Star formation in the Bullet Cluster. I. the infrared luminosity function and star formation rate

The Bullet Cluster is a massive galaxy cluster at z = 0.297 undergoing a major supersonic (Mach 3) merger event. Using data from Spitzer MIPS and the Infrared Array Camera, optical imaging, and optical spectroscopy, we present the global star formation rate (SFR) of this unique cluster. Using a 90% spectroscopically complete sample of 37 star-forming MIPS confirmed cluster members out to R > 1.7 Mpc, and the Rieke et al. relation to convert from 24 μm flux to SFR, we calculate an integrated obscured SFR of 267M _⊙ yr^-1 and a specific SFR of 28M_⊙ yr^-1 per 10¹⁴M_⊙. The clustermass normalized integrated SFR of the Bullet Cluster is among the highest in a sample of eight other clusters and cluster mergers from the literature. Five LIRGs and one ULIRG contribute 30% and 40% of the total SFR of the cluster, respectively. To investigate the origin of the elevated specific SFR, we compare the infrared luminosity function (IR LF) of the Bullet Cluster to those of Coma (evolved to z = 0.297) and CL1358+62. The Bullet Cluster IR LF exhibits an excess of sources compared to the IR LFs of the other massive clusters. A Schechter function fit of the Bullet Cluster IR LF yields L * = 44.68±0.11 erg s ^-1, which is ∼0.25 and 0.35 dex brighter than L * of evolved Coma and CL1358+62, respectively. The elevated IR LF of the Bullet Cluster relative to other clusters can be explained if we attribute the "excess" star-forming IR galaxies to a population associated with the infalling group that has not yet been transformed into quiescent galaxies. In this case, the timescale required for quenching star formation in the cluster environment must be longer than the timescale since the group's accretion-a few hundred million years.We suggest that "strangulation" is likely to be an important process in the evolution of star formation in clusters.