Evidence of major dry mergers at M* > 2 x 10^11 Msun from curvature in early-type galaxy scaling relations?

TL;DR

This paper investigates how scaling relations of early-type galaxies reveal evidence that major dry mergers become dominant in galaxy assembly above a stellar mass of 2 x 10^11 Msun, indicated by curvature in size, shape, and color relations.

Contribution

It identifies a specific stellar mass scale where major dry mergers significantly influence galaxy properties, based on analysis of multiple scaling relations.

Findings

Scaling relations with size, shape, and color show curvature above 2 x 10^11 Msun.

Relations with velocity dispersion remain linear, unaffected by mergers.

Major dry mergers likely dominate galaxy assembly above the identified mass scale.

Abstract

For early-type galaxies, the correlations between stellar mass and size, velocity dispersion, surface brightness, color, axis ratio and color-gradient all indicate that two mass scales, M* = 3 x 10^10 Msun and M* = 2 x 10^11 Msun, are special. The smaller scale could mark the transition between wet and dry mergers, or it could be related to the interplay between SN and AGN feedback, although quantitative measures of this transition may be affected by morphological contamination. At the more massive scale, mean axis ratios and color gradients are maximal, and above it, the colors are redder, the sizes larger and the velocity dispersions smaller than expected based on the scaling at lower M*. In contrast, the color-sigma relation, and indeed, most scaling relations with sigma, are not curved: they are well-described by a single power law, or in some cases, are almost completely flat. When major dry mergers change masses, sizes, axis ratios and color gradients, they are expected to change the colors or velocity dispersions much less. Therefore, the fact that scaling relations at sigma > 150 km/s show no features, whereas the size-M*, b/a-M*, color-M* and color gradient-M* relations do, suggests that M* = 2 x 10^11 Msun is the scale above which major dry mergers dominate the assembly histories of early-type galaxies.