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 W λ2796 r ≳ 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 , 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 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 M h – W λ2796 r correlation would have been observed since W λ2796 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 λ2796 r anticorrelation. When combined with measurements of the equivalent width distribution (d 2 N /d z /d W r ) , the M h – W λ2796 r anticorrelation naturally explains why absorbers with W λ2796 r ≳ 2 Å are not seen at large impact parameters. We interpret the M h – W λ2796 r anticorrelation within the starburst scenario where strong Mg  ii absorbers are produced by supernovae‐driven winds.