The Density and Spectral Energy Distributions of Red Galaxies at z ~ 3.7

We use the deep NIR imaging of the FIRES survey to investigate trends with redshift of the properties of galaxies selected to have strong Balmer/4000 Å breaks at 2 < z < 4.5. Analogous to the J - K > 1.3 (AB) color criterion designed to select red galaxies at z > 2, we propose two color criteria, J - H > 0.9 and H - K > 0.9, to select red galaxies in two redshift bins at 2 < z < 3 and 3 < z < 4.5, respectively. From the FIRES catalogs of the HDF-S (4.7 arcmin 2 ) and MS 1054-03 (26.3 arcmin 2 ) fields, we find 18 galaxies with ⟨ z phot ⟩ = 2.4 that satisfy J s - H > 0.9, H < 23.4 and 23 galaxies with ⟨ z phot ⟩ = 3.7 that satisfy H - K s > 0.9, K s < 24.6, where the flux limits are chosen to match the limiting rest-frame luminosities at the different median redshifts of the two samples. The space densities of the J s - H and H - K s samples are (1.5 ± 0.5) × 10 -4 and (1.2 ± 0.4) × 10 -4 Mpc -3 , respectively. The rest-frame U - B colors of galaxies in both samples are similarly red (as expected from the definition of the color criteria), but the rest-frame UV properties are different: galaxies in the higher redshift H - K s selected sample have blue NUV-optical colors and UV slopes similar to those of Lyman break galaxies, while the J s - H galaxies are generally red over the entire wavelength range observed. Synthetic template fits indicate that the distinct rest-NUV properties of the two samples are primarily a result of dust: we find ⟨ A V ⟩ JH = 1 mag and ⟨ A V ⟩ HK = 0.2 mag. The median stellar mass determined from the template fits decreases by a factor of ~5 from z = 2.4 to 3.7, which, coupled with the fact that the space density of such galaxies remains roughly constant, may imply that the stellar mass density in red galaxies decreases by a similar factor over this redshift range.