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 λ2796 rest-frame equivalent widths Wrλ2796 ≳ 0.3 Å. 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 b̄Mg II/b̄ 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 Mh(M ⊙)) = 11.94 ± 0.31 (stat)-0.25+0.24 (sys). We demonstrate that this statistical technique, which does not require any spectroscopic follow-up, 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 Mh -Wrλ2796 correlation would have been observed since W rλ2796 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 Mh, -Wrλ2796 anticorrelation. When combined with measurements of the equivalent width distribution (d2N/dz/dWr), the M h -Wrλ2796 anti-correlation naturally explains why absorbers with Wrλ2796 ≳ 2 Å are not seen at large impact parameters. We interpret the Mh -Wrλ2796 anticorrelation within the starburst scenario where strong Mg II absorbers are produced by supemovae-driven winds.