Starburst galaxies are characterized by a high far-infrared luminosity over the central region of about 1 kpc diameter; and are often seen in interacting galaxy pairs. A physical mechanism for triggering of a burst of star formation in interacting galaxies will be presented. This involves shock compression by overpressure on infalling pre-existing molecular gas clouds. In this model, as a disk giant molecular cloud (GMC) tumbles into the central region of a galaxy following a galactic tidal encounter, it undergoes radiative shock compression by the pre-existing high pressure of the central molecular intercloud medium. The shocked outer shell of a GMC becomes gravitationally unstable, leading to a burst of star formation. The resulting luminosity depends on the fraction of cloud mass compressed, the efficiency of star formation, and the gas infall rate. The results agree well with observations. The fraction of cloud mass shocked is shown to be high for a wide range of disk cloud and central gas parameters, thus triggering by central overpressure is a general mechanism. In merging galaxies with disk overlap such as Arp 299, the overpressure is shown to arise due to the high pressure remnants of colliding atomic hydrogen gas clouds from the two galaxies.
Galaxy Interactions at Low and High Redshift
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