Quantitative Estimates of Environmental Effects on the Star Formation Rate of Disk Galaxies in Clusters of Galaxies

A simple model is constructed to evaluate the change of star formation rate of a disk galaxy due to environmental e†ects in clusters of galaxies. Three e†ects are investigated : (1) tidal force from the potential well of the cluster, (2) increase of external pressure when the galaxy plows into the intracluster medium, and (3) high-speed encounters between galaxies. General analysis indicates that the star formation rate increases signiÐcantly when the pressure of molecular clouds rises above D3 ] 105 cm~3 K in D108 yr. This is because the pressure rise makes the destruction time of the majority of molecular clouds in the galaxy less than 108 yr. The tidal force from the potential well of the cluster accelerates molecular clouds in a disk galaxy infalling toward the cluster center. Thus, the kinetic pressure rises above D3 ] 105 cm~3 K. Before the galaxy reaches the cluster center, the star formation rate reaches a maximum. The peak is 3È4 times larger than the initial value. If this is the main mechanism of the Butcher-Oemler e†ect, blue galaxies are expected to be located within D300 kpc from the center of the cluster. However this prediction is inconsistent with the recent observations. The increase of external pressure when the galaxy plows into the intracluster medium does not change star formation rate of a disk galaxy signiÐcantly. Thus, the increase of external pressure may not be the main mechanism of the Butcher-Oemler e†ect. The velocity perturbation induced by a single high-speed encounter between galaxies is too small to a†ect star formation rate of a disk galaxy, while successive high-speed encounters (galaxy harassment) trigger star formation activity because of the accumulation of gas in the galaxy center. Therefore, the galaxy harassment remains a candidate for a mechanism of the Butcher-Oemler e†ect. Subject headings : galaxies : clusters : general È galaxies : evolution È galaxies : ISM È galaxies : stellar content È intergalactic medium È stars : formation