Evidence of major dry mergers at M* > 2 × 1011M⊙ from curvature in early-type galaxy scaling relations?

Abstract

Abstract For early-type galaxies, the correlations between stellar mass and size, velocity dispersion, surface brightness, colour, axial ratio and colour gradient all indicate that two mass scales, M*= 3 × 1010 and 2 × 1011 M⊙, are special. The smaller scale could mark the transition between wet and dry mergers, or it could be related to the interplay between supernovae (SNe) and active galactic nuclei (AGNs) feedback, although quantitative measures of this transition may be affected by morphological contamination. At the more massive scale, mean axial ratios and colour gradients are maximal, and above it, the colours are redder, the sizes larger and the velocity dispersions smaller than expected based on the scaling at lower M*. In contrast, the colour–σ relation, and indeed, most scaling relations with σ, 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, axial ratios and colour gradients, they are expected to change the colours or velocity dispersions much less. Therefore, the fact that scaling relations at σ > 150 km s −1 show no features, whereas the size–M*, b/a–M*, colour–M* and colour gradient–M* relations do, suggests that M*= 2 × 1011 M⊙ is the scale above which major mergers dominate the assembly histories of early-type galaxies.