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Title:
Gas-rich Mergers in LCDM: Disk Survivability and the Baryonic Assembly of Galaxies
Authors:
Stewart, Kyle R.; Bullock, James S.; Wechsler, Risa H.; Maller, Ariyeh H.
Affiliation:
AA(Center for Cosmology, Department of Physics and Astronomy, The University of California at Irvine, Irvine, CA, 92697, USA ), AB(Center for Cosmology, Department of Physics and Astronomy, The University of California at Irvine, Irvine, CA, 92697, USA ), AC(Kavli Institute for Particle Astrophysics & Cosmology, Physics Department and Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94305, USA ), AD(Department of Physics, New York City College of Technology, 300 Jay St., Brooklyn, NY 11201, USA)
Publication:
The Astrophysical Journal, Volume 702, Issue 1, pp. 307-317 (2009). (ApJ Homepage)
Publication Date:
09/2009
Origin:
IOP
ApJ Keywords:
cosmology: theory, dark matter, galaxies: formation, galaxies: halos, methods: N-body simulations
DOI:
10.1088/0004-637X/702/1/307
Bibliographic Code:
2009ApJ...702..307S

Abstract

We use N-body simulations and observationally normalized relations between dark matter halo mass, stellar mass, and cold gas mass to derive robust expectations about the baryonic content of major mergers out to redshift z ~ 2. First, we find that the majority of major mergers (m/M>0.3) experienced by the Milky Way size dark matter halos should have been gas-rich, and that gas-rich mergers are increasingly common at high redshifts. Though the frequency of major mergers into galaxy halos in our simulations greatly exceeds the observed early-type galaxy fraction, the frequency of gas-poor major mergers is consistent with the observed fraction of bulge-dominated galaxies across the halo mass range M DM ~ 1011-1013 M sun. These results lend support to the conjecture that mergers with high-baryonic gas fractions play an important role in building and/or preserving disk galaxies in the universe. Second, we find that there is a transition mass below which a galaxy's past major mergers were primarily gas-rich and above which they were gas-poor. The associated stellar mass scale corresponds closely to that marking the observed bimodal division between blue, star-forming, disk-dominated systems and red, bulge-dominated systems with old populations. Finally, we find that the overall fraction of a galaxy's cold baryons deposited directly via major mergers is significant. Approximately ~20%-30% of the cold baryonic material in M star ~ 1010.5 M sun (M DM ~ 1012 M sun) galaxies is accreted as cold gas or stars via major mergers since z = 2, with most of this accretion in the form of cold gas. For more massive galaxies with M star ~ 1011 M sun (M DM ~ 1013 M sun), the fraction of baryons amassed in mergers since z = 2 is even higher, ~40%, but most of these accreted baryons are delivered directly in the form of stars. This baryonic mass deposition is almost unavoidable, and provides a limit on the fraction of a galaxy's cold baryons that can originate in cold flows or from hot halo cooling.
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