On the evolution and environmental dependence of the star formation rate versus stellar mass relation since z~2
This paper discusses the evolution of the correlation between galaxy star formation rates (SFRs) and stellar mass (M*) over the last ~10 Gyrs, particularly focusing on its environmental dependence. We first present the mid-infrared (MIR) properties of the H-alpha-selected galaxies in a rich cluster Cl0939+4713 at z=0.4. We use wide-field Spitzer/MIPS24um data to show that the optically red H-alpha emitters, which are most prevalent in group-scale environments, tend to have higher SFRs and stronger dust extinction than the majority population of blue H-alpha sources. With a MIR stacking analysis, we also find that the median SFR of H-alpha emitters increases in higher-density environment at z=0.4, and this trend is confirmed for both red and blue galaxies. The trend becomes much less significant when we compare their specific SFR (SSFR), although we find that there still remains a weak, but positive correlation between SSFR and galaxy number density. We then discuss the environmental dependence of the SFR versus M* relation for star-forming galaxies since z~2, by compiling comparable H-alpha-selected galaxy samples in both distant cluster environments (from MAHALO-Subaru) and field environments (from HiZELS). Based on these large, narrow-band selected H-alpha emitter samples, we find that the SFR-M* relation does not show detectable environmental dependence since z~2. Our data also suggest that the SSFR of H-alpha-selected galaxies (at log(M*/Msun)=10) evolves as (1+z)^3, and this evolutionary speed seems to be independent of the environment. Therefore we suggest that the primary star-formation quenching mechanism in cluster environments is a fast-acting process at any time in the history of the Universe since z~2.