Radiative Shock–induced Collapse of Intergalactic Clouds

Abstract

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 (≳1 cm-3) and shock Mach numbers (≲20), 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 H2 mass fraction for the simulations that use the nonequilibrium chemistry package. This is an important measurement, since H2 is the dominant coolant for a primordial gas cloud. We find peak H2 mass fractions of ≳10-2 and total H2 mass fractions of ≳10-5 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 (~0.3 M☉ yr-1) and stellar mass (~1.2 × 107 M☉) can be explained by the interaction of a ~9 × 104 km s-1 jet with an ensemble of moderately dense (~10 cm-3), warm (104 K) intergalactic clouds in the vicinity of its associated radio galaxy at the center of the galaxy cluster.