Panoramic Hα and mid-infrared mapping of star formation in a z=0.8 cluster

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, which reveals the star formation activities down to a star formation rate (SFR) of similar to 1 M-circle dot yr-1 without extinction correction. Combining with a wide-field mid-infrared (MIR) imaging survey with the AKARI satellite, we compare in detail the unobscured and obscured star formation activities in the cluster. We find that both H alpha emitters and MIR galaxies avoid the cluster central region 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 investigate the amount of hidden star formation by calculating a ratio, SFR(IR)/SFR(H alpha), and find that A(H alpha) exceeds similar to 3 in extreme cases for actively star-forming galaxies with SFR(IR) greater than or similar to 20 M-circle dot yr-1. It is notable that most of such very dusty galaxies with A(H alpha) greater than or similar to 3 are also located in the medium-density environment. These findings suggest that dusty star formation is triggered in the infall 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 a factor of similar to 2 even after 1-mag extinction correction. We suggest that the mass-normalized cluster SFR rapidly declines since z similar to 1 following proportional to (1 + z)6, although the uncertainty is still large.