A scaling relation between merger rate of galaxies and their close pair count

TL;DR

This paper establishes a precise scaling relation between galaxy close pair counts and merger rates using simulations, accounting for various dependencies, enabling more accurate observational merger rate estimations.

Contribution

It introduces a new, accurate scaling relation linking galaxy pair counts to merger rates, considering redshift, separation, and mass, and emphasizes the use of virial masses for better accuracy.

Findings

Merger timescale for major pairs is about 0.31 Gyr at z=0.1.

Timescale varies significantly with stellar mass ratio from 1:1 to 1:4.

Single timescale assumptions are inadequate for different mass ratios.

Abstract

We study how to measure the galaxy merger rate from the observed close pair count. Using a high-resolution N-body/SPH cosmological simulation, we find an accurate scaling relation between galaxy pair counts and merger rates down to a stellar mass ratio of about 1:30. The relation explicitly accounts for the dependence on redshift (or time), on pair separation, and on mass of the two galaxies in a pair. With this relation, one can easily obtain the mean merger timescale for a close pair of galaxies. The use of virial masses, instead of stellar masses, is motivated by the fact that the dynamical friction time scale is mainly determined by the dark matter surrounding central and satellite galaxies. This fact can also minimize the error induced by uncertainties in modeling star formation in the simulation. Since the virial mass can be read from the well-established relation between the virial masses and the stellar masses in observation, our scaling relation can be easily applied to observations to obtain the merger rate and merger time scale. For major merger pairs (1:1-1:4) of galaxies above a stellar mass of 4*10^10 M_sun/h at z=0.1, it takes about 0.31 Gyr to merge for pairs within a projected distance of 20 kpc/h with stellar mass ratio of 1:1, while the time taken goes up to 1.6 Gyr for mergers with stellar mass ratio of 1:4. Our results indicate that a single timescale usually used in literature is not accurate to describe mergers with the stellar mass ratio spanning even a narrow range from 1:1 to 1:4.