Astronomy and Astrophysics – Astrophysics
Scientific paper
Mar 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apjs...91....1b&link_type=abstract
Astrophysical Journal Supplement Series (ISSN 0067-0049), vol. 91, no. 1, p. 1-78
Astronomy and Astrophysics
Astrophysics
193
Absorption Spectra, Astronomical Models, Intergalactic Media, Luminosity, Lyman Alpha Radiation, Photoionization, Proximity Effect (Electricity), Quasars, Red Shift, Spectral Line Width, Tables (Data), Hubble Space Telescope, Mean Free Path, Reflecting Telescopes, Spectrographs, Ultraviolet Radiation
Scientific paper
Moderate-resolution spectra of 34 high-redshift quasi-stellar objects (QSOs) were obtained at the Multiple Mirror Telescope and Palomar 5 m Telescope in order to study the properties of the Lyman-alpha forest absorption lines with zabs approx. = zem. For zabs much less than zem, the evolution of the number of lines per unit redshift, dN/dz is well described by the form (1 + z)gamma, with gamma = 1.89 +/- 0.28 for lines with rest equivalent width greater than 0.32 A. For lines stronger than 0.16 A, gamma = 1.32 +/- 0.24. These results imply slower evolution in the mean free path for absorption for the clouds than previously thought. When this sample is combined with Hubble Space Telescope (HST) results for low redshift (Bahcall et al. 1993), a single gamma is an acceptable fit to the data, with gammma = 1.26 +/- 0.13 for zabs = 0-4.1, although an inflection at z approx. = 1.5 may be present. In the high-redshift sample, there is a significant trend of decreasing gamma with decreasing equivalent width limit, or, alternatively, a change in equivalent width distribution with redshift. There is no significant curvature in the log (dN/dz) versus log (1 + z) relation for the strong lines, but some steepening of gamma at z approx. = 2.5 for the weak ones. While these findings for the weak lines are highly significant in a statistical sense, they may result from some subtle line-blending problem, rather than a real physical process. The lines with zabs approx. = zem are significantly depleted. The strength of the deficit of lines depends weakly on QSO Lyman limit luminosity, but not a redshift. The strength of the deficit also depends of the rest equivalent width of the lines, with weak lines showing a relatively weaker effect. These findings support previous suggestions that the 'proximity effect' results from enhanced photoionization by the QSO's extreme ultraviolet (EUV) radiation, rather than some property of the QSO or intergalactic medium which evolves with redshift. The simple photoionization model of Bajtlik, Duncan, & Ostriker describes the data well. If the ionizing background at the Lyman limit, Jnu, is assumed to be constant with redshift for z greater than 1.6 and less than 4.1, and writing Jnu = J-21 x 10-21 ergs/(sq cm sec Hz sr), then J-21 approx. = 3. This value is formally consistent with previous results, although about a factor of 3 larger. Hence, the factor by which QSOs fail to provide the EUV background at high redshifts (at lest in this simple picture), is also increased. If the redshift dependence of Jnu is parameterized as Jnu proportional to ((1 + z)j), then j is not well constrained owing to the small redshift interval sampled, and acceptable fits are obtained for j greater than -7 and less than 4. Additions to the simple model and the resulting uncertainties in these estimates of Jnu are discussed. One serious uncertainty is the neutral fraction of the clouds.
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