The Luminosity Dependence of the Galaxy Merger Rate

TL;DR

This study investigates how the galaxy merger rate depends on luminosity, combining observational data and simulations to reveal that major mergers predominantly involve galaxies fainter than L* and significantly influence galaxy evolution.

Contribution

It provides the first detailed analysis of luminosity dependence of galaxy merger rates using SDSS data and semi-analytic models, highlighting the role of fainter galaxies in mergers.

Findings

Merger rate Nc peaks near M* and declines for fainter galaxies.

Over 90% of major mergers involve galaxies fainter than L*.

Luminosity-dependent clustering affects small-scale pair counts.

Abstract

We measure the number of companions per galaxy (Nc) as a function of r-band absolute magnitude for both the Sloan Digital Sky Survey and the Croton et al. (2006) semi-analytic catalog applied to the Millennium Run simulation. For close pairs with projected separations of 5-20 h^{-1} kpc, velocity differences less than 500 km/s, and luminosity ratios between 1:2 and 2:1, we find good agreement between the observations and simulations, with Nc consistently close to 0.02 over the range -22 < M_r < -18. For larger pair separations, Nc(M_r) instead becomes increasingly steep towards the faint end, implying that luminosity-dependent clustering plays an important role on small scales. Using the simulations to assess and correct for projection effects, we infer that the real-space Nc(M_r) for close pairs peaks at about M*, and declines by at least a factor of two as M_r becomes fainter. Conversely, by measuring the number density of close companions, we estimate that at least 90% of all major mergers occur between galaxies which are fainter than L*. Finally, measurements of the luminosity density of close companions indicate that L* galaxies likely dominate in terms of the overall importance of major mergers in the evolution of galaxy populations at low redshift.