On The Survival and Abundance of Disk-dominated Galaxies

TL;DR

This paper investigates how certain galaxy merger parameters influence the survival of disk-dominated galaxies in a Lambda CDM universe, explaining their observed abundance.

Contribution

It introduces a model linking merger parameters to spheroid formation, matching observed galaxy distributions in the universe.

Findings

Survival rate of disk galaxies aligns with observations under specific merger criteria.

Merger mass ratio > 0.3 and virial velocity > 55 km/s are key for spheroid formation.

Model explains the fraction of disk-dominated galaxies in the local universe.

Abstract

We study the formation of disk-dominated galaxies in a Lambda CDM universe. Their existence is considered to be a challenge for the Lambda CDM 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 gas-rich 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 Lambda CDM 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.