THE SLOAN DIGITAL SKY SURVEY REVERBERATION MAPPING PROJECT: VELOCITY SHIFTS OF QUASAR EMISSION LINES

Quasar emission lines are often shifted from the systemic velocity due to various dynamical and radiative processes in the line-emitting region. The level of these velocity shifts depends both on the line species and on quasar properties. We study velocity shifts for the line peaks (not the centroids) of various narrow and broad quasar emission lines relative to systemic using a sample of 849 quasars from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. The coadded (from 32 epochs) spectra of individual quasars have sufficient signal-to-noise ratio (S/N) to measure stellar absorption lines to provide reliable systemic velocity estimates, as well as weak narrow emission lines. The large dynamic range in quasar luminosity (similar to 2 dex) of the sample allowed us to explore potential luminosity dependence of the velocity shifts. We derive average line peak velocity shifts as a function of quasar luminosity for different lines, and quantify their intrinsic scatter. We further quantify how well the peak velocity can be measured as a function of continuum S/N, and demonstrate that there is no systematic bias in the velocity measurements when S/N is degraded to as low as similar to 3 per SDSS pixel (similar to 69 km s(-1)). Based on the observed line shifts, we provide empirical guidelines on redshift estimation from [O II] lambda 3727, [O III] lambda 5007, [Ne V] lambda 3426, MgII, CIII], HeII lambda 1640, broad H beta, CIV, and Si IV, which are calibrated to provide unbiased systemic redshifts in the mean, but with increasing intrinsic uncertainties of 46, 56, 119, 205, 233, 242, 400, 415, and 477 km s(-1), in addition to the measurement uncertainties. These results demonstrate the infeasibility of measuring quasar redshifts to better than similar to 200 km s(-1) with only broad lines.