A cross-correlation analysis of Mg II absorption line systems and luminous red galaxies from the SDSS DR5

We analyze the cross-correlation of Mg II (λ2796, 2803) quasar absorption systems with luminous red galaxies (LRGs) from the Fifth Data Release (DR5) of the Sloan Digital Sky Survey. The absorption line sample consists of 2705 unambiguously intervening Mg II absorption systems, detected at a 4σ level, covering a redshift range (0.36≤z _abs≤0.8) and a rest equivalent-width range of 0.8 ≤ W ^λ2796_r≤5.0 . We cross-correlate these absorbers with 1,495,604 LRGs with accurate photometric redshifts in the same redshift range and examine the relationship of Mg II equivalent width and clustering amplitude. We confirm with high precision a previously reported weak anticorrelation of equivalent width and the dark-matter halo mass, measuring dark-matter halo masses of Mg II absorbers to be logM_h (M _⊙ h ^-1) = 11.29 ^0.36_0.62 for the W_r ≥ 1.4 sample and logM_h (M_⊙ h ^-1) = 12.70 ^0.53 _1.16 for absorbers with 0.8≤W_r < 1.4 . These measurements agree with previous reported values within the stated errors. Additionally, we investigate the significance of a number of potential sources of bias inherent in absorber-LRG cross-correlation measurements, including absorber velocity distributions and weak lensing of background quasars, which we determine is capable of producing a 20%-30% bias in angular cross-correlation measurements on scales less than 2′. We measure the Mg II-LRG cross-correlation for 719 absorption systems with v < 60, 000 km s ^-1 in the quasar rest frame and find that these absorbers typically reside in dark-matter halos that are 10-100 times more massive than those hosting unambiguously intervening Mg II absorbers. Furthermore, we find evidence for evolution of the redshift number density, ∂N/∂z, with 2σ significance for the strongest (W ^λ2796_r ≳ 2.0) absorbers in the DR5 sample. This width-dependent ∂N/∂z evolution does not significantly affect the recovered equivalent width-halo mass anticorrelation and adds to existing evidence that the strongest Mg II absorption systems are correlated with an evolving population of field galaxies at these redshifts, while the nonevolving ∂N/∂z of the weakest absorbers more closely resembles the LRG population.