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 H<SUB>160</SUB> (H) and Infrared Array Camera (IRAC) 4.5 μm bands, specifically H-[4.5]\gt 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\gt 3 ({JH}-{blue}) and extremely dusty galaxies at z≲ 3 ({JH}-{red}). The 350 arcmin<SUP>2</SUP> 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]\lt 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\gt 3) HIEROs, which have a median photometric redshift < z> ∼ 4.4 and stellar mass {M}<SUB>*</SUB>∼ {10}<SUP>10.6</SUP> {M}<SUB>☉ </SUB> 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}<SUB>☉ </SUB> yr<SUP>-1</SUP>, leading to a specific SFR, {{sSFR}}\equiv {{SFR}}/{M}<SUB>*</SUB>∼ 4.2 Gyr<SUP>-1</SUP>, suggesting that the sSFRs for massive galaxies continue to grow at z\gt 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}<SUB>*</SUB>≳ {10}<SUP>11.2</SUP> {M}<SUB>☉ </SUB>) compact QS galaxies at z∼ 3 and have the right number density. HIEROs make up ∼ 60% of all galaxies with {M}<SUB>*</SUB>\gt {10}<SUP>10.5</SUP> {M}<SUB>☉ </SUB> identified at z\gt 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}<SUB>*</SUB>\gt {10}<SUP>10.5</SUP> {M}<SUB>☉ </SUB> galaxies, and HIEROs are therefore crucial to understanding the very early phase of massive galaxy formation.