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 ∼ 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 HAB < 26 mag, probing lower-luminosity (lower mass) galaxies at these redshifts. We identify 32 galaxies satisfying the (J110 − H160)AB > 1.0 mag color selection, 16 of which have unblended [3.6μm] and [4.5μm] photometry from Spitzer. Using this multiwavelength dataset 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 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 redselected 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 arcmin. We place an upper limit on the space density of candidate massive evolved galaxies with 2.5 < z < 3.5 and H160(AB) ≤ 26 mag of n = 6.6 −3.0×10 Mpc with a corresponding upper limit on the stellar mass density of ρ = 5.6 −2.8 × 10 M⊙ Mpc. 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 (. 2 × 10 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. Subject headings: cosmology: observations – galaxies: evolution – galaxies: high-redshift – galaxies: stellar content – infrared: galaxies