Ultra-strong Mg  II absorbers as a signature of cool intragroup gas

We present the results of a spectroscopic survey of galaxies in the vicinity of an ultra-strong Mg II λλ2786, 2803 absorber of rest-frame absorption equivalent width W_r(2796) = 4.2 Å at z = 0.5624. This absorber was originally found at projected separation ρ = 246 kpc of a luminous red galaxy (LRG) at z = 0.5604. Inamori-Magellan Areal Camera and Spectrograph (IMAC) spectroscopy has revealed two galaxies at ρ < 60 kpc (z = 0.5623 and z = 0.5621) and a third one at ρ = 209 kpc (z = 0.5623) near the redshift of the absorber. These findings indicate that the absorbing gas resides in a group environment. Combining Sloan Digital Sky Survey (SDSS) broad-band photometry with additional B-, K_s-band images and optical spectroscopy, we perform a stellar population synthesis analysis of the group members to characterize their star formation histories, on-going star formation rates (SFR), and stellar masses. We find that the two group members at ρ < 60 kpc are best characterized by old stellar populations (>1 Gyr) and little on-going star formation activity (SFR < 2.9 M⊙ yr^(−1)), while the third object at ρ = 209 kpc exhibits [O II]- and continuum-derived SFR consistent with SFR > 3.0 M⊙ yr^(−1). Including the two ultra-strong Mg II absorbers analysed by Nestor et al., this is the third ultra-strong Mg II absorber for which a detailed study of the galactic environment is available. All three absorbers are found in galaxy groups. We examine different physical mechanisms giving rise to the absorbing gas including starburst-driven outflows, cold filaments, extended rotating discs and stripped gas. We argue that the large equivalent width observed in these absorbers is more likely due to the gas dynamics of the intragroup medium rather than driven by starburst outflows.