2019ApJ...886L..35P

We present high spatial resolution (≃2 kpc) Atacama Large Millimeter/submillimeter Array (ALMA) observations of [C II] 158 µm and dust-continuum emission from a galaxy at z=3.7978 selected by its strong H I absorption (a damped Lyα absorber, DLA) against a background QSO. Our ALMA images reveal a pair of star-forming galaxies separated by ~6 kpc (projected) undergoing a major merger. Between these galaxies is a third emission component with highly elevated (2x) [C II] 158 µm emission relative to the dust continuum, which is likely to arise from stripped gas associated with the merger. This merger of two otherwise-normal galaxies is not accompanied by enhanced star formation, contrary to mergers detected in most luminosity-selected samples. The DLA associated with the merger exhibits extreme kinematics, with a velocity width for the low-ionization metal lines of Δv₉₀~470kms^–1, that spans the velocity spread revealed in the [C II] 158 µm emission. We propose that DLAs with high Δv₉₀ values are a signpost of major mergers in normal galaxies at high redshifts, and use the distribution of the velocity widths of metal lines in high-z DLAs to provide a rough estimate the fraction of z > 3 galaxies that are undergoing a major merger.