Interaction between the intergalactic medium and galactic outflows from dwarf galaxies

We have carried out two-dimensional (2D) hydrodynamical simulations in order to study the interaction between supernova-powered gas outflows from low-mass galaxies and the local intergalactic medium (IGM). We are specifically interested in investigating whether a high-pressure IGM, such as that in clusters of galaxies, can prevent the gas from escaping from the galaxy, as suggested by Babul & Rees, We find that this is indeed the case as long as ram pressure effects are negligible. The interface between the outflow and the ambient IGM is demarcated by a dense expanding shell formed by the gas swept up by the outflow. A sufficiently high IGM pressure can bring the shell to a halt well before it escapes the galaxy. Galaxies in such high-pressure environments are, however, more likely than not to be ploughing through the IGM at relatively high velocities. Hence, they will also be subject to ram pressure, which acts to strip the gas from the galaxy. We have carried out simulations that take into account the combined impact of ram pressure and thermal pressure. We find that ram pressure deforms the shell into a tail-like structure, fragments it into dense clouds and eventually drags the clouds away from the galaxy. The clouds are potential sites of star formation and if viewed during this transient phase, the galaxy will appear to have a low-surface-brightness tail much like the galaxies with diffuse comet-like tails seen in the z = 1.15 cluster 3C324. The stars in the tail would, in time, stream away from the galaxy and become part of the intracluster environment. In contrast, the relatively unhindered outflows in low-density, low-temperature environments can drive the shells of swept-up gas out to large distances from the galaxy. Such shells, if they intersect a quasar line of sight, would give rise to Ly alpha absorption lines of the kind seen in quasar spectra. In addition, the fact that outflows from low-mass galaxies can extend out to distances of 40 kpc or more indicates that such galaxies may have played an important role in polluting the intergalactic medium with metals.