Resolving the generation of starburst winds in Galaxy mergers

Abstract

We study galaxy superwinds driven in major mergers, using pc-scale resolution simulations with detailed models for stellar feedback that can self-consistently follow the generation of winds. The models include molecular cooling, star formation at high densities in giant molecular clouds, and gas recycling and feedback from supernovae (I and II), stellar winds and radiation pressure. We study mergers of systems from Small-Magellanic-Cloud-like dwarfs and Milky Way analogues to z similar to 2 starburst discs. Multiphase superwinds are generated in all passages, with outflow rates up to similar to 1000 M-circle dot yr(-1). However, the wind mass-loading efficiency (outflow rate divided by star formation rate, SFR) is similar to that in the isolated galaxy counterparts of each merger: it depends more on global galaxy properties (mass, size and escape velocity) than on the dynamical state or orbital parameters of the merger. Winds tend to be bi- or unipolar, but multiple 'events' build up complex morphologies with overlapping, differently oriented bubbles and shells at a range of radii. The winds have complex velocity and phase structure, with material at a range of speeds up to similar to 1000 km s(-1) (forming a Hubble-like flow), and a mix of molecular, ionized and hot gas that depends on galaxy properties. We examine how these different phases are connected to different feedback mechanisms. These simulations resolve a problem in some 'subgrid' models, where simple wind prescriptions can dramatically suppress merger-induced starbursts, often making it impossible to form Ultra Luminous Infrared Galaxies (ULIRGs). Despite large mass-loading factors (greater than or similar to 10-20) in the winds simulated here, the peak SFRs are comparable to those in 'no wind' simulations. Wind acceleration does not act equally, so cold dense gas can still lose angular momentum and form stars, while these stars blow out gas that would not have participated in the starburst in the first place. Considerable wind material is not unbound, and falls back on the disc at later times post-merger, leading to higher post-starburst SFRs in the presence of stellar feedback. We consider different simulation numerical methods and their effects on the wind phase structure; while most results are converged, we find that the existence of small clumps in the outflow at large distances from the galaxy is quite sensitive to the methodology.