The Extragalactic Ionizing Background at Low Redshift

Details The Extragalactic Ionizing Background at Low Redshift The Extragalactic Ionizing Background at Low Redshift

2001-08-21

arxiv.org/abs/astro-ph/0108344v1

Astronomy and Astrophysics

Astrophysics

8 Latex pages, 3 encapsulated Postscript figures, uses newpasp.sty, to appear in "Extragalactic Gas at Low Redshfit", ASP Conf

Scientific paper

We present a measurement of the mean intensity of the hydrogen-ionizing background radiation field at low redshift using 906 Ly-alpha absorption lines in 151 quasar spectra from the archives of the Faint Object Spectrograph on the Hubble Space Telescope. Using a maximum likelihood technique and the best estimates possible for each QSO's Lyman limit flux and systemic redshift, we find J(\nu_{0})= 7.6^{+9.4}_{-3.0} x 10^{-23} ergs s^{-1} cm^{-2} Hz^{-1} sr^{-1} at 0.03 < z < 1.67. This is in good agreement with the mean intensity expected from models of the background which incorporate only the known quasar population. When the sample is divided into two subsamples, consisting of lines with z < 1 and z > 1, the values of J(\nu_{0}) found are 6.5^{+38.}_{-1.6} x 10^{-23} ergs s^{-1} cm^{-2} Hz^{-1} sr^{-1}, and 1.0^{+3.8}_{-0.2} x 10^{-22} ergs s^{-1} cm^{-2} Hz^{-1} sr^{-1}, respectively, indicating that the mean intensity of the background is evolving over the redshift range of this data set. Relaxing the assumption that the spectral shapes of the sample spectra and the background are identical, the best fit HI photoionization rates are found to be 6.7 x 10^{-13} s^{-1} for all redshifts, and 1.9 x 10^{-13} s^{-1} and 1.3 x 10^{-12} s^{-1} for z < 1 and z > 1, respectively.

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