The Rest‐Frame Extreme‐Ultraviolet Spectral Properties of Quasi‐stellar Objects

We use a sample of 332 Hubble Space Telescope spectra of 184 QSOs with z >0.33 to study the typical ultraviolet spectral properties of QSOs, withemphasis on the ionizing continuum. Our sample is nearly twice as large as thatof Zheng et al. (1997) and provides much better spectral coverage in theextreme ultraviolet (EUV). The overall composite continuum can be described bya power law with index alpha_EUV = -1.76 +/- 0.12 (f_nu ~ nu^alpha) between 500and 1200 Angstroms. The corresponding results for subsamples of radio-quiet andradio-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 oursample as possible, totaling 39 radio-quiet and 40 radio-loud QSOs. The typicalindividually measured values of alpha_EUV are in good agreement with thecomposites. We find no evidence for evolution of alpha_EUV with redshift foreither radio-loud or radio-quiet QSOs. However, we do find marginal evidencefor a trend towards harder EUV spectra with increasing luminosity forradio-loud objects. An extrapolation of our radio-quiet QSO spectrum isconsistent with existing X-ray data, suggesting that the ionizing continuum maybe represented by a single power law. The resulting spectrum is roughly inagreement with models of the intergalactic medium photoionized by theintegrated radiation from QSOs.Comment: 14 pages using emulateapj, 15 figures, accepted for publication in Ap