Host galaxies of ultrastrong Mg II absorbers at z 0.5

From a sample of 109 candidate ultrastrong Mg II (USMg II; having rest equivalent width of Mg II, W₂₇₉₆ > 3.0 Å) 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 Sloan Digital Sky Survey (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 ≤ D[kpc] ≤ 79, $0.2\le L_{[\mathrm{ O}\,~\small {\rm II}]}[ 10^{41}~\mathrm{ erg} \mathrm{ s}^{-1}]\le 4.5$, 2.59 ≤ SFR[M_⊙yr^-1] ≤ 33.51, $0.15L_B^{*}\le L_B\le 1.63L_B^{*}$, and 10.21 ≤ log[M_*/ M _⊙] ≤ 11.62, respectively. The impact parameters found are larger than that predicted by the W₂₇₉₆ versus D relationship of the general population of Mg II absorbers. At a given D, USMg II 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 same M_⋆ at z ~ 0.5. We report a correlation between $L_{[\mathrm{ O}\,\small {\rm II}]}$ and W₂₇₉₆ for the full population of Mg II absorbers, driven mainly by the host galaxies of weak Mg II absorbers that tend to have low $L_{[\mathrm{ O}\,\small {\rm II}]}$ and large impact parameters. We find at least ~33 per cent of the USMg II host galaxies (with a limiting magnitude of m_r < 23.6) are isolated and the large W₂₇₉₆ in these cases may originate from gas flows (infall/outflow) in single haloes of massive but not starburst galaxies. We also find galaxy interactions could be responsible for large velocity widths in at least ~17 per cent cases.