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Accepted for ApJLPreprint typeset using LATEX style emulateapj v. 5/2/11

LY FOREST TOMOGRAPHY FROM BACKGROUND GALAXIES:THE FIRST MEGAPARSEC-RESOLUTION LARGE-SCALE STRUCTURE MAP AT z > 2

Khee-Gan Lee1, Joseph F. Hennawi1, Casey Stark2,3, J. Xavier Prochaska4,5, Martin White2,3,David J. Schlegel5, Anna-Christina Eilers1, Andreu Arinyo-i-Prats6, Nao Suzuki7, Rupert A.C. Croft8,

Karina I. Caputi9, Paolo Cassata10,11, Olivier Ilbert11, Bianca Garilli12, Anton M. Koekemoer13,Vincent Le Brun11, Olivier Le Fe`vre11, Dario Maccagni15, Peter Nugent3,2, Yoshiaki Taniguchi14,

Lidia A.M. Tasca11, Laurence Tresse11, Gianni Zamorani15, Elena Zucca15

Accepted for ApJL

ABSTRACT

We present the first observations of foreground Lyman- forest absorption from high-redshift galax-ies, targeting 24 star-forming galaxies (SFGs) with z 2.32.8 within a 515 region of the COSMOSfield. The transverse sightline separation is 2 h1 Mpc comoving, allowing us to create a tomo-graphic reconstruction of the 3D Ly forest absorption field over the redshift range 2.20 z 2.45.The resulting map covers 6 h1 Mpc14 h1 Mpc in the transverse plane and 230 h1 Mpc along theline-of-sight with a spatial resolution of 3.5 h1 Mpc, and is the first high-fidelity map of large-scalestructure on Mpc scales at z > 2. Our map reveals significant structures with & 10 h1 Mpc extent,including several spanning the entire transverse breadth, providing qualitative evidence for the fila-mentary structures predicted to exist in the high-redshift cosmic web. Simulated reconstructions withthe same sightline sampling, spectral resolution, and signal-to-noise ratio recover the salient struc-tures present in the underlying 3D absorption fields. Using data from other surveys, we identified18 galaxies with known redshifts coeval with our map volume enabling a direct comparison to ourtomographic map. This shows that galaxies preferentially occupy high-density regions, in qualitativeagreement with the same comparison applied to simulations. Our results establishes the feasibility ofthe CLAMATO survey, which aims to obtain Ly forest spectra for 1000 SFGs over 1 deg2 ofthe COSMOS field, in order to map out IGM large-scale structure at z 2.3 over a large volume(100 h1 Mpc)3.

Subject headings: cosmology: observations galaxies: high-redshift intergalactic medium quasars: absorption lines surveys techniques: spectroscopic

1. INTRODUCTION

lee@mpia.de1 Max Planck Institute for Astronomy, Konigstuhl 17, D-69117

Heidelberg, West Germany2 Department of Astronomy, University of California at Berke-

ley, B-20 Hearst Field Annex # 3411, Berkeley, CA 94720, USA3 Lawrence Berkeley National Laboratory, 1 Cyclotron Rd.,

Berkeley, CA 94720, USA4 Department of Astronomy and Astrophysics, University of

California, 1156 High Street, Santa Cruz, CA 95064, USA5 University of California Observatories, Lick Observatory

1156 High Street, Santa Cruz, CA 95064, USA6 Institut de Cie`ncies del Cosmos, Universitat de Barcelona

(IEEC-UB), Mart Franque`s 1, E08028 Barcelona, Spain7 Kavli Institute for the Physics and Mathematics of the Uni-

verse (IPMU), The University of Tokyo, Kashiwano-ha 5-1-5,Kashiwa-shi, Chiba, Japan

8 Department of Physics, Carnegie-Mellon University, 5000Forbes Avenue, Pittsburgh, PA 15213, USA

9 Kapteyn Astronomical Institute, University of Groningen,P.O. Box 800, 9700 AV Groningen, The Netherlands

10 Instituto de Fisica y Astronomia, Facultad de Ciencias, Uni-versidad de Valparaiso, Av. Gran Bretana 1111, Casilla 5030,Valparaiso, Chile

11 Aix Marseille Universite, CNRS, LAM (LaboratoiredAstrophysique de Marseille) UMR 7326, 13388, Marseille,France

12 INAFIASF, via Bassini 15, I-20133, Milano, Italy13 Space Telescope Science Institute, 3700 San Martin Drive,

Baltimore MD 21218, USA14 Research Center for Space and Cosmic Evolution, Ehime

University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan15 INAFOsservatorio Astronomico di Bologna, via Ranzani,1,

I-40127, Bologna, Italy

The Lyman- (Ly) forest absorption seen in quasarspectra is a crucial probe of the intergalactic medium(IGM). In the modern fluctuating Gunn-Peterson sce-nario (Cen et al. 1994; Bi et al. 1995; Croft et al. 1998;Hui et al. 1997), this is from residual neutral hydrogenin photoionization-equilibrium, tracing the underlyingdensity field, allowing the study of large-scale structure(LSS) at z & 2 (e.g., Croft et al. 2002; McDonald et al.2006; Busca et al. 2013; Palanque-Delabrouille et al.2013; Delubac et al. 2014).The Ly forest observed in individual quasars probe

the IGM along 1-dimension, but using multiple spectrawith small transverse separations, it is possible to to-mographically reconstruct a 3D map of the Ly for-est absorption (Pichon et al. 2001; Caucci et al. 2008;Cisewski et al. 2014; Lee et al. 2014a, hereafter L14).The effective spatial-resolution, 3D, of such a map isdetermined by the transverse sightline separation, d.This probes Mpc scales only by exploiting UV-brightstar-forming galaxies (SFGs) as background sources inaddition to quasars. However, SFGs are faint (g & 23) even with 8-10m telescopes, only spectral S/N of afew are feasible from such objects, assuming reasonableexposure times. However, L14 argued that such data atmoderate resolutions (R /() 1000) are adequatefor Ly forest tomography that resolve the LSS on scalesof 3D 2 5 h

1 Mpc.In this Letter, we describe pilot observations for the

2 Lee et al.

COSMOS Lyman-Alpha Mapping And Tomography Ob-servations (CLAMATO) survey. The full survey is aimed

at mapping the z 2.3 IGM within 1 deg2 of theCOSMOS field (Scoville et al. 2007). The pilot observa-tions were however limited to one half-night of successfuldata, yielding moderate-resolution spectra for 24 SFGsat g 24.9 within 5 14.This data represents, to our knowledge, the first sys-

tematic attempt to exploit spectra of unlensed high-redshift SFGs for Ly forest analysis. Our backgroundsources are 2.5 3mag fainter than existing Ly for-est datasets (e.g., g 21.5 in BOSS, Dawson et al.2013), yielding 100 greater area density of sightlines

( 1000 deg2 vs 15 deg2 in BOSS). This dramaticincrease results in small average inter-sightline separa-tions (d 2.3 h

1 Mpc), enabling a tomographic re-construction of the 3D Ly forest absorption, providingan unprecedented view of the z > 2 cosmic web on scalesof several comoving Mpc. As we shall see, comparisonswith a small number of coeval galaxies as well as sim-ulated reconstructions indicate that the map is indeedtracing LSS.In this paper, we assume a concordance flat CDM

cosmology with M = 0.26, = 0.74, and H0 =70 km s1.

2. OBSERVATIONS AND DATA REDUCTION

Our observations target g-selected galaxies and AGN(using the Capak et al. 2007 photometry) at 2.3 24.5 ob-jects but these were less likely to be successfully reducedor have adequate S/N. Nevertheless, even this reducednumber of sources is sufficient to carry out Ly foresttomography, as we shall see.The position of the 24 SFGs on the sky are shown in

Figure 1. Our brightest objects are g 24.0 SFGs withS/N 34 per 1.2 A pixel, while on the faint-end we usespectra down to S/N 1.3 from g 24.8 sources. Ex-amples of the spectra are shown in Figure 2. We also at-tempted to visually identify damped Ly absorbers thatmight affect Ly forest analysis but found none.To extract the Ly forest transmission from the spec-

tra, we need to estimate the intrinsic continuum ofthe SFGs. Studies of z 3 SFG composite spectra(Shapley et al. 2003; Berry et al. 2012) suggest that thisis relatively flat in the Ly forest region, with only afew strong intrinsic absorbers visible this is corrobo-rated by high-resolution line analysis of the lensed galaxyMS1512-cB58 (Savaglio et al. 2002). From these stud-ies, we determined that the strongest intrinsic absorp-tion within the 1040 1190 A Ly forest region are atN II 1084.0, N I 1134.4, and C III 1175.7 we mask5 A around these transitions. We then adopt as ourcontinuum template the restframe composite spectrumof 59 SFGs from Berry et al. (2012), in which the Lyforest variance in the restframe 1040 1190 A regionhave been smoothed out through averaging, albeit withan overall absorption decrement.Using this template, we estimate the continuum, C,

for each individual spectrum by mean-flux regulation(Lee et al. 2012, 2013), i.e. adjusting the amplitude andslope of the 1040 1190 A continuum template untilthe mean Ly forest transmission, F (z), from eachspectrum agrees with the measurements of Becker et al.(2013). This method ensures that there is no overall biasin the resulting continua. We estimate the continuumerror to be . 10%, by considering the variation of Star-burst99 (Leitherer et al. 1999, 2010) models with respectto various physical parameters. This is adequate for ourS/N 4 spectra, but in future papers we will study SFGcontinuum-fitting in more detail.We divide the restframe 1040 1190A flux, f , from

each spectrum by the continuum to obtain the Ly for-est transmission F = f/C, and further the forest fluctu-ations:

F = F/F (z) 1. (1)

16 http://www.ucolick.org/~xavier/LowRedux/lris_cook.html

Ly Forest Tomography with z 2 3 Galaxies 3

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Fig. 1. HST ACS F814W mosaic (Koekemoer et al. 2007) of our target region. The red boxes indicate our background spectroscopicsources with Ly forest coverage over 2.15 z 2.40; source redshifts and g-magnitudes are labeled above each object. The transversearea of our map is bounded in blue; upper- and right-axes indicate the transverse comoving separation at z = 2.325 relative to the mapcoordinate origins.

We also compute the error, N = /C/F (z), where is the pixel noise reported by the reduction pipeline. Thevectors of F and N , along with the corresponding 3Dpixel positions, constitute the inputs for the tomographicreconstruction.

3. TOMOGRAPHIC RECONSTRUCTION

To create the Ly forest tomographic reconstruction,we use Wiener filtering (e.g., Wiener 1942; Press et al.1992; Zaroubi et al. 1995), where the reconstructed field,recF , is:

recF = CMD (CDD +N)1 F , (2)

where CDD +N and CMD are the data-data and map-data covariances, respectively. The noise covariance ma-trix N is assumed to have only diagonal elements set bythe noise variances, Nii =

2N,i. This term allows us to

weight each input pixel by its S/N, so lower-S/N spectraare down-weighted and avoids noise spikes from biasingthe map.Following L14 and Caucci et al. (2008), we assume that

between any two points r1 and r2, whether in the maps

4 Lee et al.

Fig. 2. Examples of SFG spectra obtained with Keck-LRIS and subsequently used for Ly forest tomographic reconstruction. From topto bottom, these represent our highest-, median-, and lowest-S/N spectra, respectively. The red curve represents the estimated pixel noise,with masked pixels (mostly intrinsic absorption-lines) set to zero. The green curve is the Shapley et al. (2003) composite LBG spectrumoverplotted at the source redshifts, while the orange curve is the estimated continuum (see text).

or skewers, CDD = CMD = C(r1, r2) and

C(r1, r2) = 2F exp

[(r)

2

2L2

]exp

[(r)

2

2L2

], (3)

where r and r are the distance between r1 andr2 along, and transverse to. the line-of-sight, respec-tively. L and L are free parameters that set the effec-tive smoothing of the reconstruction parallel and perpen-dicular to the line-of-sight, respectively, while F = 0.8sets the overall correlation strength. These parame-ters need to be matched to the data quality: we setL = 2.7 h

1 Mpc, roughly the comoving scale alongthe LOS corresponding to our spectral resolution ele-ment. For L, Caucci et al. (2008) suggested setting itto the typical transverse sightline separation d, butwe choose L = 3.5 h

1 Mpc even though our sightlineseparation is d 2.3 h

1 Mpc. This is a conservativechoice taking into account the low-S/N of our individualspectra. The choice of these reconstruction parameters issomewhat arbitrary since small changes do not qualitia-tively change the resulting map features, but in futurework we will discuss optimal choices for these parame-ters.Our map originates at [0, 0] =

[10h0022.s56,+021048.0], spanning

[6h1 Mpc, 14h1 Mpc] in the [xperp, yperp] di-rections on the sky (c.f. top- and right-axes inFigure 1); along the line-of-sight, the origin isz = 2.20 and extends = 230 h

1 Mpc upto z 2.45, giving an overall comoving vol-ume of 6 h1 Mpc 14 h1 Mpc 230 h1 Mpc =19320h3Mpc3 (27 h1 Mpc)3. Note that our mapdoes not cover the region . 211, where we expe-rienced a high failure-rate in spectral-extraction andredshift-identification due to deteriorating observingconditions. However, the two spectra in the excludedregion are still included in the map input; given ourtransverse correlation length of L = 3.5 h

1 Mpc,these spectra ( 1.5 h1 Mpc and 3 h1 Mpc fromthe lower map boundary) still contribute to the low-yperpportions of the map.We evaluated Equation 2 to solve for the output to-

mographic map, recF , using a preconditioned conjugate-gradient algorithm to carry out the matrix inversionand matrix-vector multiplication (C. Stark et al., inpreparation), sampling on a 3D comoving grid with(0.5 h1 Mpc)3 cells. For simplicity, we assumed a fixeddifferential comoving distance d/dz (evaluated at z =2.325, the mean map redshift) when setting up the out-put grid. This avoids a flared map geometry, since thetransverse comoving area increases with redshift, but

Ly Forest Tomography with z 2 3 Galaxies 5

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Fig. 3. Tomographic reconstruction of 3D Ly forest absorption from our data, shown in 3 redshift segments in 3D (top) and projectedover 3 slices along the R.A. direction (bottom panels). The color scale represents reconstructed Ly forest transmission such that negativevalues (red) correspond to overdensities. Square symbols denote positions of coeval galaxies within the map; error bars indicate thev 300 km s

1 uncertainty on their redshifts. Pink solid lines indicate where 3 of the skewers probe the volume, with inset panelsindicating the corresponding 1D absorption spectra (top-hat smoothed by 3 pixels) that contributed to the tomographic reconstruction.

within our limited redshift range this effect is small.The resulting map of the 3D Ly forest absorption,

recF , is shown in Figure 3 as 3D visualizations andslices projected over the xperp (R.A.) direction. A lotof structure is obvious even within this small volume,with overdensities (negative-recF regions) spanning co-moving distances of yperp & 10 h

1 Mpc both alongthe line-of-sight (e.g. from z 2.21 to z 2.23 atyperp 8 h

1 Mpc) and across the transverse direction(at z 2.43). The strong overdensities are typically sam-pled by multiple sightlines at different background red-shifts. This is illustrated by inset panels in the map slicesin Figure 3, where we show 3 examples of the 1D absorp-tion field, F , that went into the reconstruction theoverdensity at a comoving distance of 3950 h1 Mpcand yperp 5 h

1 Mpc can be seen as clear dips in allthree of the spectra, which is unlikely to be caused by

pixel noise. Note that in moderate-resolution Ly forestdata, significant absorbers are typically due to blendsof clustered Ly forest absorption and not individual ab-sorbers (Lee et al. 2014b; Pieri et al. 2013). One alsoclearly sees significant voids (dark blue regions) on scalesof 5 10 h1 Mpc.As validation, we performed reconstructions on mock

data sets derived from simulations (e.g., L14). Thesemocks have identical sightline configurations, resolution,and S/N as the data, including random continuum errorswith 7% RMS. The resulting reconstructions are illus-trated in Figure 4, compared with the true absorptionfield from the simulation. The good correspondence be-tween large-scale features in the true and reconstructedfields gives us confidence that the real map (Figure 3) isindeed probing LSS. However, the PDF of the simulatedreconstructions differed from the real map (c.f. black

6 Lee et al.

Mock Spectra

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Fig. 4. (Top) A slice from a tomographic reconstruction (projected over xperp = 2h1 Mpc) using a mock data set with similarspatial sampling and S/N to our data. (Middle) A reconstruction (with the same [L, L, F ]) from the full grid of noiseless spectra with

0.8h1 Mpc transverse separations. For reference, the bottom panel shows the true absorption field in the simulation. Magenta squaresindicate locations of coeval R 25.5 galaxies in the top panel we also introduced random redshift errors.

histograms in Figures 5a and b). To investigate, we ran24 mock reconstructions on independent simulation vol-umes, which showed considerable scatter in the resultingPDFs (shaded grey area in Figure 5b). This suggeststhat part of the discrepancy is due to cosmic-variancefrom our small volume. Moreover, while DM-only sim-ulations correctly reproduce Ly forest clustering, theydo not yield the right PDF (White et al. 2010), whichcould also contribute to the disagreement.L14 argue (e.g. their Figure 6) that the reconstructed

recF scales approximately linearly with the dark-matteroverdensity, dm dm/dm, smoothed on similarscales (3D 3.5 h

1 Mpc in our case), albeit with somescatter due to reconstruction noise. The most negativerecF correspond to overdensites of dm 2 while themost positive recF indicate underdensities of dm 0.2.

4. COMPARISON WITH COEVAL GALAXIES

Since galaxies are well-known tracers of LSS, wecan exploit the spectroscopically-confirmed high-redshiftgalaxies within the COSMOS field (Lilly et al. 2007;Le Fevre et al. 2014) to make a comparison with our Lyforest tomographic map. We searched an internal COS-MOS compilation of all available spectroscopic redshifts,and found 18 galaxies coeval within the map volume (4were uniquely confirmed by our observations). This small

number is clearly inadequate for mapping z & 2 LSS on Mpc scales, illustrating the challenge of using galaxyredshift surveys for this purpose, despite many hundredhours of large-telescope time. In order to make galaxymaps with comparable resolution to our tomographic re-constructions, the galaxy number density needs to beincreased dramatically, requiring 30m-class telescopes toobtain redshifts from faint (R & 26) galaxies.For these coeval galaxies, we determined their 3D po-

sitions within our map (overplotted on Figure 3) andevaluated the corresponding recF . The

recF sampled by

these galaxies are shown in Figure 5a, compared with therecF distribution from the full map; in Figure 5c, we showthe galaxy distribution as a function of the percentile ofmap ranked by flux (where larger flux percentiles rep-resent overdensities). The galaxies preferentially occupylow-recF regions (i.e. overdensities) of the map.However, at first glance it seems troublesome that sev-

eral galaxies are located in high-recF (underdense) re-gions. This could partly be due to errors in the galaxyspectroscopic redshifts: these are v 300 km s

1

(Diener et al. 2013), i.e. 3.3 h1 Mpc along the

LOS at z 2.3. This seems plausible for the galaxy at[xperp, yperp, z] = [0.5 h

1 Mpc, 0.6 h1 Mpc, 2.233] (toppanel, Figure 3), which apparently occupies a void but isin fact within 1 of two overdensities on either side. In-

Ly Forest Tomography with z 2 3 Galaxies 7

(a) Data Reconstruction

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Fig. 5. (a) PDF of our tomographic map (black) compared with that sampled by 18 coeval galaxies within our map volume (red,both PDFs normalized to unit area). (b) Similar to (a), but evaluated over 24 mock reconstructions simulating the real map. The redcurve shows the rec

Fevaluated at 2506 simulated R 25.5 galaxies within the mock reconstructions the simulated galaxies clearly also

preferentially live in low-recF

regions. Shaded regions indicate the range of map PDFs from the 24 mock reconstructions, indicating thesignificant sample variance from the small volume. (c) Distribution of coeval galaxies as a function of the map flux percentile, such thatrecF

decreases with the percentile, i.e. larger percentiles probe overdensities. The black curve indicates the predicted distribution from thesimulated galaxies within our mock reconstructions.

deed, in Figure 3 most of the galaxies are within 1 ofsignificant overdensities. Another possible reason for thisdiscrepancy could be different redshift-space distortionsexperienced by the galaxies and the forest: the latterhas been constrained by Slosar et al. (2011) but yet tobe measured for z & 2 galaxies.Tomographic reconstruction errors (c.f., Figure 4)

could also decrease the correlation between the galaxiesand 3D Ly absorption, particularly in regions poorly-sampled by sightlines. We investigate this using oursimulations, from which we extracted R 25.5 galaxiesthrough halo abundance-matching (see L14 for details),introduced the expected LOS redshift errors and thenevaluated their positions within the mock tomographicreconstructions; this is illustrated by the mock galaxiesin Figure 4. The distribution is shown in the red his-togram in Figure 5b, which shows a clear preference to-wards negative-recF (overdensities). This is also evidentin Figure 5c, which shows the distribution as a functionof flux percentiles (normalized to N = 18 as in the realdata). A two-sample Kolmogorov-Smirnov test between

the percentile distribution of the real galaxies versus thatfrom the simulations indicate 22% probability of beingdrawn from the same distribution, which is reasonableconsidering the small data set. The long tail of galaxiesin the underdensities is primarily due to a combinationof galaxy redshift errors and reconstruction noise. Theformer could be mitigated in the near-future by accuratesystemic redshifts from near-IR spectroscopy, while to ac-count for reconstruction noise we are developing methodsto estimate the map covariance and hence characterizethe uncertainties at any point within the maps.

5. CONCLUSION

We present the spectroscopic observations targeting,for the first time, high-redshift galaxies as backgroundsources for Ly forest analysis. This enabled us tocreate a tomographic map of the 3D absorption fieldwith a spatial resolution of 3D 3.5 h

1 Mpc coveringa comoving volume of (27 h1 Mpc)3 at z 2.3.Simulated tomographic reconstructions show that oursightline-sampling, resolution, and S/N should yield a

8 Lee et al.

good recovery of the underlying absorption field. Sup-porting this conclusion, a sample of 18 coeval galaxieswith known spectroscopic redshifts are found to preferen-tially occupy high-absorption regions (i.e. overdensities)in our map.These results demonstrate the feasibility and promise

of the full CLAMATO survey: 1000 SFGs at zbg

2 3 covering 1 deg2 in the COSMOS field, whichwill enable a z 2.3 Ly forest tomographic mapwith 3D 3 4 h

1 Mpc spatial resolution over a(65 h1 Mpc)2 250 h1 Mpc (100 h1 Mpc)3 comov-ing volume. This will allow us to directly characterizethe topology and morphology of z > 2 LSS for the firsttime already we see tantalizing hints of structures ex-tending across & 10 h1 Mpc in the high-redshift cosmicweb. A large-volume LSS map will also enable a searchfor progenitors of massive z 0 galaxy clusters theseprotoclusters should manifest themselves at z & 2 asoverdensities of a few over 10 h1 Mpc (Chiang et al.2013) scales. In a forthcoming paper, we will discussmethods to find protoclusters using Ly forest tomogra-phy.The proposed survey will create rich synergy with other

COSMOS datasets. We would be able to study var-ious high-redshift galaxy properties, e.g., morphology,color, star-formation rate, as a function of their envi-ronment within the cosmic web. Such studies will re-quire the full (100 h1 Mpc)3 CLAMATO volume inorder to sample enough objects to beat down the galax-ies redshift uncertainties and reconstruction errors, butpromises unique insights into galaxy formation and evo-lution during the z 2 3 epoch. Finally, CLAM-ATO will probe small-scale clustering of LSS, and willbe highly-complementary with wide-field surveys such asHETDEX (Hill et al. 2004) and DESI (Levi et al. 2013)to probe cosmological clustering over a broad range ofspatial-scales at z & 2.

KGL and ACE are grateful to the National GeographicSociety for travel support through the Waitt Grantsprogram. This research used resources of the NERSCCenter, which is supported by the Office of Science of theU.S. D.O.E. under Contract #DE-AC02-05CH11231.We would like to thank those of Hawaiian ancestry, onwhose sacred mountain we were privileged to be guests.

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