Starbursts triggered by central overpressure in interacting galaxies

Abstract

We propose a physical mechanism for the triggering of a burst of massive-star formation in the central regions of interacting spiral galaxy pairs. Our mechanism is based on the detailed evolution of the interstellar medium in a galaxy following an encounter. We show that as a disk giant molecular cloud tumbles into the central region of the galaxy following a galaxy encounter, it undergoes a radiative shock compression by the preexisting high pressure of the central molecular intercloud medium. When the growth time for the gravitational instabilities in the shocked outer shell of a cloud becomes smaller than the crossing time of the shock, the shell becomes gravitationally unstable, which results in a burst of star formation. The resulting luminosity depends linearly on the fraction of the cloud mass that is shock compressed, the efficiency of star formation in the shocked gas, and the gas infall rate from the galactic disk. If a galaxy with preencounter gas parameters as in the Galaxy were to undergo distant atidal encounter, our mechanism yields a lower limit to the expected central infrared luminosity of $\sim2-6 x 10^9 L_\odot$ and the infrared luminosity-to-gas mass ratio of $\sim$ a few $L_\odot/M_\odot$. These results are in reasonable agreement with observations of central starbursts in interacting galaxies. The evolved mergers of galaxies, with their higher central gas concentrations, would yield higher values of $\leq10^{12} L_\odot$ and $\leq/\sim100 L_\odot/M_\odot$, respectively, for the above parameters, which agree with observed values from the central regions of evolved mergers.