Calibrating galaxy redshifts using absorption by the surrounding intergalactic medium

Rest-frame UV spectral lines of star-forming galaxies are systematically offset from the galaxies' systemic redshifts, probably because of large-scale outflows. We calibrate galaxy redshifts measured from rest-frame UV lines by utilizing the fact that the mean H I Ly alpha absorption profiles around the galaxies, as seen in spectra of background objects, must be symmetric with respect to the true galaxy redshifts if the galaxies are oriented randomly with respect to the lines of sight to the background objects. We use 15 bright QSOs at z approximate to 2.5-3 and more than 600 foreground galaxies with spectroscopic redshifts at z approximate to 1.9-2.5. All galaxies are within 2 Mpc proper from the lines of sight to the background QSOs. We find that Ly alpha emission and ISM absorption redshifts require systematic shifts of Delta v(Ly alpha) = -295(-35)(+35) km s(-1) and Delta v(ISM) = 145(-35)(+70) km s(-1), respectively. Assuming a Gaussian distribution, we put 1 sigma upper limits on possible random redshift offsets of <220 km s(-1) for Lya and <420 km s(-1) for ISM redshifts. For the small subset (< 10 per cent) of galaxies for which near-IR spectra have been obtained, we can compare our results to direct measurements based on rest-frame optical, nebular emission lines, which we confirm to mark the systemic redshifts. While our Delta v(ISM) agrees with the direct measurements, our Delta v(Ly alpha) is significantly smaller. However, when we apply our method to the near-IR subsample which is characterized by slightly different selection effects, the best-fitting velocity offset comes into agreement with the direct measurement. This confirms the validity of our approach, and implies that no single number appropriately describes the whole population of galaxies, in line with the observation that the line offset depends on galaxy spectral morphology. This method provides accurate redshift calibrations and will enable studies of circumgalactic matter around galaxies for which rest-frame optical observations are not available.