MMT survey for intervening MgII absorption

We present the results from a spectroscopic survey for intervening Mg (II) absorption in the spectra of 381 background QSOs conducted at the MMT telescope. This survey complements our earlier SDSS EDR Mg (II) survey, extending our results to lower redshift (z similar or equal to 0: 15) and weaker Mg (II) lambda 2796 rest equivalent width (W-0(lambda 2796) similar or equal to 0.1 angstrom). We confirm two major results from that survey: the transition in the W-0(lambda 2796) distribution at W-0(lambda 2796) approximate to 0.3 angstrom, and the W-0(lambda 2796)-dependent evolution of the incidence of systems. The nature of partial derivative N-2/partial derivative z partial derivative W-0(lambda 2796) is consistent with the idea that multiple physically distinct components/processes contribute to the incidence of Mg (II) absorption systems in a W-0-dependent manner and evolve at different rates. A significant decrease in the total proper absorption cross section is detected in our MMT data for systems as weak as 1.0 angstrom <= W-0(lambda 2796) < 1.5 angstrom at z less than or similar to 0.4. We discuss this W-0-dependent evolution in the context of the evolution of galaxy structures, processes including superwinds and interactions, and damped-Ly alpha absorbers. We also consider the possibility that the observed redshift and W-0(lambda 2796) dependence of the incidence of absorption in spectroscopic surveys for low-ionization/neutral gas results from the effects of dust-induced extinction.