Abstract
We use simulations with realistic models for stellar feedback to study galaxy mergers. These high resolution (1 pc) simulations follow formation and destruction of individual GMCs and star clusters. The final starburst is dominated by in situ star formation, fueled by gas which flows inwards due to global torques. The resulting high gas density results in rapid star formation. The gas is self gravitating, and forms massive (~10^10 M_sun) GMCs and subsequent super-starclusters (masses up to 10^8 M_sun). However, in contrast to some recent simulations, the bulk of new stars which eventually form the central bulge are not born in superclusters which then sink to the center of the galaxy, because feedback efficiently disperses GMCs after they turn several percent of their mass into stars. Most of the mass that reaches the nucleus does so in the form of gas. The Kennicutt-Schmidt law emerges naturally as a consequence of feedback balancing gravitational collapse, independent of the small-scale star formation microphysics. The same mechanisms that drive this relation in isolated galaxies, in particular radiation pressure from IR photons, extend over seven decades in SFR to regulate star formation in the most extreme starbursts (densities >10^4 M_sun/pc^2). Feedback also drives super-winds with large mass loss rates; but a significant fraction of the wind material falls back onto the disks at later times, leading to higher post-starburst SFRs in the presence of stellar feedback. Strong AGN feedback is required to explain sharp cutoffs in star formation rate. We compare the predicted relic structure, mass profile, morphology, and efficiency of disk survival to simulations which do not explicitly resolve GMCs or feedback. Global galaxy properties are similar, but sub-galactic properties and star formation rates can differ significantly.
Highlights
A wide range of observed phenomena indicate that gas-rich mergers are important to galaxy evolution and star formation
In Hopkins, Quataert & Murray (2012b, hereafter Paper III), we showed that these same models of stellar feedback predict the elusive winds invoked in almost all galaxy formation models; the combination of multiple feedback mechanisms is critical to give rise to massive, multiphase winds having a broad distribution of velocities, with material both stirred in local fountains and unbound from the disc
As we showed in Paper I and Paper II for isolated discs, the self-regulation of the star formation rate (SFR) in these systems is a consequence of feedback; Paper I and Hopkins et al show that their location on the Kennicutt relation has almost nothing to do with the actual star formation prescription, once feedback is included
Summary
A wide range of observed phenomena indicate that gas-rich mergers are important to galaxy evolution and star formation. Even a small mass fraction of a few per cent formed in these nuclear starbursts can have dramatic implications for the mass profile structure (Mihos & Hernquist 1994a), phase-space densities (Hernquist et al 1993), rotation and higher order kinematics (Cox et al 2006b), kinematically decoupled components (Hoffman et al 2009, 2010), stellar population gradients (Kewley et al 2010; Soto & Martin 2010; Torrey et al 2012a), and growth of the central BH (Di Matteo, Springel & Hernquist 2005; Hopkins et al 2006; Hopkins & Hernquist 2009)
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