Mergers of luminous early-type galaxies

TL;DR

This study provides an observational measurement of the merger rate of luminous early-type galaxies in the local universe and estimates the resulting gravitational wave background from supermassive black hole coalescences.

Contribution

It offers the first direct measurement of the galaxy merger rate for luminous early-type galaxies and estimates the gravitational wave background from SMBH binaries with fewer assumptions.

Findings

Merger rate of ~(1.0+/-0.4)×10^{-5} Mpc^{-3} Gyr^{-1}

21% of galaxy pairs show interaction features

Estimated gravitational wave strain amplitude from SMBH binaries

Abstract

We have made a careful selection of a large complete volume-limited sample (1209) of projected close pairs (7<r_p<50 kpc) of luminous early-type galaxies (M_r<-21.5) in the local universe (z<0.12) from the SDSS data. 249 (21%) of them show interaction features, which suggests that about 0.8% of the galaxies are merging. We derived a comoving volume merger rate of ~(1.0+/-0.4)\times 10^{-5} Mpc^{-3} Gyr^{-1} for luminous early-type galaxies. This is a direct observational determination of the merger rate of luminous galaxies in the local universe. We also obtained the chirp mass distribution of supermassive black hole (SMBH) binary following log[Phi(log M /M_{odot})]=(21.7+/-4.2)-(3.0+/-0.5)log M/M_{odot}. With less assumptions than previous works, we estimated the strain amplitude of the gravitational wave (GW) background from coalescence of SMBH binaries at frequency 10^{-9}-10^{-7} Hz.