe undetected, and the maximum threshold can-not be determined. In the case of the minimumthreshold, Stark et al. (2010) have proposed thatthe equivalent width at the rest frame should beEW0 > 55 A for LAEs. More recently, these au-thors have proposed a value of 25 A (Stark et al.2011) that approaches the widely accepted mini-mum threshold of EW0 > 20 A. The intensity ofthe Lyα line and its EW may depend on intrin-sic properties of the LAEs or on geometrical at-tributes (see § 1). Moreover, the adopted thresholdis a figure that probably depends also on the ob-servational limits adopted to separate candidatesefficiently. Thus, Stark et al. (2011) note that themeasured rest-frame EWs of the Lyα line for 13spectroscopically confirmed LAEs range between9.4 A and 350 A. Therefore, we have conservedeven the most extreme values for the EW in oursimulations. Besides, these values may be usefulto make extreme-case differences between the re-spective methodologies to detect LAE candidates.

For each simulated LAE, once the power of theLyα line (LLyα) and its EW0 have been assigned,it is possible to calculate the spectral power Φc ofthe continuum:

Φc =LLyα

EW0. (2)

This spectral power is used to compute the spec-tral shape of the continuum at wavelengths λ ≥λLyα. For wavelengths λ ≤ λLyα, the continuumis assumed to be zero, as the optical depth forphotons with wavelengths shorter than the Lyαline is large. Equation (2) is also used to build acontinuum of reference for all the wavelengths ofinterest in our analysis, which will be used later to

5

estimate a broad-band continuum emission neces-sary for the criterion of detection.

The observed spectra for both, the Lyα lineand the continuum, have been calculated fromthe corresponding spectral luminosities taking intoaccount the redshift for each simulated object.These lines and continuum spectra have beenadded in order to obtain the observed flux, fromwhich we will get the photometry later in our anal-ysis. We have constructed another spectrum ofconstant intensity for each object, calculated fromthe corresponding continuum of reference.

4. Narrow-band filters

In this section we analyze the outcome pro-duced by narrow-band filters applied to our sim-ulated sample. Gronwall et al. (2007) have notedthe effect of a nonsquare bandpass in the defini-tion of survey volume and sample’s flux calibrationfor large photometric selected samples of emission-line galaxies. For these authors, the volume ofspace sampled is a strong function of line strength:objects with bright line emission can be detectedeven if the Lyα line lies in the wings of the filter,but for weak Lyα sources the line must lie near thebandpass center to be noticeable. Furthermore, ig-noring the actual position of the line in the band-pass prevents a precise estimate of the effective fil-ter transmission in the line position, affecting theflux calibration. Being aware of these constraints,we consider two narrow-band filter profiles. First,an ideal rectangular filter to study the effect of ageneral pass-band filter on the detection of LAEs,and secondly a filter with a response similar tothat of Subaru NB921 attached to the Suprime-Cam, to study the effect of a real-life filter profile.

Figure 2 shows the distribution of the simulatedsample, the ideal and the NB921 Subaru filters,and the OSIRIS-TF (see §5). The simulationsyield a number of approximately 400 objects perwavelength bin (each bin with a width of 25 A).

4.1. Ideal rectangular filter

The ideal rectangular filter is defined such as:

Tλ =

1 if 9130 ≤ λ ≤ 9262 A,

0 otherwise,(3)

where Tλ is the transmission of the filter as a func-tion of the wavelength λ. The bandwidth of the

0

200

400

a6.45 6.5 6.55 6.6 6.65 6.7

Redshift

0

200

400

b

9000 9100 9200 9300 94000

200

400

Wavelength (Å)

d0

200

400

c

Fre

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cies

Fig. 2.— Number of simulations and detections.Panel (a) shows the absolute frequency per wave-length bin of the simulated population of LAEs.Panels (b) and (c) depict the number of detectionscomputed for an ideal rectangular filter and for theSubaru NB921 filter; both panels show the trans-mission profile for the respective filter superposed.Panel (d) renders the number of detections com-puted for the OSIRIS-TF, along with the trans-mission profile of the wide filter (solid line) builtusing the band synthesis technique (see text).

filter defined above is 132 A, which equates theFWHM of the Subaru NB921 filter (see § 4.2).

Simulated LAEs are recovered according to twocriteria, one for source detection and the otherfor LAE candidate discrimination. The detection

6

criterion is fulfilled when the irradiance in thenarrow-band filter is ≥ 5 × 10−18 erg s−1 cm−2,which corresponds to the Subaru flux limit (Ouchi et al.2010). The discriminant criterion is a contrastcondition between the narrow-band and the broad-band fluxes. This criterion is in fact a simplifiedversion of the Ouchi et al. (2010) color criterion,but is sufficient to analyze the simulated data.In our case, the condition that must be accom-plished consists of the narrow-band flux being atleast a magnitude brighter than the (adjacent)broad-band flux, which provides a continuum ofreference. Computationally, this criterion leads tothe flux of the object measured through the filterbeing at least 2.5 times larger than the flux mea-sured from the constant continuum of referencecomputed in § 3.

Results are shown in Figure 2b. Roughly,due to the asymmetric continuum bias, a largernumber of detections at shorter wavelengths areexpected, and decays smoothly at longer wave-lengths. Small departures from this behavior aredue to random deviations in the original simulatedsample redshift distribution. Thus, most detec-tions are scattered over the filter pass-band, butsome simulated objects with their Lyα peaks lyingat shorter wavelengths are recovered.

Only around a fourth part of the wavelengthrange in the leftmost bin in Figure 2b lies insidethe filter profile, and almost half of the recoveredobjects in this bin correspond to simulations withthe Lyα line peak located inside the filter pass-band. In fact, most recovered objects in this bin(83 out of 160) have the peak of the Lyα line atwavelengths shorter than the filter window, butwith a fraction of the long wavelength queue ofthis line inside the filter. Note that the detectionof these objects will depend on the line parameters(peak position, FWHM and intensity), and thatthe recorded signal will be diminished by the lossof the line flux outside the filter window, and thusthese objects tend to have larger EWs that makedetections easier. On the other hand, the smoothdecay of the number of recovered LAEs along thefilter pass-band is a result of the smaller quantitiesof the continuum emission lying inside the filter asthe Lyα line peaks at longer wavelengths. For ob-jects with their Lyα line near the long wavelengthextreme of the filter, the contribution of the con-tinuum to the total flux in the band is negligible,

and part of the long wavelength queue of the linemay also lie outside the filter window, yielding asteep drop in the number of detections. There-fore these objects also tend to have large EWs tocompensate for their total loss of flux.

4.2. Subaru filter

NB921 filter is characterized by an almostGaussian profile with a central wavelength at9196 A and a FWHM of 132 A (Kashikawa et al.2011), that corresponds to a spectral resolution of70. The transmission profile is available from Sub-aru.1 A close inspection of this profile shows thatthe maximum of the recovered LAEs is slightlyoffset (9183 A) with respect to the filter peak, inagreement with the Lyα wavelength distributionof confirmed LAEs reported by Kashikawa et al.(2011).

We used the same detection criteria as in thecase of the ideal rectangular filter. The resultsare shown in Figure 2c. The number of recoveredLAEs is restrained by the filter profile along withthe same effects that have been noted in the idealrectangular filter. On the one hand there is thedecay of the continuum contribution for the ob-jects with the peak of the Lyα line closer to thelong wavelength limit of the filter, on the otherhand there also is the loss of the long wavelengthqueue of the Lyα line, which spreads to the lowtransmission region of the filter profile. There-fore, the long wavelength queue of the line ob-served through the Subaru filter is damped bythe bell-shaped transmission, in agreement withShimasaku et al. (2006) and Ouchi et al. (2008).

5. Tunable filters

Throughout this work, a distinction is drawnbetween a frame, corresponding to one set of dataread from the CCDs; an image, a number of framesat the same etalon settings which have been com-bined for analysis; a field, a stack of images of thesame area of sky at different etalon settings.

5.1. OSIRIS tunable filters

Basically, the imager/spectrograph OSIRIS-TFis a low resolution Fabry-Perot spectrograph

1http://www.naoj.org/Observing/Instruments/SCam/sensitivity.html

7

which consists of a blue and a red arm. Ourobservations were performed with the red arm,that can be centered at any wavelength between6500 and 9300 A, and we observed using a fixedbandpass of 12 A which corresponds to a spectralresolution of about 770, a set of order-sorter fil-ters to avoid contamination by neighboring orders,and a detector array consisting of two MAT 4k×2kCCDs (Cepa et al. 2003). The 12 A bandpass isthe only one currently available for wavelengthsaround 9200 A.

The observing strategy consists on sweepinga selected spectral range using steps of a halfof the FWHM (in our case, 24 images shifted6 A between 9122 and 9260 A) to avoid alias-ing. To apply this methodology to our simulateddata, we have modeled a set of 24, approximatelyLorentzian shaped, tunable filters using the follow-ing approximation that relates wavelengths andtransmissions of the OSIRIS-TF (Cepa et al. 2011,eq. 3.14):

T ≈

1 +

[2(λ− λ0)

δλ

]2−1

, (4)

where δλ is the filter FWHM and λ0 is the centralwavelength.

Figure 3 shows a simulated LAE at redshiftz = 6.6 (a), the set of OSIRIS-TF filtered spec-tra for this simulation (b), and the photomet-ric output (c). This particular simulation has arather small EW0 = 17.8 A, which is adequate tomake the continuum more apparent in the plot.In panel (c), the short wavelength edge of the linealiases because the spectrum varies at a higher fre-quency than the OSIRIS-TF spectral resolution.Nonetheless, the asymmetry of the line profile isstill noticeable.

In this paper we have adopted two criteriato retrieve LAEs from the simulations, based onthe characteristics of OSIRIS-TF. The first is adetection condition that imposes an irradiance& 4× 10−18 erg s−1 cm−2 for the field narrow-band image. This field is built through the sum ofthe 24 ultra-narrow-band filters to provide a widersynthetic filter (band synthesis technique, Cepa2009). The second condition consists of a ratiobetween the maximum and minimum of the 24filters larger than 2.5 (i.e., one magnitude). Theseconditions are equivalent to those imposed to the

0.0

0.5

1.0 a

10−

18×

ergs−

1cm

−2A−

1

0.0

0.5

1.0 b

9100 9150 9200 9250 93000.0

0.5

1.0

Wavelength (Å)

c

10−

17×

ergs−

1cm

−2

Fig. 3.— OSIRIS-TF output. Panel (a) showsthe spectral flux density of a LAE at z = 6.6simulated spectrum. Panel (b) shows the set ofOSIRIS-TF transmitted spectra for this simula-tion. Panel (c) renders the photometric outputfor each OSIRIS-TF, which is the signal integratedover each filter. Note that the units and the scaleof the last plot are different from the two previousdiagrams.

Subaru survey.

Results are shown in Figure 2d. The distri-bution of the simulated LAEs recovered usingOSIRIS-TF expands the full range of wavelengthsbetween 9122 and 9260 A. A visual inspection of

8

Table 1: High redshift OSIRIS-TF candidates

IDa RAb DECRedshift Irradiancec

Candidateshh:mm:ss.s dd:mm:ss OSIRIS-TF only OSIRIS-TF & SED

Aa 20:56:23.8 -4:40:07 6.494 4.01 ± 0.09 LAE Interloper / [O II] emitterAb 20:56:25.0 -4:37:07 6.498 1.05 ± 0.09 LAE LBGAc 20:56:23.8 -4:37:02 .6.494 1.12 ± 0.09 LAE Interloper / [O II] emitterBa 20:56:38.1 -4:40:04 &6.448 1.49 ± 0.09 LAE/LBG InterloperBb 20:56:26.4 -4:37:14 &6.512 1.67 ± 0.09 LAE/LBG Interloper / [O II] emitterCa 20:56:16.7 -4:37:53 6.513 1.00 ± 0.09 LBG Young spiral galaxyCb 20:56:30.6 -4:37:37 6.502 1.82 ± 0.09 LBG LBG

aCandidate identifier.bEpoch J2000.cIn units of 10−17 erg s−1 cm−2.

Figures 2a and 2d shows that the shape of thedistribution in this range of wavelengths closelyresembles that of the simulated LAEs, withoutany apparent bias introduced by the filter profiles.Also notice that almost all the simulated objectsin this range are recovered. In addition, there alsoare some retrieved LAEs with their Lyα line peak(λLyα) outside the 9122 — 9260 A range, which isa consequence of the OSIRIS-TF Lorentzian pro-file. These objects tend to have large EWs; andthose with λLyα > 9260 A tend to be brighter, too.

5.2. OSIRIS-TF pilot observations

Photometry was carried out at the GTC usingOSIRIS-TF with FWHM of 12 A at five con-tiguous wavelengths separated by steps of 6 A,and a field of view free of adjacent orders ofabout 8 arcminutes on each side. The observa-tions were performed with central wavelengthsλc = 9122 A, 9128 A, 9134 A, 9140 A & 9146 A.The observation run was done on 2010 September8. A binning of 2 × 2 was used in fast (standard)readout mode (200 kHz), with three dithered ex-posures per wavelength of 210 s each, and sepa-rated by a triangular offset pattern of 10 arcsec-onds to eliminate diametric ghosts during datareduction, each called a frame. The night wasphotometric and the seeing varied from 0.75 to0.82 arcseconds during the observation run.

Following the observation of the cluster in allthe wavelengths, a standard star was observedwith the same instrumental settings (but differentexposure times) for flux calibration.

Standard IRAF procedures for bias subtractionwere used on the data. Super-flats (a flat gen-erated by averaging the sky from scientific im-ages where sources have been masked) were cre-ated from and divided to the scientific frames dueto the unevenly-lit dome flats. Because of theTFs small bandpass and position-dependent wave-length, all observations contain sky (OH) emissionrings which were subtracted to all frames with theIRAF package TFred,2 which estimates the skybackground, including the sky rings which are sev-eral arcminutes in diameter. The three frame off-sets of each wavelength were aligned and combinedto generate an image with a total exposure timeof 630 s, these were later convolved to the worstseeing of 0.82 arcseconds.

Also, a field was created convolving to theworst seeing of 0.82 arcseconds, and combining allaligned images of different wavelengths (band syn-thesis technique). This field has a total integrationtime of 3150 s, and yields a detection limit irradi-ance of 9×10−18 erg s−1 cm−2 integrated over thefull wavelength range (36 A) of the synthetic band.We used SExtractor (Bertin & Arnouts 1996) tomake a catalog of detected sources in the field. Weexcluded sources from the catalog that were toobright for high-redshift galaxies. Photometry oneach monochromatic image was performed usingan aperture of 1.5 arcseconds using the positionsgathered by SExtractor. Then we selected possible

2Written by D. H. Jones for the Taurus Tunable Filter,previously installed on the Anglo-Australian Telescope;http://www.aao.gov.au/local/www/jbh/ttf/

9

Field 9122 A 9128 A 9134 A 9140 A 9146 A

Aa

Ab

Ac

Ba

Bb

Ca

Cb

Fig. 4.— OSIRIS-TF observations. Column 1 shows the field obtained adding five individual images shownin the columns 2–6 labeled by the central wavelength of the tuned filter. The horizontal line at the top leftof the fields marks a 6′′ scale. Each row corresponds to a different LAE or LBG candidate, identified by thelabels at left. The objects lie in the center of the identifying circles.

candidates based on the maximum vs minimumflux ratio, excluding all those sources with a ratiobelow 2.5 (i.e., one magnitude). The remainingobjects were carefully inspected by eye to rejectfaint cosmic rays, ghost residuals, and source con-tamination by nearby companions or located tooclose to the edge of the image; the region aroundthe gap between the detectors was particularlyclumped with fake detections. Finally, we selectedthose candidates that showed a photometric pro-file similar to those expected for LAEs and LBGswith either the Lyα line or the Lyman Break lying,at least partially, inside the observed wavelengthrange. Given the likely range of Lyα emission-linewidths, and the wavelength sampling, we expectto observe this line in more than three adjacentpassbands.

Table 1 summarizes the data for the candi-dates. Column 1 identifies the OSIRIS-TF can-didate. Columns 2 and 3 show the right ascensionand declination coordinates, respectively. Col-umn 4 shows the redshift obtained from the peakof the alleged Lyα line or Lyman break. Column 5is the total irradiance corresponding to the field.Column 6 and 7 present the classification of can-didates obtained using the OSIRIS-TF data, andthe SED fitting to Subaru, HST and Spitzer pho-tometrical data.

Figure 4 shows the OSIRIS-TF images for eachLAE and LBG candidate. Each row in this figurecorresponds to a different object located at thecenter of a guiding circle. Despite candidate Babeing near the upper border of the clipped image,it does not affect our analysis. This is also thecase for the candidate Ca, which is near the dif-fuse border defined by the dithered gap betweenthe detectors. The first column of frames in Fig-ure 4 shows the field exposure obtained by pilingup all the individual images; the rest of columnsshow the candidates observed at the wavelengthidentified in the heading row.

Figure 5 shows three LAE candidates observedwith the OSIRIS-TF at the GTC, and selectedsimulations extracted from our database that re-sembles the real data, rather than a fitted modelfor each object. The shift in wavelengths be-tween the observations and the model is due tothe dependency of the tuned wavelength of thefilter on the distance r to the optical center inthe OSIRIS-TF images (Gonzalez et al. 2013, in

9100 9110 9120 9130 9140 91500

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Fig. 5.— OSIRIS-TF LAE candidates. Panels (a),(b) and (c) correspond to candidates Aa, Ab, andAc, respectively. Circles connected by solid linesdepict OSIRIS-TF photometrical data obtained atthe GTC. Triangles linked by dotted lines show ex-amples extracted from our simulations that resem-ble the real data. Photometrical data are shiftedin wavelength with respect to the simulations dueto the wavelength dependency of the OSIRIS-TFwith the distance to the optical center.

11

9030 9050 9130 91500

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Fig. 6.— OSIRIS-TF LAE or LBG candidates. The same as in Figure 5, but the spectral profiles arecompatible to both LAEs and LBGs. Panels (a), and (b) correspond to candidates Ba, and Bb, respectively.

preparation):3

λ = λc − 5.04r2, (5)

where λc is the wavelength at the optical centerexpressed in A, and r in arcminutes.

The redshifts for these LAE candidates werecalculated identifying the brightest photometricpoint with the position of the Lyα line. For thepeak-missed LAE candidate shown in Figure 5c,the redshift estimate is therefore an upper limit(see Table 1). In fact the fraction of candidateswith either upper or lower redshift limits are ex-pected to be relatively high. This subject, andthe possible contamination of the candidate sam-ple by foreground galaxies, will be addressed in §6and §7.

The two objects shown in Figure 6 have a pho-tometric profile compatible with either LAE orLBG candidates. Their respective redshift lowerlimits are shown in Table 1. The photometricprofile for the object shown in Figure 6a may becontaminated by residuals of the sky-line subtrac-tions at wavelengths below 9050A. As in the pre-vious figure, simulated objects extracted from ourdatabase are also plotted for comparison purposes.

Objects shown in Figure 7 are two LBG candi-dates. Both objects show a steep increase in fluxthat is held at longer wavelengths, as it is expectedfor the continuum emission of LGBs.

3http://gtc-osiris.blogspot.com.es/

6. Subaru, HST and Spitzer archive data

We have searched for additional photometricdata for our LAE and LBG candidates in astro-nomical public databases, and have found somedeep images in the Subaru, Hubble Space Tele-scope (HST) and Spitzer archives that cover,at least partially, the OSIRIS-TF field aroundMS2053.7-0449. We used these data along withour observations to fit the SED of the OSIRIS-TFcandidates, obtaining a more accurate classifi-cation (see Table 1). Table 2 shows the fluxesin the different bands of Subaru/Suprime–Cam,HST/WPFC2 and Spitzer/IRAC data. This tablealso includes the fluxes of the OSIRIS-TF bandsynthesis images, that correspond to a filter ofFWHM = 36 A. In the case of OSIRIS-TF fieldsand HST fluxes, we found discrepancies based onSubaru calibrations. We then used stars in thefield to recalibrate the OSIRIS-TF and HST datato the Subaru photometry.

6.1. Subaru observations

The Subaru/Suprime–Cam data were obtainedfrom the Subaru-Mitaka Okayama-Kiso ArchiveSystem (SMOKA). They consist of data in filtersV , i' and z' observed in 2009. The wavelengthrange of the z' filter includes our OSIRIS-TF ob-servations, resulting in a helpful band to esti-mate the continuum. Data was reduced followingthe Suprime–Cam Data Reduction software (SD-FRED2). The images were flat fielded, matched

12

9110 9120 9130 91400

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Fig. 7.—OSIRIS-TF LBG candidates. Panels (a), and (b) correspond to candidates Ca, and Cb, respectively.Circles with error bars linked by solid lines depict photometrical data obtained with the OSIRIS-TF at theGTC. The spectral profiles are compatible with LBGs.

in PSF size for a predetermined target FWHM,scaled and combined. The total integration timefor V , i' and z' filters was 5040, 1920 and 1800 sec-onds, respectively. The photometry for the candi-dates was performed with the qphot IRAF pack-age, fluxes were derived by measuring inside a 1.5arcsecond aperture.

OSIRIS-TF candidates Aa, Ab, Ac and Ba areseen in the V , i' and z' filters, pointing at possibleinterlopers (i.e., a galaxy at a lower redshift andwith spectral features resembling those of LAEsover a limited range of wavelengths) or LBG can-didates. Candidates Bb, Ca and Cb are not seen inat least one Subaru/Suprime–cam filter, but theirbest SED fit suggests they are all candidate inter-lopers.

6.2. HST observations

The HST/WFPC2 data were obtained from theMultimission Archive at the Space Telescope Sci-ence Institute (MAST). They consist of two pro-grams: ID5991 (filters F702W and F814W), andID6745 (filters F606W and F814W). The propos-als cover slightly different fields. Only two of theseven candidates fall in the images, namely, Ac inthe field of the proposal ID5991, and Ca in thefield covered by ID6745.

The image reduction was performed using theIRAF/STSDASS package. First, a warm-pixel re-jection was applied to the images using the IRAF

task warmpix. The cleaned images were then com-bined with the task crrej to remove cosmic-rayshits. Finally, the background was subtracted andthe WFPC2 chips were combined using the taskwmosaic. The total integration times for filtersF702W and F814W were 2400 s and 2600 s, respec-tively for the proposal ID59991. For the proposalID6745 the total integration times were 3300 s and3200 s for filters F606W and F814W, respectively.The photometry for the seven candidates was per-formed in the HST filters with the IRAF packageapphot. The magnitudes were derived by measur-ing fluxes inside a fixed circular aperture of 7 pix-els (∼ 0.′′7). OSIRIS-TF candidates found in thesefilters suggest an interloper nature.

6.3. Spitzer/IRAC observations

A Spitzer/IRAC observation was gathered fromthe Spitzer Heritage Archive (SHA). The As-tronomical Observation Request (AOR) number18626048 (program Name/ID KTRAN-MS2053/30642, P.I. K.-V. Tran) was retrieved for this work.It is a four-channel (3.6, 4.5, 5.8 & 8µm) IRACmap mode observation consisting of 3 rows and1 column with a 42-point cycling dither pattern,and “medium” scale (median separation betweendither positions is 53 pixels; the IRAC pixel scaleis approximately the same, ∼ 1.2 arcsec in the fourcamera bands). The resulting map footprint is aroughly rectangular area of 22.9× 8.6 arcmin cen-tered at α≈ 314.113, δ≈−4.670. The major

13

Table 2: Candidate Fluxes

FILTER Aa Ab Ac Ba Bb Ca Cb

Suprime V 26± 2 < 0.70 4± 1 8.3± 0.7 · · · 36± 1 · · ·F606W · · · · · · · · · · · · · · · 41± 7 · · ·F702W · · · · · · 1.5± 0.2 · · · · · · · · · · · ·Suprime i′ 7.3± 0.3 7.5± 0.2 1.3± 0.1 11.0± 0.3 4.0± 0.2 · · · 9.7± 0.2F814W · · · · · · 0.5± 0.2 · · · · · · 9.0± 0.6 · · ·OSIRIS-TF 38± 1 18± 2 10± 1 22± 2 12.0± 0.8 15± 1 20.9± 0-3Suprime z′ 7.3± 0.4 21.0± 0.6 0.7± 0.4 16± 2 4.0± 0.4 · · · 17.0± 0.33.6 µm 0.55± 0.03 10.34± 0.04 < 0.15 0.41± 0.06 < 0.15 1.86± 0.03 0.44± 0.024.5 µm 0.47± 0.03 6.37± 0.03 < 0.08 < 0.08 < 0.08 1.15± 0.03 0.24± 0.035.8 µm 0.80± 0.07 3.47± 0.07 0.69± 0.07 < 0.14 < 0.14 0.77± 0.07 0.36± 0.078.0 µm < 0.17 1.73± 0.08 < 0.17 < 0.17 0.44± 0.09 < 0.17 < 0.17

Note.—All the fluxes in units of 10−19 erg s−1 cm−2 A−1

.

axis is oriented at P.A.≈ 168.2.

The data reduction of the four IRAC chan-nels was performed using MOPEX v18.4.9, us-ing as starting point the Corrected Basic Cali-brated Data (CBCD). The IRAC CBCD data dif-fers from the standard BCD products in the miti-gation of several instrumental artifacts, includingstray light, muxstripe, banding, muxbleed (elec-tronic ghosting), column pulldown and jail bars.The Overlap and Mosaic pipelines were run tocreate mosaic images from the individual images.The former removes image background variationsdue to foreground light sources, while the laterremoves defects and spurious pixels, reassemblesthe data onto a common pixel grid, and combinesthem into a mosaic with a corresponding noisemap.

The four-channel photometry of the sources wasperformed by means of the Astronomical Pointsource EXtractor (APEX) tool provided withinthe MOPEX software. The User List Multiframemode was used. In the APEX Multiframe mode,required for the IRAC instrument, the extractionis carried out simultaneously on the stack of in-put images rather than on a single mosaic image.The position and flux density estimate is providedfor each detected source. The User List mode waschosen, providing APEX with an input list of ob-ject positions rather than using the Detect mod-ule to automatically find the sources in the field.Point-Response Function (PRF) fitting and aper-

ture photometry was done on the input list ob-jects. The centroids of the photometrical profileswere allowed to move slightly with respect to theinput positions, resulting in small shifts between0.′′05 and 0.′′6, except for Bb (1.′′3). Only detectionswith SNR> 3 have been considered.

6.4. SEDs and photometrical redshifts

We have used an updated version of HyperZ(Bolzonella et al. 2000) and synthetic spectraltemplates of galaxies (Bruzual A. & Charlot 1993;Bruzual & Charlot 2003) and quasars (Hatziminaoglou et al.2000) to fit the SEDs of the OSIRIS-TF LAE andLBG candidates. These templates correspond todifferent types of starburst, spirals, irregular, ellip-tical galaxies and quasars. For fitting the SEDs,we used the available Subaru, HST and Spitzerdata. We included upper limits to compute theSED. We had to provide the transmission pro-files of the Spitzer/IRAC filters4, which are notincluded in the HyperZ filter database. We didnot include OSIRIS-TF data because the syn-thetic templates do not have enough resolution tofit emission lines, which may dominate over theflux in the synthetic filter. With these fittings wecheck whether the SEDs are compatible or notwith LAEs and LBGs. The results are summa-rized in Table 3. There is a prudence to be taken

4Available at NASA/IPAC Infrared Science Archive:http://irsa.ipac.caltech.edu/data/SPITZER/docs/irac/calibrationfiles/spectralresponse/

14

when using HyperZ to fit the data: the numberof photometric bands is rather small to obtain ac-curate fittings, and usually it is possible to fit theSED of various types of galaxies at different red-shifts. Therefore, in our case SED fitting is usefulto discard candidates, but provides a moderatesupport for object classification, and the fits mustbe regarded with caution. Below we present theresults of fitting SEDs to the OSIRIS-TF LAE,LAE/LBG, and LBG candidates.

6.4.1. OSIRIS-TF LAE candidates

Figure 8 shows the photometry and the SEDsof the OSIRIS-TF LAE candidates.

Aa: Candidate Aa is detected in the SubaruV , i' and z' filters; it lies out of the field ofthe HST images; it is also found in Spitzer im-ages at 3.6–5.8µm, but not in the 8µm band.Figure 8a presents the OSIRIS-TF, Subaru andSpitzer available data for this object, and theSEDs for a starburst galaxy at z = 3.03 and an[O II] interloper at z = 1.45. The OSIRIS-TFphotometric point shows an excess that may be as-cribed to [O II] emission. Figure 8a also shows theoptical high resolution SED template for a star-burst with E(B−V )< 0.1 by Calzetti et al. (1994)and Kinney et al. (1996).5. This high resolutiontemplate has been moved to the starburst galaxyredshift of z = 1.45, and it has been scaled us-ing a third-degree polynomial to the low resolutionstarburst SED template fitted by HyperZ. The re-sulting optical SED does not pretend to show theactual strengths of the emission lines, but give anidea of what should be their appearance. The χ2,in Table, 3 is not very different for the [O II] emit-ter and starburst fits. The source is an interloper,and most probably an [O II] emitter.

Ab: Photometric data and SED fitting for can-didate Ab are shown in Figure 8b. The objectis detected in Subaru filters V , i' and z'; it is islocated out of the field of the HST images; it isfound in all the Spitzer/IRAC filters.

The photometric profile using only the OSIRIS-TFobservations (see Figure 5b), suggests a LAE can-didate. The profile of the best starburst SED fit at

5Templates available at http://www.stsci.edu/hst/observatory/cdbs/cdbs kc96.html

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Fig. 8.— SED fittings for OSIRIS-TF LAEcandidates. OSIRIS-TF (hollow circle), Subaru,HST, and Spitzer (filled circles for all three) fluxesare plotted along with SEDs calculated with Hy-perZ. (a) Candidate Aa: the solid line corre-sponds to the best fit SED, a z = 3.03 star-burst galaxy; the dashed line shows the fit atz = 1.45 for an [O II] interloper, and the grayline shows a scaled high–resolution version of theprevious fit, obtained from a starburst galaxy tem-plate by (Calzetti et al. 1994; Kinney et al. 1996).(b) Candidate Ab: the best fit corresponds to az ≃ 5.4 LBG. (c) Candidate Ac: the best fit is ob-tained for a z = 3.0 (solid line) starburst galaxy;dashed and gray lines show the fit and scaled tem-plates for a z = 1.45 [O II] interloper, as for Aa.

15

Table 3: SED fits

ID Starburst [O II]b

Redshifta χ2 χ2

Aa 3.0 26.2 31.3Ab 5.4 36.3 · · ·Ac 3.0 18.3 19.4Ba 5.4 33.1 · · ·Bb 5.3 4.6 5.7Ca 2.4 10.1c · · ·Cb 5.5 3.1 · · ·

aSED fit done with Subaru, HST and Spitzer data.b[O II] candidates would be at z ≃ 1.45.cThe shown best SED χ2 fit is for a young spiral galaxy.

z ≃ 5.4 supports a candidate LBG, in which casethe OSIRIS-TF photometry point probably cor-responds to a spectral feature in the absorptionarea around the the 9000 A wavelength. There-fore, object Ab is classified as a possible LBG atz & 5.4.

Ac: Figure 8c shows the photometric data forcandidate Ac, and the SEDs of a starburst galaxyat z = 3.0 and z = 1.45. The object appears inall Subaru filters, in the field of the HST programID5991, and is detected in the Spitzer 5.8µm im-age. Figure 8c is similar to Figure 8a, with thegray line showing the optical high resolution SEDtemplate for a starburst with E(B − V )< 0.1.

The photometric profile of the OSIRIS-TF ob-servations (see Figure 5c) resembles the long wave-length queue of an emission line, but the HST,Subaru and Spitzer data are not consistent witha z ≃ 6.5 LAE candidate. The fit for a starburstgalaxy at z = 1.45 matches the OSIRIS-TF photo-metric point corresponding to the [O II]λλ3726,3729doublet. The χ2 values for the best starburst fitand the [O II] interloper are comparable; alto-gether the object is likely the later.

6.4.2. OSIRIS-TF double LAE/LBG candidates

Figure 9 shows the photometry and SED fittingfor the OSIRIS-TF double LAE/LBG candidates.

Ba: Candidate Ba (shown in Figure 9a) is de-tected in all of Subaru’s filters; it lies out of the

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Fig. 9.— SED fittings for OSIRIS-TF LAEor LBG candidates. Fluxes obtained fromOSIRIS-TF (hollow circle) and Subaru, HST, andSpitzer/IRAC (filled circles for the last three) im-ages are plotted, along with SEDs fits calculatedwith HyperZ. (a) Candidate Ba: the SED fit is fora starbusrst galaxy at redshift z ≃ 5.4. The objectis most likely and interloper. (b) Candidate Bb:photometric data have been fitted at z ≃ 5.3 (solidline) for a starburst galaxy, and at 1.45 (dashedline) for an [O II] emitter. The best fit suggests a[O II] interloper at the lowest redshift. The grayline is the same as in Aa in Figure 8.

field of the HST images; and it is found in theSpitzer 3.6µm band. Figure 9a shows the pho-tometric data available along with the best SEDfit for a starburst galaxy at z ≃ 5.4. The flux inthe OSIRIS-TF synthetic filter may be dominatedby the short wavelength queue of an emission line(Figure 6a), which explains that the OSIRIS-TFdata is well above the Suprime–Cam z' band. Theobject is most likely an interloper.

16

Bb: Figure 9b shows the photometric data forobject Bb, and the SED of a starburst galaxy atz ≃ 5.3 and at 1.45. The object is detected inthe Subaru i' and z' filters; it lies outside of theHST fields; and it is also found in the Spitzer 8µmband. Figure 9b is similar to Figure 8a, with thegray line showing the optical high resolution SEDtemplate for a starburst with E(B − V )< 0.1.

Note that, as mentioned earlier, the centroidsof the photometrical profiles of the Spitzer/IRACimages for this object are shifted 1.′′3 with respectto their optical counterparts, and thus there is apossibility that the infrared images are misidenti-fied. Both starburst galaxy SED fits are reason-able. However, the OSIRIS-TF flux is well abovethe Suprime–Cam z', suggesting that some of theshort wavelength wing of the [O II] line (see Figure6b) lies in the OSIRIS-TF wavelength range. Theobject is probably an [O II] interloper.

6.4.3. OSIRIS-TF LBG candidates

Figure 10 shows the photometry and the SEDsof the OSIRIS-TF LBG candidates.

Ca: Images for Ca are available through the Sub-aru V filter, HST program ID6745 and Spitzer 3.6–5.8µm bands, remains undetected in the 8µm ob-servations and lies outside the field of Subaru’si' and z' filters. Figure 10a shows the photo-metric data for this object (see Figure 7a for theset of OSIRIS-TF observations). The OSIRIS-TFphotometric point could correspond with an ab-sorption line. We also plot the best fitting of aSED, that corresponds to a young spiral galaxyat z ≃ 2.4. The HST optical data is incongruentwith any object at z > 4. In any case, Ca is likelya young spiral galaxy.

Cb: Figure 10b shows the photometric data andthe SED of a starburst galaxy at z = 5.5. Theobject is detected in Subaru’s i' and z' filters; itlies out of the field of view of the HST images;and it is found in the Spitzer 3.6–5.8µm filter butnot in the V and 8µm bands. The starburst SEDfits the photometric data fairly well. The spec-tral profile observed with OSIRIS-TF (Figure 7b)may in fact be an artefact due to the absorption re-gion at wavelengths longer than the Lyman Break.Nonetheless, the SED fit sustains that Cb is a reli-

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Fig. 10.— SED fittings for OSIRIS-TF LBG can-didates. Fluxes obtained from OSIRIS-TF (hol-low circles) and Subaru, HST, and Spitzer/IRAC(filled circles for the last three) images are plot-ted, along with SEDs calculated with HyperZ. (a)Candidate Ca: the best SED fit is that of a spiralgalaxy at redshift z ≃ 2.4. (b) Candidate Cb: thephotometric data are consistent with the SED ofa starburst galaxy at z = 5.5; The SED profilesuggests the object may be a LBG at z & 5.5 or ahigh–redshift interloper.

able LBG candidate at z & 5.5 rather than at 6.5as estimated using only OSIRIS-TF data.

7. Discussion

In this section we compare the results obtainedwith the different instrumental models applied toour simulated LAE sample, and we examine differ-ent explanations for the results of the OSIRIS-TFpilot observations.

17

Table 4: Simulation statistics

Instrument Parametera Nb Median Q1c Q3d

IDEAL z 1290 6.55 6.53 6.58LLyα

e · · · 6.35 4.16 10.93EW0

f · · · 94 56 175FWHM0

f · · · 1.63 1.25 2.13Subaru z 1108 6.54 6.51 6.56

LLyαe · · · 7.49 4.85 13.23

EW0f · · · 105 66 187

FWHM0f · · · 1.65 1.26 2.19

OSIRIS-TF z 2708 6.55 6.51 6.59LLyα

e · · · 6.00 3.95 11.14EW0

f · · · 67 34 140FWHM0

f · · · 1.67 1.27 2.16

aModeled parameters studied (redshift, luminosity of the Lyα line, and the rest-frame EW and FWHM).bNumber of detections over the 5000 simulations.cFirst quartile of the parameter distribution of the recovered LAEs.dIdem for the third quartile.eIn units of 1043 erg s−1.f In A.

7.1. Insights from simulated data

Table 4 summarizes the modeled performancefor detection of LAEs for the three filter modelsanalyzed in this paper. We present the statisticalresults using non-parametric scores (sample me-dian and quartiles) since most of the simulatedvariables are poorly fitted by the Gaussian distri-bution.

The main differences that can be drawn fromTable 4 correspond to the number of detectionsand the EW distributions of the LAE recoveredin each filter. The number of LAEs recoveredfrom our simulation using the NB921 filter in theSubaru Suprime camera and the ideal rectangu-lar filter are similar, but they are only about 40%of those selected using the OSIRIS-TF. Theseresults are explained by the asymmetric contin-uum bias, the filter profiles, and by the differentpower of narrow-band and ultra-narrow-band sur-vey methodologies to detect LAEs with small EWvalues.

It is difficult to detect small EW LAEs. Thus,it is very interesting that the distribution of theequivalent widths at the rest-frame for OSIRIS-TFrecovered LAEs shows a median (EW 0 = 67 A)that is very accurate with respect to the median

of the simulations (68 A), and that is significantlysmaller than those of the ideal and NB921 filters(94 and 105 A, respectively). This result showsthat the OSIRIS-TF can extent the search of LAEcandidates to objects with a relatively low con-trast between the line and the continuum fluxes,which otherwise would be underestimated or evenunnoticed in narrow-band surveys. Taking thevalues of the first quartile for the Subaru’s EWin Table 4, we estimate that many LAEs withEW (Lyα) < 60 A may remain undetected in Sub-aru’s survey. This would explain the differencebetween the EW medians for LAEs at redshiftsz ≃ 5.7 and 6.5 reported by Kashikawa et al.(2011, 89 and 74 A, respectively), as well as thelack of LAEs between redshifts 6.6 < z < 7.1 withEW (Lyα) in the range between 20 and 55 A ac-counted by Pentericci et al. (2011). In fact, thesechanges in the EW of LAEs are interpreted as afast evolution of the luminosity of the Lyα linecaused by the incomplete reionization of the Uni-verse at redshifts z > 6. If LAEs at z ≃ 6.5 andwith EW (Lyα) < 60 A were numerous, tunablefilter surveys might have a large impact on ourknowledge of their LF, changing our current view.

In addition to the EW, the mean luminosityof the Lyα line is also 5% and 20% lower for

18

simulated LAEs detected with OSIRIS-TF withrespect to those found using the ideal and Sub-aru filters, respectively. As we mentioned above,the OSIRIS-TF methodology to find LAE candi-dates presented in this paper is able to find al-most all of our simulated objects. Now we seethat OSIRIS-TF superior performance respect tothe other instruments is not only due to an unbi-ased wavelength coverage, but also to differencesof the line luminosity properties of the detectedLAEs.

The distribution of redshifts for the OSIRIS-TFcandidates yields the largest interquartile range of∆z = 0.08, in contrast to 0.05 for both the idealand the NB921 filters. This difference betweenthe OSIRIS-TF and the narrow-band filters is aconsequence of the respective filter transmissionprofiles. On one hand, narrow-band filters tendto be effective in finding LAE candidates on arather restricted range of wavelengths of the fil-ter bandpass, on the other OSIRIS-TF candidatesare evenly distributed on the swept wavelengthrange between 9122 and 9160 A. Moreover, theLorentzian profile of the OSIRIS-TF transmission,extends some filter sensitivity to find LAEs be-yond the probed wavelengths. In practice, this willtranslate in a excess of LAE candidates with upperredshift limits, and an excess of double LAE/LBGcandidates with lower redshift limits at the blueand red borders of the set of OSIRIS-TF, respec-tively, as we have seen with the observed data pre-sented in §5.2. Finally, the distributions of theFWHM at rest-frame do not change significantlyamong the different filters.

The depth achieved using narrow-band filtersin LAE surveys is severely limited by the fil-ter profile. Our results using simulations agreewith the analysis of Subaru’s data reported byKashikawa et al. (2011). Thus, the output variesdramatically along the filter band, reflecting thefilter response. In any case, the asymmetric con-tinuum profile at each side of the Lyα line intro-duces a detection bias regardless the profile of thenarrow-band filter. This bias is difficult to correct,as it may depend on several factors, such as theactual EW of the Lyα line. Therefore, narrow-band surveys are useful to find candidates in aslim volume over a large area, but the propertiesthat can be derived from follow-up spectroscopicalobservations are prone to produce biased results.

In contrast, the OSIRIS-TF can sample thewhole range of the wavelengths of interest witha spectral resolution about 10 times larger thannarrow-band filters, and thus the combined Lyαline and break features could be recognized, andthe redshift determined to a better accuracy (Fig-ure 3). This avoids the biases introduced by therelatively large bandwidth and the extended wingsof the transmission profiles of narrow-band fil-ters. LAEs with their Lyα line lying between 9122and 9260 A are almost completely recovered (Fig-ure 2). However, OSIRIS-TF also has limitations,in particular the amount of total observing timeincreases with the number of images, which is pro-portional to the desired spectral resolution. Be-sides, the OSIRIS-TF relatively small field of viewof 8.53× 8.67 arcmin2 with a small shadowed areain one side, cannot compare to the wide field ofSubaru Suprime-Cam (34× 27 arcmin2). As a lowresolution Fabry-Prot spectrographa, the field ofview of tunable filter instruments is limited by thedependence of the effective wavelength on the dis-tance to the optical center (see eq. 5), that wouldspoil the desired monochromaticity in wide fieldimages (although this effect can be compensatedby wavelength scanning at the expense of telescopetime). Thus, OSIRIS-TF is an instrument suitablefor pencil-beam surveys spanning a relatively largevolume, and for assessing the biasses produced bystandard narrow-band filter surveys.

We have looked for relationships between thesimulated variables. Aside the obvious depen-dences imposed by candidate selection criteria(e.g. detection and EW limits), there are no prac-tical differences between the detected and non-detected sets of simulated data, regardless of thefilter characteristics. The only tiny effects thatwe have found involves LAES with the largest ob-served FWHM and located near any of the edgesof the filter. Thus, objects near the blue edge, butwith the Lyα peak off the filter range, may be stilldetected because part of the flux lies in the fil-ter. On the other hand, objects near the filter rededge may be undetected because the long wave-length queue of the Lyα line extends beyond thetransmitted wavelengths.

We have also investigated the effect of the num-ber of filters used to detect LAEs. This effect maybe present in observational strategies using severalnarrow bands to detect LAE candidates (or in gen-

19

Table 5: Dependence of detections on the numberof filters.

FWHM NumberDetections

EW0a

(A) of filters (A)

12 24 2535 66±220 14 2542 66±230 9 2497 67±250 5 2282 72±260 4 1799 74±2

100 2 168 51±6

aMedian and error. The errors (e) are computed from theinterquartile range(rq) by e = 0.7413rqN−1/2, whereN is the number of detections.

eral any emission objects), such as the OSIRIS-TFprocedure discussed in this paper. The particularfilter profile may render small changes on the re-sults, and thus we have used ideal filter sets (ratherthan Gaussian or Lorentzian profiles) to charac-terize the several sets with different numbers offilters. The filter passband is different for eachset, thus they cover the same spectral region andeffective volume. In any case, the separation be-tween the centers of two adjacent filter is half ofthe FWHM of the filter passband, as in the caseof the OSIRIS-TF.

Table 5 and Figure 11 summarize the resultsobtained with different sets of ideal filters. Thewavelength range (r) at the filters FWHM coveredfor all the sets is the same, and it is given by theexpression:

r =n+ 1

2w = 150 A, (6)

where n is the number of filters in the set and wis the filter FWHM. We notice that the numberof detections, around 2500 objects, decays slightly(3%) from 24 to 9 filters. However, for 5 filtersthere is a sharp cut-off in this number, and forthe set of two filters, only 168 are detected. Forthe line luminosity and redshift, the change alsostarts at 5 filters, with increments in their values.The rest-frame EW shows also an increment for 5filters, but abruptly decays for 2 filter sets. Thereason for this decay is that the only 168 objectsdetected are very luminous in both line and con-tinuum emission. The rest-frame FWHM of theline, not shown in Figure 11, may also show an

0

1000

2000

3000

Num

ber

of d

etec

tions

a

6.55

6.60

6.65

z

b

0 5 10 15 20 25 40

50

60

70

80

Number of filters

EW

(A)

d 0

1

2L

(×1042

erg

s−1) c

Fig. 11.— Dependences on the number of filters.This figure shows the results obtained using differ-ent sets of ideal (rectangular) filters on the LAEsimulation dataset. Each of these sets consist ofdifferent number of filters, but they cover the samewavelength range than the original OSIRIS-TFset. Filters in a given set share the same FWHM,and their centers differ in steps of half this FWHM.For detections, we have adopted the same criteriaas in the case of OSIRIS-TF. Panels show (a) thenumber of detections, (b) the median redshift, (c)the line luminosity, and (d) the restframe equiv-alent width. The results for 7 or more filters aresimilar, but for smaller number of filters the num-ber of detections falls dramatically, and the otherparameters also change significantly.

20

Table 6: OSIRIS-TF Survey Expected Quantities

Observationsa

Planed Accomp.

Slicesb 24 5Irradiance limitc 4 9LLyα limitd 2.1 4.7Volume LAEse 8760 1503Expected LAEsf 4.2 0.1L[O II] limitg 2.9 6.4Volume [O II]e 122 21Expected [O II] (Takahashi)h 7.4 0.8Expected [O II] (Dressler)i 2.5 0.2

. .

aState of the observations: Planned or Accomplished.bNumber of wavelength slices.cIrradiance lower limit (×10−18 erg s−1 cm−2).dLyα luminosity lower limit (×1042 erg s−1).eProper volume covered (Mpc−3).fNumber of expected LAEs obtained through the Schechterfunction with Kashikawa et al. (2011) parameters. Numbersare given with a precission of one decimal place, rather thanintegers, to ensure that at least one significant digit is shown.

g[O II] luminosity lower limit (×1040 erg s−1).hTakahashi et al. (2007)iDressler et al. (2011)

increment, but it is less significant than for theprevious variables. We note that for the 2 filtersset the distribution of detected objects becomesbimodal, rather than reflecting the original distri-bution. This occurs because a line located at themiddle of the wavelength range lies in two filters,and our detection algorithm is unable to recognizesuch a line when the number of filters is small.

For Lyα and in general for other single andisolated emission lines, the previous analysis indi-cates that we might save significant telescope time,and still obtain similar results, observing through9 filters with FWHM of 30 A rather than 24 filterswith FWHM of 12 A. In return, the lower spec-tral resolution may increase the number of inter-lopers. However, currently the OSIRIS-TF reso-lution cannot be changed in the wavelength rangeof our observations. It is worth noting that forother studies where line deblending is necessary,such as mapping Hα+[N II] or the [S II] doublet,it may be convenient to maintain a filter band-pass lower than 15 A (e.g. Lara-Lopez et al. 2010;Cedres et al. 2013).

7.2. Excess of candidates and interlopers

The pilot observations obtained with theOSIRIS-TF embrace 5 out of 24 adjacent wave-length slices included in our complete program tofind LAE candidates. Table 6 shows the values ofsome calculated and expected quantities.

Given the LAEs and [O II] interlopers LFs, wehardly expected to find any of these objects inthe limited set of observations presented in thispaper. Regardless of this prospect, we have ex-tracted five candidates for which OSIRIS-TF dataare congruent with LAEs, two of them also LBGcandidates (in total, there were four LBG candi-dates). All of the LAE candidates were rejectedafter fitting broad-band SEDs, but then three ofthem showed as possible [O II] interlopers, whichis also a number of objects higher than expecta-tions. These discrepancies between the numberof expected LAEs and [O II] interlopers, and theactual number of candidates, is explained due tothe low number of filters used in our current ob-servations. Thus, the available data expands onlya very small range of wavelengths (the FWHMof the synthetic OSIRIS-TF is 36 A rather than150 A for the complete set of 24 filters), preventinga reliable sampling of the line and the continuumto both sides of the line. As a result, we can-not distinguish between small and large FWHMlines because only a part of the wing of a line isobserved, or even discriminate between emissionlines and partially observed absorption features.These problems can be easily solved by observingthrough the complete set of filters. Meanwhile,we need to rely on archive data to improve can-didate selection. Under these circumstances, eventhe limited number of broad-band archive data isuseful to reject candidates with SED profiles notcompatible with LAEs or LBGs, but insufficientto confirm the nature of the objects.

Dressler et al. (2011) have calculated the lumi-nosity function for [O II], [O III], and Hα inter-lopers for LAEs at z ≃ 5.7. These LFs have asharp cut-off for luminosities & 3 × 1041 erg s−1,being [O II] emitters the most numerous of theforeground sources. Considering a similar cut-offfor z ≃ 6.5 interlopers, it corresponds to irra-diances > 2.2 × 10−17 erg s−1 cm−2 for [O II]atz = 1.45, > 9.1 × 10−17 erg s−1 cm−2 for [O III]at z = 0.82, and > 5.6 × 10−17 erg s−1 cm−2 for

21

Table 7: Observer-frame FWHM for z ≃ 6.5 LAEsand interlopers.

Line RedshiftVelocity FWHM(km s−1) (A)

Lyαλ1216 6.50 400 12.16[O II]λλ3726−9 1.45 – ∼ 10[O III]λ5007 0.82 100 3.04Hαλ6563 0.39 100 3.04

Hα at z = 0.39. Our LAE candidates listed in Ta-ble 1 have irradiances below all these flux cut-offvalues, the only exception being Aa which slightlyexceeds the [O II] flux cut-off. Therefore, interlop-ers can enhance the number of fake candidates, inaccordance with the results obtained when fittingthe SEDs in §6.

Lyα observed lines at z = 6.5 are rather wide,with FWHM around 10 A. Interlopers’ emissionlines have observed widths that usually are wellbelow this value. Table 7 shows the observedFWHM for Lyα and possible interlopers. Forthe interlopers, a fiducial velocity for the emis-sion lines arising from Star Formation Regions of100 km s−1 has been chosen. Of course, the in-terlopers have redshifts z ≪ 6.5, and thus lowerdoppler broadening than Lyα. Then, all the sin-gle lines, but the unresolved [O II] blend, haveobserved FWHM easily distinguishable from theLyα with the OSIRIS spectral resolution. In thecase of the [O II] blend, the separation betweenthe individual line peaks, rather than the veloci-ties, dominates the observed FWHM.

Given the detection limit for these observa-tions (9 × 10−18 erg s−1 cm−2), [O II] interlop-ers at z ≃ 1.45 with line luminosities brighterthan L[O II] > 1.166 × 1041 erg s−1 will be de-tected. This yields a number of 0.2 – 0.8 ex-pected [O II] interlopers in our data, dependingon the luminosity function adopted (Dressler et al.2011; Takahashi et al. 2007, respectively). Ex-pected numbers for the full set of planed obser-vations are shown in Table 6. From Dressler et al.(2011), we expect a final efficiency of about 2/3 tofind LAEs, i.e., 2 LAEs for every [O II] interloper,when the program is fulfilled. The OSIRIS MultiObject Spectrograph-Mode, soon available at theGTC, could be used for follow-ups if necessary.

The lines ratio [O II]λ3726/[O II]λ3729 be-

tween the individual lines that conform the [O II]blend feature depends on the electronic densityNe. Extreme cases have values limNe→0 = 1.5and limNe→∞ = 0.35, and thus this lines ratio canbe used to calculate the electronic density whenit is in the range 2 < log(Ne) < 4 (Pradhan et al.2006). Different values of [O II] ratios have a directincidence on the unresolved blend FWHM mea-sured with OSIRIS-TF, which makes even moredifficult to distinguish between LAEs and [O II]interlopers. There are few studies on the LF of[O II], and all of them deal with the blend asa single feature (Hogg et al. 1998; Gallego et al.2002; Teplitz et al. 2003; Takahashi et al. 2007;Dressler et al. 2011).

An effect to take into account is the distor-tion of the observed LAEs and [O II] interlop-ers LF, and thus their number counts, due tothe redshift dependent magnification bias (e.g.Bartelmann 2010; de Diego et al. 2011). This ef-fect increases the number of observed faint sources,but reduces their number density enlarging the an-gular distance between the sources. The overallresult depends on the steepness of the number-count function. For the high luminosity LAEs,such as those observed at z ∼ 6.5, this func-tion is steep and more sources become observ-able. The situation is more complex for [O II]interlopers, for which the high luminosity objectsdo not dominate the number counts. Thus, themagnification bias may reduce the number of ob-served [O II] interlopers with luminosities below ≃3×1041 erg s−1, and enhances the counts for moreluminous sources. In our case, the field observedwith the OSIRIS-TF is dominated by the clusterof galaxies MS 2053.7-0449 at redshift z = 0.583,that expands about 4×7 arcmin2. The cluster con-tains a gravitational lensed arc (Luppino & Gioia1992; Tran et al. 2005). The strong gravitationallens model has been discussed by Verdugo et al.(2007), and the weak-lensing signature of the clus-ter was detected by Hoekstra et al. (2002), who es-timated a cluster velocity dispersion of 886 km s−1.

Following Hildebrandt et al. (2011), we haveused the singular isothermal sphere approximationto calculate the lens magnification as a function ofthe angular separation θ from the cluster center:

µ(θ) =θ

θ − θE, (7)

22

where θE is the Einstein radius (Narayan & Bartelmann1996) for the cluster:

θE = 4π(σv

c

)2 Dds

Ds

, (8)

and where σv is the one-dimensional velocity dis-persion (we have adopted Hoekstra et al. 2002, es-timate of 886 km s−1), Dds is the angular diameterdistance from the lensing cluster to the source, andDs is the angular diameter distance from the ob-server to the source. We have taken a commonredshift of z = 6.5 for LAEs and z = 1.45 for[O II] interlopers. In our case, the Einstein radiusfor MS 2053.7-0449 is θE = 17′′ and 11′′ for LAEsand [O II] interlopers, respectively.

The mean magnification 〈µ〉wl in the weaklensing regime of the OSIRIS-TF field aroundMS2053.7-0449 can be calculated considering anangular separation 3θE ≤ θ ≤ 4′ which rangesfrom the weak lensing limit to the edge of thedetector:

〈µ〉wl =

∫ 4′

3θEµ(θ) dθ

∫ 4′

3θEdθ

, (9)

yielding mean magnifications of 1.17 for LAEs and1.13 for [O II] interlopers. Figure 12 shows thechange of the magnification with the distance tothe cluster center. For the planed sample, thismagnification yields an increase from 4.2 to 5.7 forexpected LAE counts. However, for the currentlyobserved bands, the expected number of LAEs hasa negligible increase of about 0.1 objects. As dis-cussed above, the case of [O II] interlopers is morecomplicated, and depends on the actual profile oftheir LF.

A major concern with photometric searches ofLAE candidates is that only a tight range of red-shifts is probed, yielding small samples prone tocosmic variance due to large-scale density fluc-tuations. Cosmic variance accounts for devia-tions from the factual or expected values of thenumber-counts of rare phenomena, and of ob-jects found in small volume surveys. FollowingTrenti & Stiavelli (2008), we have calculated thatthe uncertainty on the number-counts is σcounts =1 for both LAEs and [O II] interlopers. More-over, [O II] emitters are likely to show strong cross-correlation positions (Dressler et al. 2011). There-fore, we do not discard the possibility that someinterloper candidates are actually [O II] emitters.

0 100 200 3001.0

1.1

1.2

1.3

θ (arcsec)

|µ|

Fig. 12.—Magnification as a function of the angu-lar separation to the center of the cluster of galax-ies. The dotted line shows the absolute value ofthe magnification µ as a function of the angularseparation θ for the cluster MS 2053.7-0449, usinga singular isothermal sphere model. Filled circlescorrespond to [O II] interloper candidates.

8. Conclusions

Narrow-band surveys allow to sample large skyareas and have been successful in finding LAE can-didates. However, we have shown that the asym-metrical profile of the continuum around the Lyαline in LAEs yields a detection bias that affectsthe performance to find objects at redshifts wherethe Lyα line lies near the long wavelength edge ofnarrow-band photometric filters. Therefore, theSubaru survey and others that might be conductedusing narrow-band filters, are prone to yield a bi-ased LF. Besides, this methodology tends to ignoreor underestimate small EW objects.

In the case of ultra-narrow-band surveys, oursimulations show that the overall performance forLAE detection using OSIRIS-TF is not affectedby the filter transmission profile, and the LFof LAEs could be accurately calculated. More-over, OSIRIS-TF can recover simulated LAEswith Lyα line EWs significantly smaller thanthe objects recovered with narrow-band filters.Nonetheless, tunable filters do not produce largemonochromatic images, and the number of ultra-narrow-band images increases in proportion tothe required spectral resolution, thus the size ofthe studied area is limited for practical reasons.

23

Therefore, both narrow-band and ultra-narrow-band surveys have different strengths and weak-nesses, and thus they must be regarded as comple-mentary strategies to study high-redshift LAEs.

We are carrying on a program to find LAE can-didates with the OSIRIS-TF at the GTC. Partof this program is devoted to find candidates atredshift z ≃ 6.5, with an strategy based on ourstudy with Monte Carlo data. We have alreadyperformed pilot observations of five sets of imageswith the GTC and the OSIRIS-TF instrument atadjacent wavelengths, which is a fraction of the24 wavelength slices that we plan to observe. TheOSIRIS-TF images are separated by wavelengthsteps of 6 A, and have a bandpass of 12 A, cov-ering a wavelength range of about 36 A. The to-tal exposure time in each wavelength is 630 s, ren-dering a detection level of 9× 10−18 erg s−1 cm−2,which is about two times less sensitive than thatreported for the Subaru LAE surveys. AvailableSubaru, HST/WFPC2 and Spitzer/IRAC archivedata have been employed to build SED mod-els. Because of the limited wavelength range ofOSIRIS-TF observations, these models have beenvery helpful to reject LAE and LBG candidatesand to identify interlopers, although the low num-ber of bands used in the fits prevents accurate ob-ject classification and redshift estimate.

We have calculated the number of expectedLAEs in the OSIRIS-TF field with our observa-tional conditions. For this purpose, we have takeninto account the weak lensing effect introduced bya nearby cluster of galaxies, and the effect of thecosmic variance. Thus, we expected no more thanone possible LAE and [O II] interloper showing upin our data. Actually, we have identified three pos-sibly [O II] interlopers, and one LBG candidate.The possible overabundance of [O II] interlopersin the field might be a result of cross-correlationpositions (Dressler et al. 2011). In any case, theseresults support the capabilities of OSIRIS-TF toperform an accurate survey of emission-line ob-jects.

Future work We plan to complete the set ofOSIRIS-TF 24 wavelength slices to extract a sam-ple of high-redshift candidates, namely LAEs,LBGs and high-redshift interlopers. The sam-ple will be studied to confirm the nature of theobjects using OSIRIS Multi-Object Spectrograph

when available.

This research has been partially funded bythe UNAM-DGAPA-PAPIIT IN110013 Program.J.A.D. and M.A.D. are grateful for support fromCONACyT grant CB-128556. J.A.D. is gratefulfor support from grant SAB2010-0011 awardedby the Spanish MIED through the “ProgramaNacional de Movilidad de Recursos Humanos”included in the Plan Nacional de I-D+i 2008-2011. T.V. acknowledges support from CONA-CYT grant 165365 through the program “Es-tancias posdoctorales y sabaticas al extranjeropara la consolidacion de grupos de investigacion.”This work has been partially funded by the Span-ish Ministry of Science and Innovation (MICINN)under the Consolider-Ingenio 2010 Program grantCSD2006-00070: First Science with the GTC(http://www.iac.es/consolider-ingenio-gtc),AYA2011-29517-C03-01, and AYA2011-29517-C03-02. Observations presented in this paper weremade with the Gran Telescopio Canarias (GTC),instaled in the Spanish Observatorio del Roquede los Muchachos of the Instituto de Astrofısicade Canarias, in the island of La Palma. The au-thors are thankful to the anonymous referee forthe critical and constructive suggestions.

Facilities: GTC (OSIRIS).

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