Host galaxies of Ultra-Strong MgII absorbers at $z \sim 0.7$

Abstract: We report spectroscopic identification of the host galaxies of 18 ultra-strong MgII systems (USMgII) at $0.6 \leq z \leq 0.8$. We created the largest sample by merging these with 20 host galaxies from our previous survey within $0.4 \leq z \leq 0.6$. Using this sample, we confirm that the measured impact parameters ($\rm 6.3\leq D[kpc] \leq 120$ with a median of 19 kpc) are much larger than expected, and the USMgII host galaxies do not follow the canonical $\rm W_{2796}-D$ anti-correlation. We show that the presence and significance of this anti-correlation may depend on the sample selection. The $\rm W_{2796}-D$ anti-correlation seen for the general MgII absorbers show a mild evolution at low $\rm W_{2796}$ end over the redshift range $0.4 \leq z \leq 1.5$ with an increase of the impact parameters. Compared to the host galaxies of normal MgII absorbers, USMgII host galaxies are brighter and more massive for a given impact parameter. While the USMgII systems preferentially pick star-forming galaxies, they exhibit slightly lower ongoing star-forming rates compared to main sequence galaxies with the same stellar mass, suggesting a transition from star-forming to quiescent states. For a limiting magnitude of $m_r < 23.6$, at least $29\%$ of the USMgII host galaxies are isolated, and the width of the MgII absorption in these cases may originate from gas flows (infall/outflow) in isolated halos of massive star-forming but not starbursting galaxies. We associate more than one galaxy with the absorber in $\ge 21\%$ cases where interactions may cause wide velocity spread.