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J. Scott, J. Bechtold (Steward Observatory, University of Arizona), A. Dobrzycki (CfA), V. Kulkarni (Steward Observatory, University of Arizona)
The proximity effect is the deficit of Lyman \alpha absorption lines in a QSO spectrum near the QSO Lyman \alpha emission line. This is thought to be caused by enhanced photoionization of the Lyman \alpha forest absorbers in the immediate vicinity of the QSO due to the flux of ultraviolet photons from the QSO itself. At large distances from the QSO, the proximity effect dies away and the ionization state of the absorbers is mitigated only by the ambient extragalactic UV radiation field. Thus, the proximity effect has been used as a tool for quantifying the intensity of this UV background at the Lyman limit, JLL. We present measurements of JLL from the proximity effect analysis based on previously unpublished, moderate-resolution spectra of 40 QSOs, supplemented by 59 QSO spectra from the literature.
The proximity effect analysis is fraught with systematic uncertainties; and we make an attempt to account for several of these in this work. The first important issue is that of QSO systemic redshifts. Redshifts based on the [OIII]\lambda5007 emission line for 19 objects in our sample show an average blueshift of ~1200 km s-1 of Ly\alpha emission with respect to the the systemic redshifts of the QSOs as defined by these lines. Using redshifts based on [OIII] or Mg II for the 35 objects for which they are measured and adding 1200 km s-1 to the remaining QSO Ly\alpha redshifts gives a value for JLL of 7.5 x 10-22 ergs s-1 cm-2 Hz-1 sr-1.
Allowing for the fact that individual QSOs have different spectral indicies which may also be different from that of the background, we use the standard proximity effect method to solve for the HI photoionization rate, \Gamma, and the parameters describing its evolution with redshift. And lastly, we use simulated Lyman \alpha forest spectra including the proximity effect to investigate curve-of-growth effects in the photoionization model used in the analysis.
This large absorption line sample and these techniques for measuring the background and understanding the systematics involved allows us to place what we believe are the firmest limits on the background at these redshifts. We conclude with a comparison of our results to previous work and to models for the evolution of the UV background with an emphasis on whether a contribution from star-forming galaxies is needed to account for the value of JLL we measure.
The author(s) of this abstract have provided an email address for comments about the abstract: jscott@as.arizona.edu