Toward unbiased determination of the redshift evolution of Lyman-alpha forest clouds

Details Toward unbiased determination of the redshift evolution of Lyman-alpha forest clouds Toward unbiased determination of the redshift evolution of Lyman-alpha forest clouds

May 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...426..502l&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 426, no. 2, p. 502-512

Astronomy and Astrophysics

Astrophysics

10

Absorption Spectra, Intergalactic Media, Line Spectra, Lyman Alpha Radiation, Quasars, Red Shift, Spectrum Analysis, Bias, Cosmology, Errors, Estimates, Evolution (Development)

Scientific paper

The possibility of using DA, the mean depression of a quasar spectrum due to Ly-alpha forest absorption, to study the number density evolution of the Ly-alpha forest clouds is examined in some detail. Current DA measurements are made against a continuum that is a power-law extrapolation from the continuum longward of Ly-alpha emission. Compared to the line-counting approach, the DA-method has the advantage that the DA measurements are not affected by line-blending effects. However, we find using low-redshift quasar spectra obtained with the Hubble Space Telescope (HST), where the true continuum in the Ly-alpha forest can be estimated fairly reliably because of the much lower density of the Ly-alpha forest lines, that the extrapolated continuum often deviates systematically from the true continuum in the forest region. Such systematic continuum errors introduce large errors in the DA measurements. The current DA measurements may also be significantly biased by the possible presence of the Gunn-Peterson absorption. We propose a modification to the existing DA-method, namely, to measure DA against a locally established continuum in the Ly-alpha forest. Under conditions that the quasar spectrum has good resolution and S/N to allow for a reliable estimate of the local continuum in the Ly-alpha forest, the modified DA measurements should be largely free of the systematic uncertainties suffered by the existing DA measurements. We also introduce a formalism based on the work of Zuo (1993) to simplify the application of the DA-method(s) to real data. We discuss the merits and limitations of the modified DA-method, and conclude that it is a useful alternative. Our findings that the extrapolated continuum from longward of Ly-alpha emission often deviates systematically from the true continuum in the Ly-alpha forest present a major problem in the study of the Gunn-Peterson absorption.

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