FORMATION OF DISC GALAXIES IN THE PRESENCE OF A BACKGROUND UVX RADIATION-FIELD

In order to investigate the effects of a hard ultraviolet (UVX) photon field on the formation of large disc galaxies, we have performed 3D h ydrodynamic and N-body simulations of the formation of a Milky Way-siz ed galaxy. We use a TREE-SPH code to follow the collapse of an overden se region with 95 per cent collisionless dark matter and 5 per cent ba ryonic gas. The principal effect of the UVX flux is to ionize the gas, significantly reducing its ability to dissipate energy by line-coolin g mechanisms. We present the results of two simulations starting from the same initial conditions: one of the simulations includes the prese nce of a UVX radiation field while the other does not. Based on the re sults of these and other simulations, we conclude that a background UV X radiation field can significantly affect not only the formation of s mall galaxies, as proposed by Efstathiou, but also the global properti es of large disc galaxies.