The optical–UV emissivity of quasars: dependence on black hole mass and radio loudness

Abstract

We analyzed a large sample of radio-loud and radio-quiet quasar spectra at redshift 1.0 ≤ z ≤ 1.2 to compare the inferred underlying quasar continuum slopes (after removal of the host galaxy contribution) with accretion disk models. The latter predict redder (decreasing) α_3000 continuum slopes ( L_ν ∝ ν^α at 3000Å) with increasing black hole mass, bluer α_3000 with increasing luminosity at 3000Å, and bluer α_3000 with increasing spin of the black hole, when all other parameters are held fixed. We find no clear evidence for any of these predictions in the data. In particular we find that: (i) α_3000 shows no significant dependence on black hole mass or luminosity. Dedicated Monte Carlo tests suggest that the substantial observational uncertainties in the black hole virial masses can effectively erase any intrinsic dependence of α_3000 on black hole mass, in line with some previous studies. (ii) The mean slope α_3000 of radio-loud sources, thought to be produced by rapidly spinning black holes, is comparable to, or even redder than, that of radio-quiet quasars. Indeed, although quasars appear to become more radio loud with decreasing luminosity, we still do not detect any significant dependence of α_3000 on radio loudness. The predicted mean α_3000 slopes tend to be bluer than in the data. Disk models with high inclinations and dust extinction tend to produce redder slopes closer to empirical estimates. Our mean α_3000 values are close to the ones independently inferred at z < 0.5 suggesting weak evolution with redshift, at least for moderately luminous quasars.