Merger histories of galaxy halos and implications for disk survival

We study the merger histories of galaxy dark matter halos using a high-resolution Lambda CDM N-body simulation. Our merger trees follow similar to 17,000 halos with masses M-0 = 10(11)-10(13) h(-1) M-circle dot at z = 0 and track accretion events involving objects as small as m similar or equal to 10(10) h(-1) M-circle dot. We find that mass assembly is remarkably self-similar in m/M-0 and dominated by mergers that are similar to 10% of the final halo mass. While very large mergers, m greater than or similar to 0.4M(0), are quite rare, sizeable accretion events, m similar to 0.1M(0), are common. Over the last similar to 10 Gyr, an overwhelming majority (similar to 95%) of Milky Way-sized halos with M-0 = 10(12) h(-1) M-circle dot have accreted at least one object with greater total mass than the Milky Way disk (m > 5 x 10(10) h(-1) M-circle dot), and approximately 70% have accreted an object with more than twice that mass (m > 10(11) h(-1) M-circle dot). Our results raise serious concerns about the survival of thin-disk-dominated galaxies within the current paradigm for galaxy formation in a Lambda CDM universe. In order to achieve a similar to 70% disk-dominated fraction in Milky Way-sized Lambda CDM halos, mergers involving m similar or equal to 2 x 10(11) h(-1) M-circle dot 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.