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 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.