Late-stage galaxy mergers in COSMOS to z~1

The role of major mergers in galaxy and black hole formation is not well constrained. To help address this, we develop an automated method to identify late-stage galaxy mergers before coalescence of the galactic cores. The resulting sample of mergers is distinct from those obtained using pair-finding and morphological indicators. Our method relies on median-filtering of high-resolution images in order to distinguish two concentrated galaxy nuclei at small separations. Using mock images, we derive statistical contamination and incompleteness corrections for the fraction of late-stage mergers. We apply our new method to a magnitude-limited (I < 23) sample of 44,164 galaxies from the COSMOS HST/ACS catalog. Using a mass-complete sample with \(\log M_*/M_\odot > 10.6\) and \(0.25 < z \leq 1.00\), we find ~5% of systems are late-stage mergers with separations between 2.2 and 8 kpc. Correcting for incompleteness and contamination, the fractional merger rate increases strongly with redshift as \((1+z)^{3.8\pm0.9}\), in agreement with earlier studies and with dark matter halo merger rates. Separating the sample into star-forming and quiescent galaxies shows that the merger rate for star-forming galaxies increases strongly redshift, \((1+z)^{4.5\pm1.3}\), while the merger rate for quiescent galaxies is consistent with no evolution, \((1+z)^{1.1\pm1.2}\). Limiting our sample to galaxies with spectroscopic redshifts from zCOSMOS, we find that the star formation rates and X-ray selected AGN activity in likely late-stage mergers are enhanced by factors of ~2 relative to a control sample. Combining our sample with more widely separated pairs, we find that \(8\pm5\%\) of star formation and \(20\pm8\%\) of AGN activity is triggered by close encounters (