TIDAL TRIGGERING OF STAR FORMATION BY THE GALAXY CLUSTER POTENTIAL

摘要

We have modeled the effect on star formation in a disk galaxy falling radially into the potential well of a cluster of galaxies due to the radial and transverse tidal accelerations from the cluster potential. We evaluated these accelerations for four different cluster mass distributions: (1) the canonical King model, (2) an isothermal X-ray gas, (3) an adiabatic X-ray gas distribution, which is more centrally peaked than the King model, and (4) that inferred from gravitational lensing, which is the most centrally peaked. The latter two peaked potentials show a progressively more significant lateral compression of the galaxy disk from the tidal field within 250 kpc of the cluster center. The tidal component is negligible for the canonical King model or isothermal X-ray potential within this radius because of the flattened central mass distribution which characterizes these distributions. For the centrally peaked potentials, inside of approximately 250 kpc, tidal accelerations from the cluster potential on galaxies will be important and will trigger collisions of neutral hydrogen clouds which will increase the level of expected star formation in a disk galaxy. The rotation of the galaxy will result in the entire disk experiencing the maximum transverse tidal acceleration within only 1/4 of a disk rotation period. Galaxies on radial orbits are very likely to pass within a radius of approximately 250 kpc as a cluster undergoes collapse or merger. Tidal triggering will thus increase the rate of star formation in galaxies during collapse. The central potential derived from lensing (which indicates a centrally concentrated mass profile) comes primarily from clusters at the epoch at which the population of galaxies seems to undergo considerable metamorphosis (the Butcher-Oemler effect). Therefore, the lensing potential may be a more realistic potential for the central region of a cluster at this epoch. Another possible effect on galaxy evolution is the ram pressure of the intracluster medium on the galaxy. Observations indicate that there are fewer X-ray luminous clusters at the Butcher-Oemler epoch relative to the present and suggest that the gas is undergoing density and temperature evolution. Therefore, we have calculated the ram-pressure effect for two cases: (1) a low-density intracluster gas which assumes that the intracluster gas experiences a significant density increase between the z = 0.2 epoch and the present and (2) the high-density gas which is similar to the most X-ray luminous clusters. For the latter case, the tidal effect of the cluster potential contributes to star formation triggering but is dominant only inside of 50 kpc, while in the former it dominates inside of ～250 kpc. If ram pressure were the primary triggering mechanism, then a correlation of star formation with the X-ray luminosity should be apparent at some level in all of the clusters showing the Butcher-Oemler effect. However, no clear correlation is seen in the available data. We suggest that the tidal effect of the cluster potential contributes, along with galaxy-galaxy interactions to increasing the star formation rate in galaxies at the z = 0.2 epoch. We show that the cluster tidal field works with gas removal processes to enhance the level of star formation, increasing the number of blue galaxies, and also to ultimately shorten the timescale over which the star formation is intensified, focusing the epoch of intense star formation. Observational tests are suggested which will test this hypothesis.