Major Merging: The Way to Make a Massive, Passive Galaxy

TL;DR

The study shows that galaxies with stellar mass above 10^{11} M_sol are predominantly spheroidal due to major mergers, indicating this process as the main pathway for forming massive, passive galaxies.

Contribution

This paper provides observational evidence linking major mergers to the formation of massive, quiescent, spheroidal galaxies, highlighting a mass-dependent morphological transition.

Findings

Galaxies above 10^{11} M_sol are mostly spheroidal with few disk-like shapes.

A sharp change in axial ratio distribution occurs at ~10^{11} M_sol.

Major mergers are likely the primary mechanism for creating massive, passive galaxies.

Abstract

We analyze the projected axial ratio distribution, p(b/a), of galaxies that were spectroscopically selected from the Sloan Digital Sky Survey (DR6) to have low star-formation rates. For these quiescent galaxies we find a rather abrupt change in p(b/a) at a stellar mass of ~10^{11} M_sol: at higher masses there are hardly any galaxies with b/a<0.6, implying that essentially none of them have disk-like intrinsic shapes and must be spheroidal. This transition mass is ~3-4 times higher than the threshold mass above which quiescent galaxies dominate in number over star-forming galaxies, which suggests these mass scales are unrelated. At masses lower than ~10^{11} M_sol, quiescent galaxies show a large range in axial ratios, implying a mix of bulge- and disk-dominated galaxies. Our result strongly suggests that major merging is the most important, and perhaps only relevant, evolutionary channel to produce massive (>10^{11} M_sol), quiescent galaxies, as it inevitably results in spheroids.