Merger Histories of Galaxy Halos and Implications for Disk Survival

The authors study the merger histories of galaxy dark matter halos using a high resolution LambdaCDM N-body simulation. The merger trees follow approx 17, 000 halos with masses Msub 0 = (10sup 11 - 10sup 13) hsup -1Msub {circledot} at z = 0 and track accretion events involving objects as small as m approxₑqual 10sup 10 hsup -1Msub {circledot}. They find that mass assembly is remarkably self-similar in m/Msub 0, and dominated by mergers that are approx10% of the final halo mass. While very large mergers, m approx> 0. 4 Msub 0, are quite rare, sizeable accretion events, m approx 0. 1 Msub 0, are common. Over the last approx 10 Gyr, an overwhelming majority (approx 95%) of Milky Way-sized halos with Msub 0 = 10sup 12 hsup -1Msub {circledot} have accreted at least one object with greater total mass than the Milky Way disk (m > 5 x 10sup 10 hsup -1Msub {circledot}), and approximately 70% have accreted an object with more than twice that mass (m > 10sup 11 hsup -1Msub {circledot}). The results raise serious concerns about the survival of thin-disk dominated galaxies within the current paradigm for galaxy formation in a LambdaCDM universe. in order to achieve a approx 70% disk-dominated fraction in Milky Way-sized LambdaCDM halos, mergers involving m approxₑqual 2 x 10sup 11 hsup -1Msub {circledot} objects must not destroy disks. Considering that most thick disks and bulges contain old stellar populations, the situation is even more restrictive: these mergers must not heat disks or drive gas into their centers to create young bulges.