Models of star formation in interacting and merging disk galaxies.

Abstract

To better understand the role that interactions between galaxies plays in their overall evolution requires a clear picture of the types of interactions that trigger starburst events as well as an understanding of the specific star formation process in these starbursts. While the general occurrence of tidally triggered starbursts seems well demonstrated observationally, the specific response of star formation to a gravitational encounter is much less clear. In an effort to better understand the nature of the starburst trigger in interacting galaxies, this work constructs numerical models of star forming interacting galaxies, which are then compared to observations of specific interacting systems. The models show that in general, galaxy interactions trigger only mild increases in the global star formation rates (SFRs) of the participating galaxies. Starburst strengths for flyby interactions are a factor of a few at best and are strongest for prograde interactions. Mergers prove effective at triggering order-of-magnitude starbursts in the central regions of the merging galaxies; however, the relative scarcity of mergers ensures that very strong interaction-induced starbursts are rare. The starburst triggering mechanism is varied. The most intense starbursts are driven by gas inflow into the nuclear regions via a merger event or tidally-triggered bar formation. Less intense star formation may be induced through compressions of gas along the tidal arms, or direct collisions between the interacting ISMs. None of the mergers modeled were able to trigger starbursts much greater than an order of magnitude; this result makes it difficult to explain the emission from ultraluminous infrared galaxies through merger-induced star formation alone unless the merging galaxies were very gas rich. Furthermore, the merging process which produces such objects must occur rapidly; models of slow mergers depleted their ISM through star formation before developing the nuclear gas concentrations which could fuel an intense starburst. The models link these two criteria by showing that through a process of "gasdynamical braking," gas rich galaxies may merge more quickly than gas poor systems.