The Sloan Digital Sky Survey Reverberation Mapping Project: Mg ii Lag Results from Four Years of Monitoring

We present reverberation mapping results for the Mg ii λ2800 Å broad emission line in a sample of 193 quasars at $0.35<z<1.7$ with photometric and spectroscopic monitoring observations from the Sloan Digital Sky Survey Reverberation Mapping project during 2014–2017. We find significant time lags between the Mg ii and continuum lightcurves for 57 quasars, and define a "gold sample" of 24 quasars with the most reliable lag measurements. We estimate false-positive rates for each lag that range from 1% to 24%, with an average false-positive rate of 11% for the full sample and 8% for the gold sample. There are an additional ∼40 quasars with marginal Mg ii lag detections, which may yield reliable lags after additional years of monitoring. The Mg ii lags follow a radius–luminosity relation with a best-fit slope that is consistent with $\alpha =0.5$, but with an intrinsic scatter of 0.36 dex that is significantly larger than found for the Hβ radius–luminosity relation. For targets with SDSS-RM lag measurements of other emission lines, we find that our Mg ii lags are similar to the Hβ lags and ∼2–3 times larger than the C iv lags. This work significantly increases the number of Mg ii broad-line lags and provides additional reverberation-mapped black hole masses, filling the redshift gap at the peak of supermassive black hole growth between the Hβ and C iv emission lines in optical spectroscopy.