The b Distribution and the Velocity Structure of Absorption Peaks in the Lyα Forest
Abstract
A theory is developed that relates the observed b parameter of a Lyα absorption line to the velocity curvature of the corresponding peak in the optical depth fluctuation. Its relation to the traditional interpretation of b as the thermal broadening width is discussed. It is demonstrated that, independent of the details of the cosmological model, the differential b distribution has a highb asymptote of dN/db~b^{m}, where m>=5, when we make the reasonable assumption that lowcurvature fluctuations are statistically favored over highcurvature ones. There in general always exist lines much broader than the thermal width. We develop a linear perturbative analysis of the optical depth fluctuation, which yields a singleparameter prediction for the full b distribution. In addition to exhibiting the highvelocity tail, it qualitatively explains the observed sharp lowb cutoffa simple reflection of the fact that highcurvature fluctuations are relatively rare. Although the existence of the highb asymptote, which is independent of the validity of the linear expansion, is consistent with the observed b distribution, a detailed comparison of the linear prediction with six observational data sets indicates that higher order corrections are not negligible. The perturbative analysis nonetheless offers valuable insights into the dependence of the b distribution on cosmological parameters such as Ω and the power spectrum. A key parameter is the effective smoothing scale of the optical depth fluctuation, which is in turn determined by three scales: the thermal broadening width, the baryon smoothing scale (approximately the Jeans scale), and the observation/simulation resolution. The first two are determined by reionization history, but are comparable in general, whereas the third varies by about an order of magnitude in current hydrodynamic simulations. Studies with nonresolutiondominated b distributions can be used to probe the reionization history of the universe.
 Publication:

The Astrophysical Journal
 Pub Date:
 June 1999
 DOI:
 10.1086/307202
 arXiv:
 arXiv:astroph/9709100
 Bibcode:
 1999ApJ...517..541H
 Keywords:

 COSMOLOGY: THEORY;
 GALAXIES: INTERGALACTIC MEDIUM;
 GALAXIES: QUASARS: ABSORPTION LINES;
 Cosmology: Theory;
 Galaxies: Intergalactic Medium;
 Galaxies: Quasars: Absorption Lines;
 Astrophysics
 EPrint:
 18 pages, Latex with aaspp4.sty, submitted to ApJ