The Near-Infrared Luminosity Function of Galaxies in Close Major-Merger Pairs and the Mass Dependence of the Merger Rate

A sample of close major-merger pairs (projected separation 5 h-1 kpc ≤ r ≤ 20 h-1 kpc, Ks-band magnitude difference δKs ≤ 1 mag) is selected from the matched Two Micron All Sky Survey (2MASS)/Two-Degree Field Galaxy Redshift Survey catalog of Cole et al. The pair primaries are brighter than Ks = 12.5 mag. After corrections for various biases, the comparison between counts in the paired galaxy sample and counts in the parent sample shows that for the local "M* galaxies" sampled by flux-limited surveys, the fraction of galaxies in the close major-merger pairs is 1.70% ± 0.32%. Using 38 paired galaxies in the sample, a Ks-band luminosity function (LF) is calculated. This is the first unbiased LF for a sample of objectively defined interacting/merging galaxies in the local universe, while all previously determined LFs of paired galaxies are biased by mistreating paired galaxies as singles. A stellar mass function (MF) is translated from the LF. Compared to the LF/MF of 2MASS galaxies, a differential pair fraction function is derived. The results suggest a trend in the sense that less massive galaxies may have a lower chance to be involved in close major-merger pairs than more massive galaxies. The algorithm presented in this Letter can be easily applied to much larger samples of 2MASS galaxies with redshifts in the near future.