SIMULATIONS OF COLLISIONS BETWEEN 2 GAS-RICH GALAXY DISKS WITH HEATING AND COOLING

Particle hydrodynamics (SPH) simulations are presented of direct colli sions between two model galaxies, most consisting of a rigid halo and a gas disk. Local self-gravity is also computed in the gas. The compan ion galaxy in these simulations is about one-third of the mass of the primary, and its disk is half the size. An adiabatic equation of state is combined with simple approximations for the effects of radiative c ooling and local heating due to young star activity, which allows a co ntinuous range of thermal phases to develop. These terms and multiple phases have not generally been included in galaxy collision simulation s to date. Their effects are assessed in part by repeating runs with a n isothermal equation of state and comparing the results. One model wi th a star plus gas disk is also included for comparison. These models are most relevant to interactions involving low surface brightness, or other late-type galaxies with extensive gas disks, including the prec ursors to well-known ring galaxies like the Cartwheel and VII Zw 466. In the simulations, the companion impact is slightly off center in the target disk, as is probably the case in these systems. In all cases, clear ring waves develop in the primary despite the disruption of part s of the disk by impact shocks. The gas density in the disk of the pri mary is initialized to values slightly below the gravitational instabi lity threshold throughout, and the ring waves induce star formation in all the heating and cooling models. The structure of the waves and ot her interaction morphologies are found to be quite similar on large sc ales in both isothermal and heating/cooling cases, despite the fact th at at certain stages large quantities of gas are heated above the init ial temperature in the latter. On a finer scale, there are clear diffe rences, including the fact that star formation heating in ring waves i ncreases the vertical scale height of the primary gas disk and delays spoke development. The companion disk is largely disrupted in most of these simulations, and a substantial mass of gas is splashed out into a bridge connecting the two potential centers. The companion disk refo rms by accreting gas out of the bridge, though generally in a differen t plane than its initial one. There is also a good deal of infall back onto the primary disk. Although heated by impact, the gas in the brid ge cools rapidly. However, kinematic expansion prevents it from reachi ng threshold density, and there is no star formation heating there. A comparison run with a diskless companion produced no significant bridg e, so in this type of collision the bridge is primarily a hydrodynamic phenomenon. The amount of material pushed out into the splash bridge and how much of it comes from each galaxy depends on the relative orie ntation of the disks at impact. This orientation also affects how much bridge material accretes onto each galaxy. The onset of accretion is initially delayed but then accelerates to a peak and declines thereaft er in both galaxies. The infall is spatially asymmetric and is primari ly located in well-defined streams. Most of the accreted gas ends up i n the central regions of the model galaxies, but only after spiraling around the center and passing through one or more shocks. Accretion he ating is substantial, and is shown to inhibit or delay global star for mation enhancements. The thermal effects of the impact between galaxie s are short-lived, but the models predict that accretion and young sta r heating effect the global thermal phase balance for a much longer pe riod. The magnitude and duration of these effects also depend on the r elative orientation of the disks at impact. Thus, the postcollision Hu bble type of the companion is a sensitive function of initial orientat ion.