The cross-correlation of Mg II absorption and galaxies in BOSS

We present a measurement of the cross-correlation of MgII absorption and massive galaxies, using the Data Release (DR)11 main galaxy sample of the Baryon Oscillation Spectroscopic Survey (BOSS) of Sloan Digital Sky Survey III (SDSS-III; CMASS galaxies), and the DR7 quasar spectra of SDSS-II. The cross-correlation is measured by stacking quasar absorption spectra shifted to the redshift of galaxies that are within a certain impact parameter bin of the quasar, after dividing by a quasar continuum model. This results in an average MgII equivalent width as a function of impact parameter from a galaxy, ranging from 50 kpc to more than 10 Mpc in proper units, which includes all Mg II absorbers. We show that special care needs to be taken to use an unbiased quasar continuum estimator, to avoid systematic errors in the measurement of the mean stacked MgII equivalent width. The measured cross-correlation follows the expected shape of the galaxy correlation function, although measurement errors are large. We use the cross-correlation amplitude to derive the bias factor of MgII absorbers, finding b_{Mg II} = 2.33 ± 0.19, where the error accounts only for the statistical uncertainty in measuring the mean equivalent width. This bias factor is larger than that obtained in previous studies and may be affected by modelling uncertainties that we discuss, but if correct it suggests that MgII absorbers at redshift z ≃ 0.5 are spatially distributed on large scales similarly to the CMASS galaxies in BOSS.