Infrared color selection of massive galaxies at z > 3

Abstract

We introduce a new color selection technique to identify high-redshift, massive galaxies that are systematically missed by Lyman-break selection. The new selection is based on the H160 (H) and Infrared Array Camera (IRAC) 4.5 μm bands, specifically H – [4.5] > 2.25 mag. These galaxies, called "HIEROs," include two major populations that can be separated with an additional J − H color. The populations are massive and dusty star-forming galaxies at z > 3 (JH – blue) and extremely dusty galaxies at z ≲ 3 (JH – red). The 350 arcmin^2 of the GOODS-North and GOODS-South fields with the deepest Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) near-infrared and IRAC data contain as many as 285 HIEROs down to [4.5] < 24 mag. Inclusion of the most extreme HIEROs, not even detected in the H band, makes this selection particularly complete for the identification of massive high-redshift galaxies. We focus here primarily on JH – blue(z>3) HIEROs, which have a median photometric redshift <z> ~ 4.4 and stellar mass M_* ~ 10^10.6 M_⨀ and are much fainter in the rest-frame UV than similarly massive Lyman-break galaxies (LBGs). Their star formation rates (SFRs), derived from their stacked infrared spectral energy distributions (SEDs), reach ~240 M_⨀ yr^-1, leading to a specific SFR, sSFR ≡ SFR/M_* ~ 4.2 Gyr^-1, suggesting that the sSFRs for massive galaxies continue to grow at z > 2 but at a lower growth rate than from z = 0 to z = 2. With a median half-light radius of 2 kpc, including ~20% as compact as quiescent (QS) galaxies at similar redshifts, JH – blue HIEROs represent perfect star-forming progenitors of the most massive M_* ≳ 10^11.2 M_⨀) compact QS galaxies at z ~ 3 and have the right number density. HIEROs make up ~60% of all galaxies with M_* > 10^10.5 M_⨀ identified at z > 3 from their photometric redshifts. This is five times more than LBGs with nearly no overlap between the two populations. While HIEROs make up 15%–25% of the total SFR density at z ~ 4 –5, they completely dominate the SFR density taking place in M_* > 10^10.5 M_⨀ galaxies, and HIEROs are therefore crucial to understanding the very early phase of massive galaxy formation.