The Grism Lens-Amplified Survey from Space (GLASS). IX. The dual origin of low-mass cluster galaxies as revealed by new structural analyses

Using deep Hubble Frontier Fields imaging and slitless spectroscopy from the Grism Lens-Amplified Survey from Space, we analyze 2200 cluster and 1748 field galaxies at $0.2\leq z\leq0.7$ to determine the impact of environment on galaxy size and structure at $\log M_*/M_\odot>7.8$, an unprecedented limit at these redshifts. Based on simple assumptions-$r_e=f(M_*)$-we find no significant differences in half-light radii ($r_e$) between equal-mass cluster or field systems. More complex analyses-$r_e=f(M_*,U-V,n,z,\Sigma$)-reveal local density $(\Sigma$) to induce only a $7\% \pm 3\%$ ($95\%$ confidence) reduction in $r_e$ beyond what can be accounted for by $U-V$ color, Sersic index ($n$), and redshift ($z$) effects.Almost any size difference between galaxies in high- and low-density regions is thus attributable to their different distributions in properties other than environment. Indeed, we find a clear color-$r_e$ correlation in low-mass passive cluster galaxies ($\log M_*/M_\odot<9.8$) such that bluer systems have larger radii, with the bluest having sizes consistent with equal-mass star-forming galaxies. We take this as evidence that large-$r_e$ low-mass passive cluster galaxies are recently acquired systems that have been environmentally quenched without significant structural transformation (e.g., by ram pressure stripping or starvation).Conversely, $\sim20\%$ of small-$r_e$ low-mass passive cluster galaxies appear to have been in place since $z\sim3$. Given the consistency of the small-$r_e$ galaxies' stellar surface densities (and even colors) with those of systems more than ten times as massive, our findings suggest that clusters mark places where galaxy evolution is accelerated for an ancient base population spanning most masses, with late-time additions quenched by environment-specific mechanisms are mainly restricted to the lowest masses.Comment: The accepted version. The catalog is available through the GLASS web page (http://glass.astro.ucla.edu), or https://www.astr.tohoku.ac.jp/~mtakahiro/Publication/Morishita17

Demonstrating Diversity in Star Formation Histories with the CSI Survey

We present coarse but robust star formation histories (SFHs) derived from spectro-photometric data of the Carnegie-Spitzer-IMACS Survey, for 22,494 galaxies at 0.3<z<0.9 with stellar masses of 10^9 Msun to 10^12 Msun. Our study moves beyond "average" SFHs and distribution functions of specific star formation rates (sSFRs) to individually measured SFHs for tens of thousands of galaxies. By comparing star formation rates (SFRs) with timescales of 10^10, 10^9, and 10^8 years, we find a wide diversity of SFHs: 'old galaxies' that formed most or all of their stars early; galaxies that formed stars with declining or constant SFRs over a Hubble time, and genuinely 'young galaxies' that formed most of their stars since z=1. This sequence is one of decreasing stellar mass, but, remarkably, each type is found over a mass range of a factor of 10. Conversely, galaxies at any given mass follow a wide range of SFHs, leading us to conclude that: (1) halo mass does not uniquely determine SFHs; (2) there is no 'typical' evolutionary track; and (3) "abundance matching" has limitations as a tool for inferring physics. Our observations imply that SFHs are set at an early epoch, and that--for most galaxies--the decline and cessation of star formation occurs over a Hubble-time, without distinct "quenching" events. SFH diversity is inconsistent with models where galaxy mass, at any given epoch, grows simply along relations between SFR and stellar mass, but is consistent with a 2-parameter lognormal form, lending credence to this model from a new and independent perspective.Comment: 17 pages, 10 figures; accepted by ApJ; version 2 - no substantive changes; clarifications and correction