ORIGIN OF THE SPREAD IN THE B-K COLOR OF QUASARS

Recently, Webster et al. have shown that there is excess reddening in radio-selected quasars relative to the optically selected population. If this reddening is universal to all quasars, then the optical survey s that depend on the UV-excess selection criteria, say,would be seriou sly incomplete. Using data compiled from the Literature and various st atistical tests, we show that there is no significant correlation betw een B-K colors and other reddening indicators in the case of optically selected quasars. The distribution of emission-line equivalent widths and line ratios of radio-loud and radio-quiet quasars in the opticall y selected sample are consistent with their being drawn from the same parent population. Our results suggest that there is an intrinsic spre ad in B-K color of similar to 2 mag in optically selected bright quasa rs due to effects other than dust. Our model calculations suggest that the required amount of reddening cannot be produced by dust in the in tervening damped Lye: absorbers. We estimate, for different extinction curves, the optical depth of dust intrinsic to quasars that is requir ed to produce the observed spread in optical-to-near-IR colors. Result s of photoionization models suggest that, for a wide range of ionizati on parameter and metallicity, the gas associated with dust will produc e Lyman limit as well as saturated heavy-element absorption at the red shift of the quasar. Observation of such associated absorption in very red quasars will confirm reddening due to dust intrinsic to the quasa r, but the data presently available suggest that the similar to 2 mag spread in B-K color in the sample is due to effects other than dust ex tinction. We present marginal evidence for the aspect dependence of op tical-to-near-IR colors in radio-loud quasars. This points to beaming as a possible reason for the reddening, but further investigation usin g a homogeneous sample is necessary before a conclusion can be reached .