Radiative shock-induced collapse of intergalactic clouds

Accumulating observational evidence for a number of radio galaxies suggests an association between their jets and regions of active star formation. The standard picture is that shocks generated by the jet propagate through an inhomogeneous medium and trigger the collapse of overdense clouds, which then become active star-forming regions. In this contribution, we report on recent hydrodynamic simulations of radiative shock-cloud interactions using two different cooling models: an equilibrium cooling-curve model assuming solar metallicities and a nonequilibrium chemistry model appropriate for primordial gas clouds. We consider a range of initial cloud densities and shock speeds in order to quantify the role of cooling in the evolution. Our results indicate that for moderate cloud densities (greater than or similar to1 cm(-3)) and shock Mach numbers (less than or similar to20), cooling processes can be highly efficient and result in more than 50% of the initial cloud mass cooling to below 100 K. We also use our results to estimate the final H-2 mass fraction for the simulations that use the nonequilibrium chemistry package. This is an important measurement, since H-2 is the dominant coolant for a primordial gas cloud. We find peak H-2 mass fractions of greater than or similar to10(-2) and total H-2 mass fractions of greater than or similar to10(-5) for the cloud gas, consistent with cosmological simulations of first star formation. Finally, we compare our results with the observations of jet-induced star formation in "Minkowski's Object," a small irregular starburst system associated with a radio jet in the nearby cluster of galaxies Abell 194. We conclude that its morphology, star formation rate (similar to0.3 M-circle dot yr(-1)) and stellar mass (similar to1.2 x 10(7) M-circle dot) can be explained by the interaction of a similar to9 x 10(4) km s(-1) jet with an ensemble of moderately dense (similar to10 cm(-3)), warm (10(4) K) intergalactic clouds in the vicinity of its associated radio galaxy at the center of the galaxy cluster.