A SCALING RELATION BETWEEN MERGER RATE OF GALAXIES AND THEIR CLOSE PAIR COUNT

Abstract

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