The orientation-dependence of the optical continuum emission in radio-loud
quasars is investigated using a new, complete sample of low-frequency-selected
quasars, the Molonglo Quasar Sample (MQS). The optical continuum is found to be
highly anisotropic, brightening continuously from lobe- to core-dominated
quasars by 3-5 mag. It is argued that aspect-dependent extinction, rather than
relativistic boosting as has been previously proposed, provides the simplest
explanation consistent with the data. The reddening hypothesis is supported by
both the steeper optical slopes and the larger Balmer decrements found in
lobe-dominated quasars, as well as the stronger anisotropy seen at blue
wavelengths. The dust responsible is shown to be physically associated with the
quasar, lying mostly at radii between the broad and narrow-line regions in a
clumpy distribution. Such a geometry is reminiscent of a torus. However,
substantial numbers of dust clouds must lie within the torus opening angle,
contributing to an increasing average optical depth with increasing viewing
angle away from the jet axis.Comment: 14 pages, 21 figures, MNRAS accepte
