On the Incidence and Kinematics of Strong Mg II Absorbers

Abstract

We present the results of two investigations into the nature of strong (rest equivalent width > 1.0 Ang) Mg II absorption systems at high redshift. The first line of questioning examines the complete SDSS- DR3 set of quasar spectra to determine the evolution of the incidence of strong Mg II absorption. The redshift evolution of the comoving line-of-sight number density, l(x), is characterized by a roughly constant value at z > 0.8, indicating that the product of the number density and gas cross-section of halos hosting strong Mg II is unevolving at these redshifts. At z < 0.8 there is a decline in l(x), which we interpret as a decrease in the gas cross-section to strong Mg II absorption and therefore a decline in the physical processes relevant to such absorption. This evolution roughly tracks the global evolution of the SFR density. The decrease of l(x) is more pronounced for larger equivalent width systems. We also present the results of a search for strong Mg II absorption in 91 high resolution quasar spectra, which allow us to investigate the kinematics of such systems. These systems are characterized by the presence of numerous components spread over a ~200 km/s velocity width and the presence of more highly ionized species, which display kinematic profiles similar to the corresponding Mg II absorption. We consider these results in light of two competing theories to explain strong Mg II absorption: post-starburst, SN-driven galactic winds and accreting gas in the halos of massive galaxies. The later model is disfavored by the absence of evolution in l(x) at z > 1. We argue that the strong Mg II phenomenon primarily arises from feedback processes in relatively low mass galactic halos related to star formation.