Merger Histories of Galaxy Halos and Implications for Disk Survival

TL;DR

This study uses high-resolution simulations to analyze galaxy halo merger histories, revealing frequent significant mergers that challenge the survival of thin galactic disks in the CDM universe.

Contribution

It provides detailed merger statistics for galaxy-sized halos, highlighting the frequency of large accretion events and their implications for disk galaxy survival.

Findings

Major mergers (>10% of halo mass) are common in Milky Way-sized halos.

Approximately 70% of such halos have experienced mergers capable of destroying thin disks.

Most halos have accreted objects larger than the Milky Way disk in the last 10 Gyr.

Abstract

We study the merger histories of galaxy dark matter halos using a high resolution LCDM N-body simulation. Our merger trees follow ~17,000 halos with masses M_0 = (10^11--10^13) Msun at z=0 and track accretion events involving objects as small as m = 10^10 Msun. We find that mass assembly is remarkably self-similar in m/M_0, and dominated by mergers that are ~10% of the final halo mass. While very large mergers, m > 0.4 M_0, are quite rare, sizeable accretion events, m ~ 0.1 M_0, are common. Over the last 10 Gyr, an overwhelming majority (~95%) of Milky Way-sized halos with M_0 = 10^12 Msun have accreted at least one object with greater total mass than the Milky Way disk (m > 5x10^10 Msun), and approximately 70% have accreted an object with more than twice that mass (m > 10^11 Msun). Our results raise serious concerns about the survival of thin-disk dominated galaxies within the current paradigm for galaxy formation in a CDM universe. In order to achieve a ~70% disk-dominated fraction in Milky Way-sized CDM halos, mergers involving m ~ 2x10^11 Msun 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.