Resolving The Generation of Starburst Winds in Galaxy Mergers
We study galaxy super-winds 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 GMCs, and gas recycling and feedback from SNe (I & II), stellar winds, and radiation pressure. We study mergers of systems from SMC-like dwarfs and Milky Way analogues to starburst disks. Multi-phase super-winds are generated in all passages, with outflow rates up to . However, the wind mass-loading efficiency (outflow rate divided by star formation rate) 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 uni-polar, 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 (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 “sub-grid” models, where simple wind prescriptions can dramatically suppress merger-induced starbursts, often making it impossible to form ULIRGs. Despite large mass-loading factors () in the winds simulated here, the peak star formation rates 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 disk 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.
keywords:galaxies: formation — galaxies: evolution — galaxies: active — star formation: general — cosmology: theory
It is well-established that feedback from stars is a key component of galaxy formation models. Absent strong stellar feedback, gas in cosmological models quickly cools and turns into stars, predicting galaxies with much larger stellar masses than observed (e.g. katz:treesph; somerville99:sam; cole:durham.sam.initial; springel:lcdm.sfh; keres:fb.constraints.from.cosmo.sims, and references therein). “Slowing down” star formation does not eliminate this problem; the real issue is that the amount of baryons in real galactic disks is much lower than the universal baryon fraction, which is the predicted amount of gas and stars found in cosmological simulations of low-mass galaxies without strong feedback (white:1991.galform; for a recent review see keres:fb.constraints.from.cosmo.sims). Observational constraints from IGM enrichment further make clear that many of those baryons must have at one point entered galaxy halos and disks, and been enriched, then ejected (aguirre:2001.igm.metal.evol.sims; pettini:2003.igm.metal.evol; songaila:2005.igm.metal.evol; martin:2010.metal.enriched.regions). Galactic super-winds are therefore implied, with large mass-loading factors of several times the SFR that are required in cosmological simulations to reproduce these observations (e.g. oppenheimer:outflow.enrichment). Such mass-loading factors are also observationally inferred in many local galaxies and massive star-forming regions at (martin99:outflow.vs.m; martin06:outflow.extend.origin; heckman:superwind.abs.kinematics; newman:z2.clump.winds; sato:2009.ulirg.outflows; chen:2010.local.outflow.properties; steidel:2010.outflow.kinematics; coil:2011.postsb.winds).
Until recently, however, numerical simulations have generally been unable to produce, from an a priori model, winds with large mass-loading factors (as well as a plausible scaling of wind mass-loading with galaxy mass or other properties); this is especially true of models which include only thermal or “kinetic” feedback via supernovae, which is very inefficient in the dense regions where star formation occurs (see e.g. guo:2010.hod.constraints; powell:2010.sne.fb.weak.winds; brook:2010.low.ang.mom.outflows; nagamine:2010.dwarf.gal.cosmo.review; bournaud10, and references therein). More recent simulations have, with higher resolution and/or stronger feedback prescriptions, seen strong winds, but generally find it is critical to include (usually simplified) prescriptions for cooling suppression and/or “pre-supernovae” feedback (see governato:2010.dwarf.gal.form; maccio:2012.cuspcore.outflows; teyssier:2013.cuspcore.outflow). This should not be surprising: feedback processes other than supernovae are critical for suppressing star formation in dense gas; these include protostellar jets, HII photoionization, stellar winds, and radiation pressure from young stars. Including these mechanisms self-consistently maintains a reasonable fraction of the ISM at densities where the thermal heating from supernovae has a larger effect; moreover there are many regimes where these mechanisms can directly drive winds, independent of and with greater mass loading than supernovae.
This conclusion implies that (not surprisingly) an accurate treatment of galactic winds requires a more realistic treatment of the stellar feedback processes that maintain the multi-phase structure of the ISM of galaxies. Motivated by these problems, in hopkins:rad.pressure.sf.fb (Paper I) and hopkins:fb.ism.prop (Paper II), we developed a new set of numerical models to follow feedback on small scales in GMCs and star-forming regions, in simulations with pc-scale resolution.111Movies of these simulations are available at http://www.tapir.caltech.edu/~phopkins/Site/Movies_sbw_mgr.html These simulations include the momentum imparted locally (on sub-GMC scales) from stellar radiation pressure, radiation pressure on larger scales via the light that escapes star-forming regions, HII photoionization heating, as well as the heating, momentum deposition, and mass loss by SNe (Type-I and Type-II) and stellar winds (O star and AGB). The feedback is tied to the young stars, with the energetics and time-dependence taken directly from stellar evolution models. Our models also include cooling to temperatures K, and a treatment of the molecular/atomic transition in gas and its effect on star formation (following krumholz:2011.molecular.prescription). We showed that these feedback mechanisms produce a quasi-steady ISM in which giant molecular clouds form and disperse rapidly, after turning just a few percent of their mass into stars. This leads to an ISM with phase structure, turbulent velocity dispersions, scale heights, and GMC properties (mass functions, sizes, scaling laws) in reasonable agreement with observations.
In hopkins:stellar.fb.winds (Paper III), we show 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, multi-phase winds having a broad distribution of velocities, with material both stirred in local fountains and unbound from the disk.
However, in Paper III we examine only idealized isolated disk galaxies. Although this is probably representative of much of a galaxy’s lifetime, a great deal of observational study has focused on winds in “starburst” galaxies, often in interacting or merging systems. Indeed, a wide range of phenomena indicate that gas-rich mergers are important to galaxy formation and star formation. These systems dominate the most intense starburst populations: ULIRGs at low redshift (joseph85; sanders96:ulirgs.mergers), and Hyper-LIRGs and bright sub-millimeter galaxies at high redshifts (papovich:highz.sb.gal.timescales; younger:smg.sizes; tacconi:smg.maximal.sb.sizes; schinnerer:submm.merger.w.compact.mol.gas; chapman:submm.halo.clustering; tacconi:smg.mgr.lifetime.to.quiescent). They are powered by compact concentrations of gas at enormously high densities (scoville86; sargent87), which provides material to fuel BH growth and boost the concentration and central phase-space density of merging disks to match those of ellipticals (hernquist:phasespace; robertson:fp; hopkins:cusps.mergers). Various studies have shown that the mass involved in these starburst events is critical for explaining the relations between spirals, mergers, and ellipticals, and has a dramatic impact on the properties of merger remnants (e.g., LakeDressler86; Doyon94; ShierFischer98; James99; Genzel01; tacconi:ulirgs.sb.profiles; dasyra:mass.ratio.conditions; dasyra:pg.qso.dynamics; rj:profiles; rothberg.joseph:kinematics; hopkins:cusps.ell; hopkins:cores).
With central densities as large as times those in Milky Way giant molecular clouds (GMCs), these systems also provide a laboratory for studying star formation, the ISM, and the generation of galactic winds under the most extreme conditions. In hopkins:stellar.fb.mergers (Paper IV), we therefore extend the models from Paper I-Paper III to include major galaxy mergers. We showed there that the same feedback mechanisms can explain the self-regulation of starbursts and extension of the Kennicutt-Schmidt relation to the highest gas surface densities observed. We also show how this controls the star formation rates and their spatial distributions, the formation of clusters, and the formation and destruction of giant molecular clouds in the ISM. In this paper, we further investigate the phase structure and generation of galactic superwinds in these models, and how they relate to merger dynamics and star formation histories.