Bivariate Luminosity Function of Galaxy Pairs

Abstract

We measure the bivariate luminosity function (BLF) of galaxy pairs and use it to probe and characterize the galaxy–galaxy interaction between pair members. The galaxy pair sample is selected from the main galaxy sample of Sloan Digital Sky Survey and supplied with a significant number of redshifts from the LAMOST spectral and GAMA surveys. We find the BLFs depend on the projected distance d p between pair members. At large separation d p  > 150 h −1 kpc, the BLF degenerates into a luminosity function of single galaxies, indicating few interactions between pair members. At 100 h −1 kpc ≤  d p  ≤ 150 h −1 kpc, the BLF starts to show a correlation between pair members, in the sense that the shape of the conditional luminosity function (CLF) of one member galaxy starts to depend on the luminosity of the other member galaxy. Specifically, the CLF with a brighter companion has a steeper faint-end slope, which becomes even more significant at 50 h −1 kpc ≤  d p  ≤ 100 h −1 kpc. This behavior is consistent with the scenario—and also is the observational evidence—that dynamic friction drives massive major merger pairs to merge more quickly. At close distance d p  ≤ 50 h −1 kpc, aside from the merging timescale effect, the BLF also shows an overall brightening of Δ M r  ≥ 0.04 mag, which reveals the enhanced star formation of the close-pair phase. By combining this with the statistical conclusion that the star formation rate of late-type galaxies in close pairs is enhanced at a level of about 40%, we further conclude that the average starburst timescale of close pairs is as long as 0.4 Gyr.