INFRARED COLOR SELECTION OF MASSIVE GALAXIES AT z > 3

Abstract

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 H 160 (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 arcmin2 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.