New Astronomy Reviews 49 (2005) 420–423

www.elsevier.com/locate/newastrev

Spitzer 24 lm observations of optical/near-IR selectedextremely red galaxies: Evidence for assembly of

massive galaxies at z � 1–2?

Lin Yan

Spitzer Science Center, MS220-6, Caltech, Pasadena, CA 91108, United States

Available online 3 October 2005

Abstract

Optical/near-IR colors have been widely used to select z > 1 galaxies with old stellar populations. However, becausea single color can not separate dust reddening from old stars, the relative fractions of dusty starbursts and old early typegalaxies in an optical/near-IR selected sample are unknown. In this talk, I present the first, direct measurement of thefraction of dusty sources in a sample of extremely red galaxies with (R � Ks) P 5.3 mag and Ks < 20.2 mag, using 24 lmmid-infrared data from the Spitzer Space Telescope. Combining deep 24 lm, Ks- and R-band data over an area of �64sq.arcmin in the ELAIS N1 field of the Spitzer First Look Survey (FLS), we find that 50 ± 6% of our ERO sample havemeasureable 24 lm flux above the 3r flux limit of 40 lJy. This flux limit corresponds to a SFR of 12 Mx/yr at z � 1,much more sensitive than any previous long wavelength measurement. The significant implication is that our deep 24lm observations have revealed for the first time galaxies being assembled together at z � 1. Some of the starburst EROsare in the midst of violent transformation to become massive early type galaxies at the epoch of z � 1–2.� 2005 Published by Elsevier B.V.

Contents

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1. Background and data reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4212. Results and Implications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421

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2.1. The 24 lm detected EROs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4212.2. The nature of the EROs with 24 lm emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4212.3. Summary and implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422

473/$ - see front matter � 2005 Published by Elsevier B.V.

1016/j.newar.2005.08.016

ail address: lyan@ipac.caltech.edu.

Fig. 1. The two panels show log10 (mfm(24 lm)/mfm(0.7 lm))versus log10 (mfm(24 lm)/mfm(8 lm)). Panel (a) presents the datafrom ELAIS N1. In this panel, red/green squares representEROs with/without 24 lm detections. For the non-24 lmEROs, the green crosses have only upper limits in the R-band.The black/blue points represent the full Ks-selected sample with/without 24 lm detections. The plotted colors of sources notdetected at 24 lm are upper limits and could move down tolower-right corner of the diagram, as illustrated by the green

and blue arrows. The black solid line in both panels representsconstant (R � Ks) = 5.3 mag color. Panel (b) shows thecomputed color–color tracks as a function of redshift for aset of SED templates of local, well-known galaxies. The subsetof the FLSV sample with measured spectroscopic redshifts areoverplotted for three different redshift bins: z < 0.5 (magenta

open triangles); 0.5 < z < 1.0 (orange open squares); and z > 1.0(cyan open circles). (For interpretation of the references to colorin this figure legend, the reader is referred to the web version ofthis article.)

L. Yan / New Astronomy Reviews 49 (2005) 420–423 421

1. Background and data reduction

Optical/near-IR colors, such as (R � Ks) or(I � Ks) have been commonly used in wide-areasurveys to select old stellar populations at z � 1–2(see Cimatti et al., 2002; McCarthy, submitted,for the review of this subject). However, these colorselections are also sensitive to dust-reddened, star-forming systems, and examples of both passivelyevolving ellipticals and dusty starburst EROs havebeen found (see McCarthy, submitted, for refer-ences). The relative contribution of these two gal-axy types is unknown. MIPS 24 lm data from theSpitzer Space Telescope offer the first opportunityto directly address this critical issue. Dusty, star-forming galaxies are clearly distinguished fromearly-type galaxies at mid-IR wavelengths. The re-sults presented here have been published in theApJS Spitzer special issue (Yan et al., in press).The primary dataset used in this work include deepoptical R, near-IR K and Spitzer 24 lm data in theELAIS N1 field. R and Ks were taken at the KPNOand Palomar, and reach 3r of 25.2 and 21 mag,respectively. The 24 lm images were taken withroughly 4000 s exposure time and have the final3r flux limit of 40 lJy.

2. Results and Implications

2.1. The 24 lm detected EROs

We produced a catalog of 129 galaxies with(R � Ks > 5.3) mag over 64 sq.arcmin. Of the 129EROs, 65 (50 ± 6%) have 24 lm emission with fluxgreater than 40 lJy. The fraction of 24 lm detectedEROs becomes slightly higher for redder sources,with 56 ± 15% and 67 ± 30% for (R � Ks) P 6.0and (R � Ks) P 6.5 mag, respectively. However,the errors of these fractions are large due to theshallowR band limit. Deeper data would be neededto reduce the uncertainties. We find that 70% of the24 lm EROs have f24 lm > 90 lJy and that themean 24 lm flux of the ERO sample is 167 lJy.

2.2. The nature of the EROs with 24 lm emission

What types of galaxies are the 24 lm detectedEROs? In Fig. 1(a), we present a 24–0.7 and 24–

2.2 lm color–color diagram showing that 24 lm-detected EROs (red squares) are clearly separatedfrom EROs without 24 lm emission (green points),as well as the general non-ERO population (blackand blue). This suggests that the MIR/Optical andMIR/NIR colors of 24 lm-detected EROs are un-ique, distinguishing them from other populations.Fig. 1(b) presents the expected colors as a functionof redshift for various SED templates, assumingno evolution. The SEDs for M51, M82 andMrk231 are taken from Silva et al. (1998), D. Faddaet al., and Chary and Elbaz (2001), and M31 bulgeSED is from Soifer et al. (1986) combined with a10 Gyr theoretical SED from Bruzual and Charlot.

422 L. Yan / New Astronomy Reviews 49 (2005) 420–423

For comparison, a sample of 112, 24 lm sourceswith known spectroscopic redshifts are markedto show the actual redshift range of the 24 lmEROs in ELAIS N1. Of these 112 redshifts, 10are 24 lm-detected EROs with 0.8 < zspec < 1.3,and the optical spectra of these 10 sources all have[OII]3727 A emission line.

Comparing Figs. 1(a) and (b), we reach the fol-lowing conclusions: (1) 24 lm-detected EROs inthe ELAIS N1 field with (R � Ks) P 5.3 mag,Ks < 20.2 mag, and fm(24 lm) > 40 lJy are infraredbright sources at z P 1. They have colors similarto starbursts like M82 at z P 1, or dust reddenedAGN like Mrk231 at z P 0.7. Their colors aretoo red to be explained by any normal spiral orold stellar populations at any redshifts. (2) Thelikely redshift range of these 24 lm EROs isz P 1, as predicted from the model SEDs ofM82 and Mrk231. This is further confirmed bycomparison to the Keck spectroscopic samplefrom the FLSV. (3). The remaining half of theERO population are probably galaxies with oldstellar populations at z � 1, as suggested by thetracks in Fig. 1(b). In a forthcoming analysis, weshould be able to set better constraints on the agesand masses of these systems by combining with theSpitzer IRAC photometry.

Half of the ELAIS N1 ERO sample have 24 lmdust emission. The key question is what their infra-red luminosities are. In the previous section, weconclude that the 24 lm-detected EROs have col-ors similar to M82, however, Fig. 1(a) does not setany constraints on either their luminosities ormasses. At z � 1 the observed 24 lm luminosityroughly corresponds to the rest-frame IRAS 12lm luminosity, within a factor of 2. The Spitzer24 lm filter is narrower than the IRAS 12 lm fil-ter, but ignoring this difference, a crude conversionof the observed Spitzer 24 lm flux can be made toplace a lower limit on the rest-frame IRAS 12 lmluminosity. We can infer the total infrared lumi-nosity of 24 lm sources at z � 1 by using the cor-relation between the 12 lm luminosity (mLm(12lm)) and the infrared luminosity LIR(8–1000lm), LIR = 0.89 · (mLm(12 lm))1.094 Lx (Soiferet al., 1989). The mean flux (167 lJy) of the 24lm-detected EROs in ELAIS N1 implies theLIR � 3 · 1011 Lx at z = 1.0 and LIR � 1012 Lx

at z = 1.5, similar to that of local LIRGs andULIGs. The corresponding SFR is 50–170 Mx/yr,using SFR = 1.71 · 10�10LIR (Mx/yr). Weemphasize that the ELAIS N1 24 lm data is verydeep so that 3r flux limit of 40 lJy correspondsto a SFR of 12 Mx/yr, which is more sensitivethan the deepest 1.4 GHz observation (Smailet al., 2002).

2.3. Summary and implications

Our result suggests that a significant fractionof EROs are extremely active starbursts (LIRGsor ULIGs). If the time scale of this starburstingphase is on the order of 108/yr, as inferred fromlocal LIRGs and ULIGs, we can set a lower lim-it to the mass of these 24 lm-detected EROs asSFR · s = 50�170 · 108 = 5 · 109–2 · 1010 Mx.If the EROs without detectable 24 lm are indeedmassive systems with old stellar populations atz � 1 as measured by several recent surveys(Glazebrook et al., in print), one plausible con-nection between the starburst and early-typeERO populations is that the former may be inthe process of transforming into the latter, asinitially postulated by Kormendy and Sanders(1992). In the hierarchical clustering paradigm,it could be interpreted that our deep 24 lmobservations are capturing a massive galaxypopulation in the midst of violent transformation– possibly in the process of assembly via merg-ers/starbursts at the epoch of z � 1–2. The accu-rate determination of the stellar and dynamicalmasses of these starburst EROs at z � 1–2 willbe critical for the resolution of this question.Finally, the measurement of volume averagedmass density at z � 1–2 would require a betterunderstanding of the physical source of the inte-grated K-band light – whether from dusty sys-tems or from old stars.

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