Unions with UNIONS: Using Galaxy–Galaxy Lensing to Probe Galaxy Mergers

We use galaxy–galaxy lensing to investigate how the dark matter (DM) haloes and stellar content of galaxies with 0.012 ≤ z ≤ 0.32 and change as a result of the merger process. To this end, we construct two samples of galaxies obtained from the Ultraviolet Near Infrared Optical Northern Survey, comprising 1623 postmergers and ∼30,000 nonmerging controls, that live in low-density environments to use as our lenses. These samples are weighted to share the same distributions of stellar mass, redshift, and geometric mean distance to a galaxy’s three nearest neighbors to ensure differences in the lensing signal are due to the merger process itself. We do not detect a statistically significant difference in the excess surface density profile of postmergers and nonmerging controls with current data. Fitting haloes composed of a pointlike stellar mass component and an extended DM structure described by a Navarro–Frenk–White profile to the lensing measurements yields, for both samples, halo masses of Mhalo ∼ 4 × 1012M⊙ and a moderately negative correlation between Mhalo and concentration c. This allows us to rule out, at the 95% confidence level, merger-induced starbursts in which more than 60% of the stellar mass is formed in the burst. The application of our methods to upcoming surveys that are able to provide samples ∼10× larger than our current catalog is expected to detect the weak-lensing signatures of mergers and further constrain their properties.