The Sloan Digital Sky Survey Reverberation Mapping Project: First broad-line Hβ and Mg II lags at z ≳ 0.3 from six-month spectroscopy

Abstract

Reverberation mapping (RM) measurements of broad-line region (BLR) lags in z > 0.3 quasars are important for directly measuring black hole masses in these distant objects, but so far there have been limited attempts and success given the practical difficulties of RM in this regime. Here we report preliminary results of 15 BLR lag measurements from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project, a dedicated RM program with multi-object spectroscopy designed for RM over a wide redshift range. The lags are based on the 2014 spectroscopic light curves alone (32 epochs over six months) and focus on the Hβ and Mg II broad lines in the 100 lowest-redshift (z <0.8) quasars included in SDSS-RM; they represent a small subset of the lags that SDSSRM (including 849 quasars to z ∼ 4.5) is expected to deliver. The reported preliminary lag measurements are for intermediate-luminosity quasars at 0.3 ≲ z <0.8, including nine Hβ lags and six Mg II lags, for the first time extending RM results to this redshift-luminosity regime and providing direct quasar black hole mass estimates over approximately half of cosmic time. The Mg II lags also increase the number of known Mg II lags by several fold and start to explore the utility of Mg II for RM at high redshift. The location of these new lags at higher redshifts on the observed BLR size-luminosity relationship is statistically consistent with previous Hβ results at z <0.3. However, an independent constraint on the relationship slope at z > 0.3 is not yet possible owing to the limitations in our current sample. Our results demonstrate the general feasibility and potential of multi-object RM for z > 0.3 quasars.