Measuring the halo mass of Mg ii absorbers from their cross-correlation with Luminous Red Galaxies

We study the cross-correlation between 716 Mg ii quasar absorption systems and ∼ 100,000 Luminous Red Galaxies (LRGs) selected from the Sloan Digital Sky Survey Data Release 3 in the redshift range 0.4 6 z 6 0.8. The Mg ii systems were selected to have λλ2796 & 2803 rest-frame equivalent widths > 1.0 Å and identifications confirmed by the Fe ii λ2600 or Mg i λ2852 lines. Over co-moving scales 0.2–13h Mpc, the Mg ii–LRG cross-correlation has an amplitude 0.69 ± 0.09 times that of the LRG–LRG auto-correlation. Since LRGs have halomasses of 10 M⊙, this strong cross-correlation signal implies that the absorber host-galaxies have halo-masses 1–2× 10 M⊙.