The Impact of Mergers on the Survival and Abundance of Disk-Dominated Galaxies

TL;DR

This study investigates how galaxy mergers influence the formation and survival of disk-dominated galaxies within the LCDM cosmology, proposing a model that matches observed galaxy distributions based on merger parameters.

Contribution

The paper introduces a model linking galaxy merger parameters to spheroid formation, explaining the observed abundance of disk-dominated galaxies in the universe.

Findings

Survival rate of disk galaxies aligns with observations if mergers with specific mass ratios and velocities are considered.

A merger criterion (mass ratio > 0.3 and virial velocity > 55 km/s) explains the fraction of disk-dominated galaxies.

Dependence of galaxy type on mass offers a test for the merger-based formation hypothesis.

Abstract

We study the formation of disk-dominated galaxies in a LCDM universe. Their existence is considered to be a challenge for the LCDM cosmology, because galaxy mergers isotropize stellar disks and trigger angular momentum transport in gas disks, thus fostering the formation of central stellar spheroids. Here, we postulate that the formation of stellar spheroids from gasrich disks is controlled by two parameters that characterize galaxy mergers, the mass ratio of merging dark matter halos, and the virial velocity of the larger merging halo. We utilize merger histories generated from realizations of the cosmological density field to calculate the fraction of dark matter halos that have avoided spheroid formation, and compare the derived statistics with the spheroid occupation fractions in surveys of nearby galaxies. We find, for example, that the survival rate of disk-dominated galaxies in LCDM is just high enough to explain the observed fractional representation of disk-dominated galaxies in the universe if the only mergers which lead to central spheroid formation are those with mass ratios M2/M1 > 0.3 and virial velocities Vvir,1 > 55 km/s. We discuss the physical origin of this criterion, and show that the dependence of the disk-dominated fraction on galaxy mass provides a further test of the merger hypothesis. [For additional details, see, Koda et al. (2009).]