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We present composite broad-line region (BLR) reverberation-mapping lagmeasurements for \halpha, \hbeta, \HeII\,$\lambda4686$ and \MgII\ for a sampleof 144, $z\lesssim 1$ quasars from the Sloan Digital Sky Survey ReverberationMapping (SDSS-RM) project. Using only the 32-epoch spectroscopic light curvesin the first 6-month season of SDSS-RM observations, we compilecorrelation-function measurements for individual objects and then coadd them toallow the measurement of the average lags for our sample at mean redshifts of$0.4$ (for \halpha) and $\sim 0.65$ (for the other lines). At similar quasarluminosities and redshifts, the sample-averaged lag decreases in the order of\MgII, \halpha, \hbeta\ and \HeII. This decrease in lags is accompanied by anincrease in the mean line width of the four lines, and is roughly consistentwith the virialized motion for BLR gas in photoionization equilibrium. Theseare among the first RM measurements of stratified BLR structure at $z>0.3$.Dividing our sample by luminosity, \halpha\ shows clear evidence of increasinglags with luminosity, consistent with the expectation from the measured BLRsize-luminosity relation based on \hbeta. The other three lines do not show aclear luminosity trend in their average lags due to the limited dynamic rangeof luminosity probed and the poor average correlation signals in the dividedsamples, a situation that will be improved with the incorporation of additionalphotometric and spectroscopic data from SDSS-RM. We discuss the utility andcaveats of composite-lag measurements for large statistical quasar samples withreverberation-mapping data.

The Sloan Digital Sky Survey Reverberation Mapping Project: Composite Lags at $z\lesssim 1$