Quasars in the Extreme UV: Spectral Indices and Potential Emission Lines, and Implications for He II Reionization

Details Quasars in the Extreme UV: Spectral Indices and Potential Emission Lines, and Implications for He II Reionization Quasars in the Extreme UV: Spectral Indices and Potential Emission Lines, and Implications for He II Reionization

Jan 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21714216s&link_type=abstract

American Astronomical Society, AAS Meeting #217, #142.16; Bulletin of the American Astronomical Society, Vol. 43, 2011

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The extreme UV (EUV, <912 A) region of quasar spectra is poorly understood, because the high opacity of intervening hydrogen absorption systems makes it very difficult to observe. In the course of identifying a large number of new sightlines for the He II Gunn-Peterson test, we have amassed an unprecedentedly large collection of quasar spectra covering 250-500 A (rest frame). This allows us to examine emission lines in this largely unexplored region, as well as the average EUV spectral index. This continuum slope is poorly constrained, but vitally important for understanding the ionizing UV background in the IGM, as well as the progress of He II reionization. Our sample complements that of the few other EUV spectral index studies, which mostly extrapolate from longer wavelengths. Cloudy photoionization models of the broad emission line region (BELR) over a high-dimensional parameter space predict detectable He II Ly-alpha emission, with any other lines absent or very weak. Observationally, we find He II Ly-alpha is only rarely present, and sometimes other emission lines are present. We consider a number of possible explanations for weak He II Ly-alpha, including absorption from the IGM and absorption intrinsic to the quasar, and find that although no single explanation suffices, together they may. We find that BELR turbulence is an effective way of increasing the strength of metal lines relative to H and He, and may plausibly explain the presence of EUV metal lines. We offer a few, very tentative identifications of EUV metal lines seen in some spectra. Updates to ongoing HST He II studies will also be briefly described.
We gratefully acknowledge NASA/STScI funding for HST program numbers 10907, 11215, 11982, and 12178.

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