Effects of Ram Pressure from the Intracluster Medium on the Star Formation Rate of Disk Galaxies in Clusters of Galaxies

Using a simple model of molecular cloud evolution, we have quantitatively estimated the change of star formation rate (SFR) of a disk galaxy falling radially into the potential well of a cluster of galaxies. The SFR is affected by the ram pressure from the intracluster medium (ICM). As the galaxy approaches the cluster center, the SFR increases to twice the initial value, at most, in a cluster with high gas density and a deep potential well, or with a central pressure of ~10^-2 cm^-3 keV, because the ram pressure compresses the molecular gas of the galaxy. However, this increase does not affect the color of the galaxy significantly. Further into the central region of the cluster (≲1 Mpc from the center), the SFR of the disk component drops rapidly owing to the effect of ram-pressure stripping. This makes the color of the galaxy redder and makes the disk dark. These effects may explain the observed color, morphology distribution, and evolution of galaxies in high-redshift clusters. By contrast, in a cluster with low gas density and a shallow potential well, or with the central pressure of ~10^-3 cm^-3 keV, the SFR of a radially infalling galaxy changes less significantly, because neither ram-pressure compression nor stripping is effective. Therefore, the color of galaxies in poor clusters is as blue as that of field galaxies if other environmental effects such as galaxy-galaxy interaction are not effective. The predictions of the model are compared with observations.