The Effect of Dry Mergers on the Color-Magnitude Relation

TL;DR

This study models how dry mergers influence the color-magnitude relation of early-type galaxies, showing they can cause the observed tilt and reduced scatter at the bright end, aligning with SDSS observations.

Contribution

It introduces a simple toy model demonstrating that dry mergers can explain the tilt and tightness of the observed CMR in early-type galaxies.

Findings

Dry mergers cause the CMR to tilt towards bluer colors at the bright end.

Dry mergers lead to a tighter CMR with less scatter at high luminosities.

The observed small scatter does not rule out significant dry merger activity.

Abstract

We investigate the effect of gas-poor (so-called "dry") mergers on the color-magnitude relation (CMR) of early-type galaxies through a simple toy model and compare with low-z observations from the Sloan Digital Sky Survey (SDSS). The observed red sequence shows a tilt towards bluer colors and a decrease in scatter at the bright end. These characteristics are predicted by our model, based on merger trees from a semi-analytic model of galaxy formation. We assume galaxies move onto a "creation red sequence" when they undergo major gas-rich mergers. Subsequent dry mergers move galaxies along the relation by increasing their mass, but also make them slightly bluer. This occurs because bright galaxies are most likely to merge with one of the more numerous fainter and consequently bluer galaxies that lie further down the relation. Bright galaxies undergo a higher fraction of dry mergers than faint galaxies, which causes a change in the slope of the CMR. A more realistic model that includes scatter in the initial relation shows that dry merging causes a tightening of the CMR towards the bright end. The small scatter in the observed CMR thus cannot be used to argue against significant mass growth from dry merging.