The shape of the blue/UV continuum of B3-VLA radio quasars: Dependence on redshift, blue/UV luminosity and radio power

Abstract

We present UBVR photometry of a sample of 73 radio quasars, about 80 per cent complete, with redshifts 0.4±2.8. From these data the shape of the spectral energy distribution (SED) in the rest-frame blue/ultraviolet is analysed, using the individual sources as well as through broad-band composite SEDs. The SEDs of the individual sources are generally well ®tted with power laws, with slopes a ranging from 0.4 to ±1.7 (Sn ~ na). Two sources with a < ÿ1:6 were excluded from the general study for having very red SEDs, signi®cantly deviating with respect to the remaining sources. The composite SEDs cover the range .1300±4500 AÊ and the only emission feature apparent from the broad-band spectra is the C iv l1549 line, in agreement with expectations from line equivalent width measurements of radio-loud quasars from the literature. The shape of the composites in the log Sn±log n plane exhibits a break at around 3000 AÊ where the spectrum changes from ablue 0:11 6 0:16 at l > 3000 AÊ to aUV ÿ0:66 6 0:15 at l < 3000 AÊ . Although the broad-band spectral points are expected to include some masked contamination from emission lines/bumps, the break cannot be explained by line/bump emission, and most likely re¯ects an intrinsic trend in the continuum. The continuum shape is shown to depend on redshift. For the quasars with z < 1:2 we ®nd ablue 0:21 6 0:16 and aUV ÿ0:87 6 0:20, i.e. a higher steepening. For z > 1:2, aUV is more ¯at, ÿ0:48 6 0:12, and there are too few spectral points longward of 3000 AÊ to obtain ablue and analyse the presence of the 3000-AÊ break. A trend similar to that between aUV and zis found between aUV and luminosity at 2400 AÊ , L2400, with luminous quasars exhibiting a harder spectrum. The data show an intrinsic correlation between L2400 and the radio power at 408 MHz, not related to selection effects or independent cosmic evolution. The correlations aUV ±z, aUV ± L2400 and L2400 ±z appear to be consistent with accretion disc models with approximately constant black hole mass and accretion rates decreasing with time. If the trends L2400 ±z and P408 ±z are predominantly related to a selection bias, rather than cosmic evolution, only one of the correlations aUV ±L2400 or aUV ±z needs to be intrinsic.