Panoramic Hα and mid-infrared mapping of star formation in a cluster

Abstract

We present the first wide-field H-alpha imaging survey around the distant cluster RXJ1716.4+6708 at z=0.81 with a narrow-band filter on MOIRCS/Subaru, down to SFR(Ha)~1Msun/yr. Combining with a wide-field MIR imaging survey with AKARI, we compare the unobscured and obscured star formation activities in the cluster. We find that both H-alpha emitters and MIR galaxies avoid the cluster centre and their spatial distribution is quite similar. Most of the H-alpha emitters show blue colours, but we find some H-alpha emitters on the red sequence. The MIR galaxies tend to be systematically redder than the H-alpha emitters probably due to heavy dust extinction. Interestingly, the red H-alpha emitters and the red MIR galaxies (i.e. dusty red galaxies) are most commonly seen in the medium-density environment such as cluster outskirts, groups and filaments, where optical colours of galaxies change. We also find that A(Ha) exceeds ~3 in extreme cases and that such very dusty galaxies are also located in the medium-density environment. These findings suggest that dusty star formation is triggered in the in-fall region of the cluster, implying a probable link between galaxy transition and dusty star formation. We finally calculate the cluster total SFR and find that the cluster total SFR based on H-alpha alone can be underestimated more than factor ~2 even after 1 mag extinction correction. We suggest that the mass-normalized cluster SFR rapidly declines since z~1 following ~(1+z)^6, although the uncertainty is still large.