The effects of a hot gaseous halo in galaxy major mergers

Authors: Benjamin P. Moster, Andrea V. Maccio', Rachel S. Somerville, Thorsten Naab, Thomas J. Cox

This paper presents the first attempt to simulate a major galaxy merger, including a component of hot gas. Galaxy properties including SFR, burst efficiency and B/T evolution are presented for mergers with and without a hot halo and with or without winds. These are compared with the equivalent simulation of an isolated galaxy.

The most interesting results are that, perhaps unsurprisingly, winds suppress the efficiency of the merger-induced starburst, causing the enhanced SFR to be suppressed but last longer, and that a hot gas halo leads to an enhanced SFR in both isolated and merger cases, but this leads to a reduced difference between the two. More interestingly perhaps, cooling from the hot halo is suppressed after the merger due to its increased temperature and spin-up (the latter effect may relate to the unusual coplanar merger geometry). With the enhanced "background" SFR from the cooling of the hot halo, and the winds suppressing any strong star burst, the strong dependence of burst efficiency on the progenitor cold gas fraction (as widely reported by Hopkins, Cox et al.) is reduced, or even negated (though it must be said, the gas fraction is defined at the beginning of the simulation, not directly before the merger).

The paper makes an important step in the direction of a multi-phase treatment of galaxy mergers. However the restriction to one (unusual) orbital configuration means these results cannot be used to provide any empirical relation describing the results of mergers. Indeed, the complexity of including winds and hot gas cooling in simulations, and dependence on the prescription used, implies that such empirical relations will be difficult to establish for sensitive parameters (e.g. star formation). On the other hand, the final bulge mass, and to a certain extent the internal dynamics of the merger remnant, appear to be fairly robust, and dependent primarily on the already formed stars in the progenitor galaxies with only a secondary dependence on the merger gas physics.