2008ApJ...677..828S

We study the nature of faint red-selected galaxies at z∼2-3 using the Hubble Ultra Deep Field (HUDF) and Spitzer Infrared Array Camera (IRAC) photometry. Given the magnitude limit of the HST data, we detect candidate galaxies to H_AB<26 mag, probing lower luminosity (lower mass) galaxies at these redshifts. We identify 32 galaxies satisfying the J₁₁₀-H_160AB>1.0 mag color selection, 16 of which have unblended [3.6 µm] and [4.5 µm] photometry from Spitzer. Using this multiwavelength data set, we derive photometric redshifts, masses, and stellar population parameters for these objects. We find that the selected objects span a diverse range of properties over a large range of redshifts, 1≲z≲3.5. A substantial fraction (11/32) of the (J₁₁₀-H₁₆₀)_AB>1.0 mag population appear to be lower redshift (z≲2.5), heavily obscured dusty galaxies or edge-on spiral galaxies, while others (12/32) appear to be galaxies at 2≲z≲3.5 whose light at rest-frame optical wavelengths is dominated by evolved stellar populations. We argue that longer wavelength data (≳1 µm, rest frame) are essential for interpreting the properties of the stellar populations in red-selected galaxies at these redshifts. Interestingly, by including Spitzer data, many candidates for galaxies dominated by evolved stellar populations are rejected, and for only a subset of the sample (6/16) do the data favor this interpretation. These objects have a surface density of ∼1/arcmin2. We place an upper limit on the space density of candidate massive evolved galaxies with 2.5<z<3.5 and H_AB160≤26 mag of n=6.6^+2.0_–3.0x10^–4/Mpc3, with a corresponding upper limit on the stellar mass density of ρ^*=5.6^+4.4_–2.8x10⁷ M_☉/Mpc3. The z>2.5 objects that are dominated by evolved stellar populations have a space density at most one-third that of z∼0 red early-type galaxies. Therefore, at least two-thirds of present-day early-type galaxies assemble or evolve into their current configuration at redshifts below 2.5. We find a dearth of candidates for low-mass (≲2x10¹⁰ M_☉) galaxies at 1.5<z≲3 that are dominated by passively evolving stellar populations even though the data should be sensitive to them; thus, at these redshifts, galaxies whose light is dominated by evolved stellar populations are restricted to only those galaxies that have assembled high stellar mass.