New perspectives on strong z≃ 0.5 Mg ii absorbers: are halo mass and equivalent width anticorrelated?

We measure the mean halo mass of z 0.5 Mg II absorbers using the cross-correlation (over comoving scales 0.05-13 h -1 Mpc) between 1806 Mg II quasar absorption systems and 250 000 luminous red galaxies (LRGs), both selected from the Sloan Digital Sky Survey Data Release 3. The Mg II systems have 2796 rest-frame equivalent widths W 2796 r 0.3 . From the ratio of the Mg II-LRG cross-correlation to the LRG-LRG auto-correlation, we find that the bias ratio between Mg II absorbers and LRGs is b Mg II /b LRG = 0.65 0.08, which implies that the absorber host galaxies have a mean halo mass 20-40 times smaller than that of the LRGs; the Mg II absorbers have haloes of mean mass log M h (M ) = 11.94 0.31 (stat) +0.24 -0.25 (sys). We demonstrate that this statistical technique, which does not require any spectroscopic followup, does not suffer from contaminants such as stars or foreground and background galaxies. Finally, we find that the absorber halo mass is anticorrelated with the equivalent width. If Mg II absorbers were virialized in galaxy haloes, a positive M h -W 2796 r correlation would have been observed since W 2796 r is a direct measure of the velocity spread of the Mg II subcomponents. Thus, our results demonstrate that the individual clouds of a Mg II system are not virialized in the gaseous haloes of the host galaxies. We review past results in the literature on the statistics of Mg II absorbers and find that they too require a M h -W 2796 r anticorrelation. When combined with measurements of the equivalent width distribution (d 2 N/dz/dW r ), the M h -W 2796 r anticorrelation naturally explains why absorbers with W 2796 r 2 are not seen at large impact parameters. We interpret the M h -W 2796 r anticorrelation within the starburst scenario where strong Mg II absorbers are produced by supernovae-driven winds.