Infrared Color Selection of Massive Galaxies at z > 3

A new ASTRODEEP science paper by  Wang, T. et al., 2016.

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 $H-[4.5]gt 2.25$ 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 $zgt 3$ (${JH}-mathrm{blue}$) and extremely dusty galaxies at $zlesssim 3$ (${JH}-mathrm{red}$). 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 $[4.5]lt 24$ 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 ${JH}-mathrm{blue}$ ($zgt 3$) HIEROs, which have a median photometric redshift $langle zrangle sim 4.4$and stellar mass ${M}_{*}sim {10}^{10.6}$ ${M}_{odot }$ and are much fainter in the rest-frame UV than similarly massive Lyman-break galaxies (LBGs).

Left panel: color–color diagram for the HIERO selection based on the $H-[4.5]$ colors. Evolutionary tracks of a set of theoretical galaxy SED templates between z = 5 and z = 2 are shown, including an instantaneous burst (SSP) model formed at z = 10 and a constant star formation model (CSF) of an age of 300 Myr with different levels of reddening. The solid horizontal line shows the HIERO selection criterion adopted.The diagonal dashed line separates pure $zgt 3$ dusty galaxies from passive galaxies at $zgt 3$ and extremely dusty galaxies at lower redshifts. Open circles denote galaxies with spectroscopic redshifts $zgt 3$ in the two GOODS fields. These are mostly UV-bright galaxies with lower levels of attenuation, i.e., LBGs. Right panel: the distribution of HIEROs with detections (>5σ) in both J- and H-selected in the GOODS fields in the $H-[4.5]$ vs. J − H color–color diagram, color-coded by their redshifts. The diagonal dashed line separates ${JH}-mathrm{blue}$ and ${JH}-mathrm{red}$ HIEROs as given by Equations (1) and (2) (the same dashed line as shown in the left panel). Galaxies detected at 24 μm (${F}_{24mu {rm{m}}}gt 30$ μJy) are shown by cyan squares. Note that 24 μm detected sources are prevalently star-forming galaxies at $zlt 3$ and classified as ${JH}-mathrm{red}$ HIEROs, as expected. Galaxies not detected in the F125W (J) band are shown with their 3σ upper limits.

Left panel: color–color diagram for the HIERO selection based on the $H-[4.5]$ colors. Evolutionary tracks of a set of theoretical galaxy SED templates between z = 5 and z = 2 are shown, including an instantaneous burst (SSP) model formed at z = 10 and a constant star formation model (CSF) of an age of 300 Myr with different levels of reddening. The solid horizontal line shows the HIERO selection criterion adopted.The diagonal dashed line separates pure $zgt 3$ dusty galaxies from passive galaxies at $zgt 3$ and extremely dusty galaxies at lower redshifts. Open circles denote galaxies with spectroscopic redshifts $zgt 3$ in the two GOODS fields. These are mostly UV-bright galaxies with lower levels of attenuation, i.e., LBGs. Right panel: the distribution of HIEROs with detections (>5σ) in both J- and H-selected in the GOODS fields in the $H-[4.5]$ vs. J − H color–color diagram, color-coded by their redshifts. The diagonal dashed line separates ${JH}-mathrm{blue}$ and ${JH}-mathrm{red}$ HIEROs as given by Equations (1) and (2) (the same dashed line as shown in the left panel). Galaxies detected at 24 μm (${F}_{24mu {rm{m}}}gt 30$ μJy) are shown by cyan squares. Note that 24 μm detected sources are prevalently star-forming galaxies at $zlt 3$ and classified as ${JH}-mathrm{red}$ HIEROs, as expected. Galaxies not detected in the F125W (J) band are shown with their 3σ upper limits.

Their star formation rates (SFRs), derived from their stacked infrared spectral energy distributions (SEDs), reach ~240 ${M}_{odot }$ yr−1, leading to a specific SFR, ${rm{sSFR}}equiv {rm{SFR}}/{M}_{*}sim 4.2$ Gyr−1, suggesting that the sSFRs for massive galaxies continue to grow at $zgt 2$ but at a lower growth rate than from z = 0 to z = 2. With a median half-light radius of 2 kpc, including $sim 20%$ as compact as quiescent (QS) galaxies at similar redshifts, ${JH}-mathrm{blue}$ HIEROs represent perfect star-forming progenitors of the most massive (${M}_{*}gtrsim {10}^{11.2}$ ${M}_{odot }$) compact QS galaxies at $zsim 3$ and have the right number density. HIEROs make up $sim 60%$ of all galaxies with ${M}_{*}gt {10}^{10.5}$ ${M}_{odot }$ identified at $zgt 3$ 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 $zsim 4$–5, they completely dominate the SFR density taking place in ${M}_{*}gt {10}^{10.5}$ ${M}_{odot }$ galaxies, and HIEROs are therefore crucial to understanding the very early phase of massive galaxy formation.

Further details can be found in Wang, T. et al., 2016, ApJ, 816, 84