A Composite HST Spectrum of Quasars

We construct a composite quasar spectrum from 284 HST FOS spectra of 101 quasars with redshifts z > 0.33. The spectrum covers the wavelengths between 350 and 3000 Å in the rest frame, with a peak S/N level of ~130 per Å at ~1200 Å. Since ~90% of the sample quasars have redshift z < 1.5, the spectrum is suitable for studying the wavelength region shortward of Lyα without large effects from intervening Lyα forest absorption. Data in the waveband between 350 and 600 Å are mainly from the spectra of z > 1.5 quasars, for which significant corrections for the accumulated Lyman-series line and continuum absorption have been applied. There is a significant steepening of the continuum slope around 1050 Å. The continuum between 1050 and 2200 Å can be modeled as a power law fν ∝ να with α = -0.99 ± 0.05. For the full sample the power-law index in the extreme ultraviolet (EUV) between 350 and 1050 Å is α = -1.96 ± 0.15. For the radio-loud subsample (60 objects), the EUV spectral index is α ≃ -2.2, while for the radio-quiet subsample (41 objects) it is α ≃ -1.8. The continuum flux in the wavelengths near the Lyman limit shows a depression of ~10%. The break in the power-law index and the slight depression of the continuum near the Lyman limit are features expected in Comptonized accretion-disk spectra. Comptonization produces a power-law tail in the wavelength band shortward of ~1000 Å and smears out the Lyman-limit edge of the intrinsic accretion-disk spectrum. In the EUV waveband, we detect several possible emission features, including one around 690 Å that may be O III + N III produced by the Bowen fluorescence effect. Comparing our composite spectrum with one made at higher redshifts by Francis et al., we find that the equivalent widths of Lyα and high-ionization emission lines are larger in our sample, reflecting a known luminosity dependence. The equivalent widths of low-ionization lines do not exhibit such a dependence, suggesting that the quasar EUV continuum above ~50 eV is steeper at higher luminosity. Radio-quiet quasars appear to show a slightly harder continuum and lower ionization levels in their emission lines.