REVEALING THE EFFECTS OF ORIENTATION IN COMPOSITE QUASAR SPECTRA

摘要

We present low-resolution, composite optical spectra for 60 quasars drawn from the 408 MHz-selected Molonglo Quasar Sample. Individual spectra have been co-added according to the ratio of radio core-to-lobe flux density R in order to show global changes as a function of quasar orientation. Compact steep-spectrum (CSS) quasars have been combined separately. With increasing implied viewing angle to the radio-jet axis, we find that (1) the optical continuum steepens, (2) the 3000 A broad emission feature decreases in relative strength, and (3) the narrow-line equivalent widths, broad-line widths, and Balmer decrements increase. The composites show that reddening is considerable in most lobe-dominated quasars (A_V ～ 2-4). Moreover, the extinction appears to be aspect-dependent, which suggests an intrinsic origin for the dust. The CSS composite spectrum is particularly striking, showing evidence for significant reddening, a steep power-law continuum, relatively strong low-ionization narrow-line emission, and no discernible 3000 A bump.