New perspectives on strong z similar or equal to 0.5 Mg II absorbers: are halo mass and equivalent width anticorrelated?

We measure the mean halo mass of z similar or equal to 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 similar to 250 000 luminous red galaxies (LRGs), both selected from the Sloan Digital Sky Survey Data Release 3. The Mg-II systems have lambda 2796 rest-frame equivalent widths W-r(lambda 2796) greater than or similar to 0.3 angstrom. 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(MgII)/b(LRG) = 0.65 +/- 0.08, which implies that the absorber host galaxies have a mean halo mass similar to 20-40 times smaller than that of the LRGs; the Mg-II absorbers have haloes of mean mass < log M-h(M-circle dot)> = 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-r(lambda 2796) correlation would have been observed since W-r(lambda 2796) 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-r(lambda 2796) anticorrelation. When combined with measurements of the equivalent width distribution (d(2)N/dz/dW(r)), the M-h -W-r(lambda 2796) anticorrelation naturally explains why absorbers with W-r(lambda 2796) greater than or similar to 2 angstrom are not seen at large impact parameters. We interpret the M-h -W-r(lambda 2796) anticorrelation within the starburst scenario where strong Mg-II absorbers are produced by supernovae-driven winds.