The Environments of Ultrastrong Mg II Absorbers^*

We present r'- or i'-band WIYN images of the fields of 15 Sloan Digital Sky Survey quasars that have spectra exhibiting intervening Mg II absorption-line systems with rest equivalent widths 2.7 Å ≤ W ≤ 6.0 Å and redshifts 0.42 < z_abs < 0.84. Such systems are rare and exhibit projected absorption velocity spreads in excess of ≈300-650 km s^-1. Approximately 60% are expected to be damped Lyα systems. In each of our fields we detect at least one galaxy that, if at the absorption redshift, would have impact parameter b ≲ 40 kpc and luminosity L ≳ 0.3L*. We measure a significant excess of galaxies at low b to the sight lines over a large range of luminosity. Many of the sight lines are found to pass either through or close to the optically luminous extent of a galaxy. Considering the very large velocity spreads seen in absorption, this suggests that these absorbing regions are more kinematically complex than local spirals such as the Milky Way. Our data indicate that interactions and galaxy pairs may be a contributing factor to the production of such large velocity spreads. Finally, we also find evidence that a population of galaxies with luminosities in the range 4L* ≲ L ≲ 13L* may contribute to the presence of ultrastrong Mg II absorption. Thus, some of the absorbing galaxies may represent a population intermediate to the very luminous high-redshift Lyman break galaxies and the fainter local starburst population.