ALMA C ii 158 μm Imaging of an H i-selected Major Merger at z ∼ 4

Abstract

We present high spatial resolution (≈2 kpc) Atacama Large Millimeter/submillimeter Array (ALMA) observations of [C ii ] 158 μ m and dust-continuum emission from a galaxy at selected by its strong H i absorption (a damped Ly α absorber, DLA) against a background QSO. Our ALMA images reveal a pair of star-forming galaxies separated by kpc (projected) undergoing a major merger. Between these galaxies is a third emission component with highly elevated (2×) [C ii ] 158 μ m 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 , that spans the velocity spread revealed in the [C ii ] 158 μ m emission. We propose that DLAs with high 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.