Quasar Metallic Absorption Systems Irradiated by a Steep-Spectrum UV Background

Details Quasar Metallic Absorption Systems Irradiated by a Steep-Spectrum UV Background Quasar Metallic Absorption Systems Irradiated by a Steep-Spectrum UV Background

Dec 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995pasj...47..877d&link_type=abstract

Publications of the Astronomical Society of Japan, v.47, p.877-888.

Astronomy and Astrophysics

Astronomy

1

Diffuse Radiation, Galaxies: Intergalactic Medium, Quasars: Absorption Lines, Ultraviolet: Galaxies

Scientific paper

We re-examine the spectral shape of the UV background at z =~ 2--3 using the following observations: the ``proximity effect'' of a Lyman alpha forest; the HeII Gunn-Peterson optical depth, tentatively detected by Jakobsen et al. (1994, Nature 370, 35); the soft X-ray background at the present epoch; and the observed ionization states of the Lyman-limit systems (LLSs) along the line of sight to quasar HS 1700+6416. We conclude that the spectral shape (I_nu ~ nu (-alpha ) ) of the optimum ionizing radiation should have the following three features: 1) a break at the HeI ionization edge, by a factor ~ 5, 2) a steep (alpha =~ 3) spectrum up to the HeII ionization edge, and 3) a flat (|alpha | <~ 1) spectrum in the energy-band range from far UV to soft X-ray. When we consider the spectral modification due to the intervening absorption by Lyman alpha clouds and LLSs, the spectral shape of the integrated UV radiation from QSOs is roughly similar to that of our optimum ionizing background, but with a somewhat smaller break at the HeII edge, and a negligible break at the HeI edge. A combination of the UV absorption feature and (soft) X-ray constraints is crucial to conclude whether or not photoionization is the main ionization mechanism of highly ionized metal ions observed in the QSO metallic absorption systems. Moreover, the LLSs at the relevant redshifts may originate in gaseous halos, having a typical gas mass of 10(10) MO with a metallicity of about (1 -- 3) times 10(-2) solar value and a radius of 50 -- 100 kpc.

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