Citation:
Date Published:
April 1, 2008Abstract:
We study the nature of faint red-selected galaxies at z~2-3 using theHubble Ultra Deep Field (HUDF) and Spitzer Infrared Array Camera (IRAC)photometry. Given the magnitude limit of the HST data, we detectcandidate galaxies to HAB<26 mag, probing lower luminosity(lower mass) galaxies at these redshifts. We identify 32 galaxiessatisfying the (J110-H160)AB>1.0 magcolor selection, 16 of which have unblended [3.6 μm] and [4.5 μm]photometry from Spitzer. Using this multiwavelength data set, we derivephotometric redshifts, masses, and stellar population parameters forthese objects. We find that the selected objects span a diverse range ofproperties over a large range of redshifts, 1<~z<~3.5. Asubstantial fraction (11/32) of the(J110-H160)AB>1.0 mag populationappear to be lower redshift (z<~2.5), heavily obscured dusty galaxiesor edge-on spiral galaxies, while others (12/32) appear to be galaxiesat 2<~z<~3.5 whose light at rest-frame optical wavelengths isdominated by evolved stellar populations. We argue that longerwavelength data (>~1 μm, rest frame) are essential forinterpreting the properties of the stellar populations in red-selectedgalaxies at these redshifts. Interestingly, by including Spitzer data,many candidates for galaxies dominated by evolved stellar populationsare rejected, and for only a subset of the sample (6/16) do the datafavor this interpretation. These objects have a surface density of ~1arcmin-2. We place an upper limit on the space density ofcandidate massive evolved galaxies with 2.5AB160<=26 mag ofn=6.6+2.0-3.0×10-4Mpc-3, with a corresponding upper limit on the stellar massdensity ofρ*=5.6+4.4-2.8×107Msolar Mpc-3. The z>2.5 objects that aredominated by evolved stellar populations have a space density at mostone-third that of z~0 red early-type galaxies. Therefore, at leasttwo-thirds of present-day early-type galaxies assemble or evolve intotheir current configuration at redshifts below 2.5. We find a dearth ofcandidates for low-mass (<~2×1010 Msolar)galaxies at 1.5