BEINGMgII. I. galaxies with USMgII absorbers

With a prerequisite of having a minimum of four HSC passbands available, we find a detection rate of ~38% at an average impact parameter of 11.4kpc. We find that galaxies hosting USMgII systems are typically star-forming main sequence galaxies, with 21% exhibiting a starburst nature. The non-zero [OII] emission along the 'clear' sightlines, with no stellar counterpart, hints that the USMgII absorbers may likely emanate from the unseen faint galaxies near the quasar.

The USMgII absorbers preferentially align along the major and minor axes of the galaxy, which suggests that they originate in the disk or large-scale wind. We show that the distribution of W_2796_ as a function of impact parameter indicates a discernible radial dependence for the 'disk' and 'wind' subsets, with the observed large scatter in W_2796_ potentially attributed to large-scale outflows. The quasar sightline hosting USMgII systems show a factor three higher galaxy surface density at impact parameters of <~50kpc, highlights the multiple pathways giving rise to USMgII absorption.