Environmental Induced Star Formation in Galactic Cooling Flows

Summary

ABSTRACT

A 2D hydrodynamic Eulerian code is applied to study the effect of induced star formation in galactic cooling flows. Our main findings are: 1) Star formation activity is induced by ram pressure when the mass loss is dominated by late type giants. 2) Star formation activity is insensitive to the environment when the mass loss is dominated by early type giants. The present calculation supports the claim that the Butcher-Oemler effect is caused by star formation activity and provides a natural explanation for its source.

INTRODUCTION

The intra-cluster gas (ICG) is a hot rarefied gas that experiences negligible cooling during a Hubble time. This fact changes as the gas flows into the inner parts of elliptical galaxies and encounters the hot inter stellar medium (ISM). The cooling time in the ISM dominated region may become 6 to 7 orders of magnitude smaller then the Hubble time (cf. Portnoy et al. 1993, hereafter PPS). Regions in which cooling times are very short are also found in the centers of many cluster of galaxies (for a review cf. Sarazin 1988 and references therein). These high density and low temperatures regions are strong sources of thermal X-ray radiation, and are known as cooling flows.

The ISM Removal Problem

In contrast to cluster cooling flows that were discovered observationally (cf. Sarazin 1988), galactic cooling flows (in normal ellipticals in contrast to CDs) were discovered numerically in simulations performed by Shaviv and Salpeter (1982, here-after ShS). The main motivation for their numerical simulation was the introduction of the cooling process of the gas into the simulations. As a consequence ShS frequently encountered a cooling instability.