RED STAR-FORMING GALAXIES AND THEIR ENVIRONMENT AT z = 0.4 REVEALED BY PANORAMIC Hα IMAGING

We present a wide-field Hα imaging survey of the rich cluster CL0939+4713 (A851) at z = 0.41 with Suprime-Cam on the Subaru Telescope, using the narrow-band filter NB921. The survey is sensitive to active galaxies with star formation rates (SFRs) down to ∼0.3 M☉ yr−1 throughout the 27′ × 27′ field. We identified 445 Hα emitters along the large-scale structures around the cluster. Using this sample, we find that (1) the fraction of Hα emitters is a strong function of environment and shows a clear decline toward the cluster central region, and (2) the color of Hα emitters is clearly dependent on environment. The majority of the Hα emitters have blue colors with B − I < 2, but we find Hα emitters with red colors as well. Such red emitters are very rare in the cluster center or its immediate surrounding regions, while they are most frequently found in groups located far away from the cluster center. These groups coincide with the environment where a sharp transition in galaxy color distribution is seen. This may suggest that dusty star formation activity tends to be involved in galaxy truncation processes that are effective in groups, and that it is probably related to the “pre-processing” that generates present-day cluster S0 galaxies. Finally, we confirm that (3) the mass-normalized integrated SFR in clusters (i.e., the total SFR within 0.5 × R200 from the cluster center divided by the cluster dynamical mass) rapidly increases with look-back time following approximately ∝(1 + z)6 and is also correlated with the cluster mass.