ALMA [CII] 158um imaging of an HI-Selected Major Merger at z~4

TL;DR

This study uses high-resolution ALMA imaging to reveal a major galaxy merger at z~4, characterized by distinct [CII] emission features and extreme kinematics, suggesting DLAs with high velocity widths are indicators of such mergers.

Contribution

First detailed ALMA imaging of a high-redshift galaxy merger associated with a DLA, linking DLA kinematics to merger activity in normal galaxies.

Findings

Detected a pair of merging galaxies separated by ~6kpc.

Identified a third emission component likely from stripped gas.

Linked high DLA velocity widths to major mergers at high redshift.

Abstract

We present high spatial-resolution (~2kpc) Atacama Large Millimeter/submillimeter Array (ALMA) observations of [CII] 158um and dust-continuum emission from a galaxy at z=3.7978 selected by its strong HI absorption (a damped Lya absorber, DLA) against a background QSO. Our ALMA images reveal a pair of star-forming galaxies separated by ~6kpc (projected) undergoing a major merger. Between these galaxies is a third emission component with highly elevated (2x) [CII] 158um 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 Dv90~470km/s that spans the velocity spread revealed in the [CII] 158um emission. We propose that DLAs with high Dv90 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.