Baryonic Ecosystem IN Galaxies (BEINGMgII)

Context. We study the galaxies hosting ultra-strong Mg II (USMgII) absorbers at small impact parameters of ∼2″ (5−20 kpc) spanning a redshift range of 0.4 ≤ z ≤ 1.7 using deep high-resolution images from the Hyper Suprime-Cam Subaru Strategic Survey and spectra from the Sloan Digital Sky Survey (SDSS). Aims. Our aim is to explore the physical origin of the USMgII absorbers and characterize the associated galaxies. Methods. We performed a galaxy spectral energy distribution (SED) fitting using optical and near-IR multiband data to identify potential absorber host galaxies. Further, we searched for the [O II ] nebular emission line from absorber galaxies in the SDSS fiber spectra. Results. From a total of 418 USMgII absorbers with W 2796 ≥ 3 Å along 412 quasar sight lines, we detected 50 galaxies based on [O II ] λλ 3727, 3729 nebular emission detected at the ≥2 σ level. Utilizing the [O II ] emission from the stacked spectrum and employing the best-fit galaxy SED template, we further identified 86 galaxies, leading to a total of 136 bona fide USMgII galaxies. With a prerequisite of having a minimum of four HSC passbands available, we found a detection rate of ∼38% at an average impact parameter of 11.4 kpc. We find that galaxies hosting USMgII systems are typically star-forming main sequence galaxies, with 21% exhibiting a starburst nature. The non-zero [O II ] emission along the “clear” sight lines, with no stellar counterpart, indicates 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 the 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 sight line hosting USMgII systems show a factor of three higher galaxy surface density at impact parameters of ≲50 kpc, highlighting the multiple pathways that give rise to USMgII absorption.