On the stellar populations in faint red galaxies in the Hubble Ultra Deep Field

We study the nature of faint red-selected galaxies at z similar to 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 (J(110) similar to H(160)) (AB)> 1: 0 mag color selection, 16 of which have unblended [3.6 mu m] and [4.5 mu m] 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 less than or similar to z less than or similar to 3.5. A substantial fraction (11/32) of the (J(110) similar to H(160))(AB) > 1: 0 mag population appear to be lower redshift (z less than or similar to 2.5), heavily obscured dusty galaxies or edge-on spiral galaxies, while others (12/32) appear to be galaxies at 2 less than or similar to z less than or similar to 3.5 whose light at rest-frame optical wavelengths is dominated by evolved stellar populations. We argue that longer wavelength data (greater than or similar to 1 mu m, 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 similar to 1 arcmin(-2). 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(-3.0)(+2. 0) x 10(-4) Mpc(-3), with a corresponding upper limit on the stellar mass density of rho* = 5.6(-2.8)(+4.4) x 10(7) M(circle dot) Mpc(-3). The z > 2: 5 objects that are dominated by evolved stellar populations have a space density at most one-third that of z similar to 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 (less than or similar to 2 x 10(10) M(circle dot)) galaxies at 1.5 < z less than or similar to 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.