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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<span class="external-identifiers"> <a target="_blank" rel="external noopener noreferrer" href="https://doi.org/10.1088/0004-6256/148/6/137">doi:10.1088/0004-6256/148/6/137</a> <a target="_blank" rel="external noopener" href="https://fatcat.wiki/release/lhb7gaxoqzgbdjnygn7lz7jdse">fatcat:lhb7gaxoqzgbdjnygn7lz7jdse</a> </span>
more »... e 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 M_*/M_ > 10.6 and 0.25 < z ≤ 1.00, we find 5 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±0.9, 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±1.3, while the merger rate for quiescent galaxies is consistent with no evolution, (1+z)^1.1±1.2. 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±5% of star formation and 20±8% of AGN activity is triggered by close encounters (<143 kpc) or mergers, once more suggesting that major mergers are not the only channels for star formation and black hole growth. (abridged)
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