Host Galaxies of Ultra Strong Mg II absorbers at z $\sim$ 0.5

Abstract

From a sample of 109 candidate Ultra-Strong Mg II (USMgII; having rest equivalent width of Mg II absorption, $W_{2796}>3.0$ Angstrom) systems at z=0.4-0.6, we confirm 27 and identify host galaxies of 20 systems based on associated nebular line emission from our SALT observations or from SDSS fiber spectra. The measured impact parameter, [O II] luminosity, star formation rate, B-band luminosity and stellar mass are in the ranges $7.3\le D[kpc]\le79$, $0.2\le L_{[O II]}[ 10^{41}~erg s^{-1}]$ $\le 4.5$, $2.59\le SFR[M_\odot yr^{-1} ]\le 33.51$, $0.15L_B^*\le L_B\le1.63L_B^*$ and $10.21\le log[M_*/M_\odot]\le11.62$ respectively. The impact parameters found are larger than that predicted by the $W_{2796}$ vs D relationship of the general population of Mg II absorbers. At a given D, USMgII host galaxies are more luminous and massive compared to typical Mg II absorbers. However, the measured SFRs are slightly lower than that of main-sequence galaxies with the same M$_\star$ at $z\sim0.5$. We report a correlation between $L_{[O II]}$ and W$_{2796}$ for the full population of Mg II absorbers, driven mainly by the host galaxies of weak Mg II absorbers that tend to have low $L_{[O II]}$ and large impact parameters. We find at least $\sim$33% of the USMgII host galaxies (with a limiting magnitude of $m_r<23.6$) are isolated and the large $W_{2796}$ in these cases may originate from gas flows (infall/outflow) in single halos of massive but not starburst galaxies. We also find galaxy interactions could be responsible for large velocity widths in at least $\sim$17% cases.