The Merger Histories of ΛCDM Galaxies: Disk Survivability and the Deposition of Cold Baryons Via Mergers.

Details The Merger Histories of ΛCDM Galaxies: Disk Survivability and the Deposition of Cold Baryons Via Mergers. The Merger Histories of ΛCDM Galaxies: Disk Survivability and the Deposition of Cold Baryons Via Mergers.

Jan 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21335402s&link_type=abstract

American Astronomical Society, AAS Meeting #213, #354.02; Bulletin of the American Astronomical Society, Vol. 41, p.495

Astronomy and Astrophysics

Astronomy

Scientific paper

We employ a high resolution ΛCDM N-body simulation to study the merger histories of galaxy-halos and the evolution of the merger rate with redshift. We confirm the existence of a 'universal' halo merger rate, and provide a slightly modified fitting formula as a function of halo mass, redshift, and merger mass ratio. We find that the majority of Milky Way-size halos have experienced at least one major merger (defined either as mass ratio > 1:3, or in terms of absolute mass m > 10^11 Msun/h), which raises concerns about the survivability of disk dominated galaxies in a LCDM universe.
We go on to explore the baryonic content of these mergers using direct empirical constraints to assign statistically likely stellar and gas masses to the central galaxies within the halos of our simulations. We find that the vast majority of mergers into Milky-Way size halos at z>1; are very gas rich (gas fraction > 50%). If we presume that gas rich-mergers such as these may result in disk dominated galaxies (as has been suggested based on direct numerical simulations), we find that only 20% of Milky Way-size galaxies have experienced a "destructive" gas poor major merger since z=2, suggesting a possible explanation to the problem of disk survivability. We also measure the total deposition of cold baryons into galaxies via mergers and find that Milky Way-size galaxies have accreted approximately 30% of their current cold baryonic mass directly from major mergers since z=2, the majority of which is gaseous. Whether this deposited material is labeled to be a "cold flow" is subject to definition, but it seems almost empirically inevitable that direct cold gas deposition of this kind must occur.

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