The rest-frame extreme-ultraviolet spectral properties of quasi-stellar objects

We use a sample of 332 Hubble Space Telescope spectra of 184 quasi-stellar objects (QSOs) with z>0.33 to study the typical ultraviolet spectral properties of QSOs, with emphasis on the ionizing continuum. Our sample is nearly twice as large as that from previous work by W. Zheng and colleagues and provides much better spectral coverage in the extreme-ultraviolet (EUV). The overall composite continuum can be described by a power law with index alpha(EUV) = -1.76+/-0.12 (f(nu) proportional to nu(alpha)) between 500 and 1200 Angstrom. The corresponding results for subsamples of radio-quiet and radio-loud QSOs are alpha(EUV) = -1.57+/-0.17 and alpha(EUV) = -1/96+/-0.12, respectively. We also derive alpha(EUV) for as many individual objects in our sample as possible, totaling 39 radio-quiet and 40 radio-loud QSOs. The typical individually measured values of alpha(EUV) are in good agreement with the composites. We find no evidence for evolution of alpha(EUV) with redshift for either radio-loud or radio-quiet QSOs. However, we do find marginal evidence for a trend toward harder EUV spectra with increasing luminosity for radio-loud objects. An extrapolation of our radio-quiet QSO spectrum is consistent with existing X-ray data, suggesting that the ionizing continuum may be represented by a single power law. The resulting spectrum is roughly in agreement with models of the intergalactic medium photoionized by the integrated radiation from QSOs.