Lyman Alpha Clouds: A Consistent Distribution in Column Density}

Details Lyman Alpha Clouds: A Consistent Distribution in Column Density} Lyman Alpha Clouds: A Consistent Distribution in Column Density}

May 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aas...182.7808c&link_type=abstract

American Astronomical Society, 182nd AAS Meeting, #78.08; Bulletin of the American Astronomical Society, Vol. 25, p.930

Astronomy and Astrophysics

Astronomy

Scientific paper

We propose a convenient analytical approximation for curve-of-growth analysis of the Lyman alpha series, which is valid over a very broad range of parameters of absorbing clouds. The equivalent width of the line as a function of the line center optical depth, τ0 =<= ft({sqrt pi e(2) mec} right ) ({λ0fNb} right ), is given by W = frac{sqrt 2 b λ0}{c} left [ln left (1+ fracπ{2} τ0 2 right) right ] 1/2 + frac{sqrt N}{Nd}, where Nd = ({mec(2) 2pi e(2}) right) (f λ0 right)(-1) ({g2 2g1} right)({{1)2}}. This expression gives the correct asymptotics in the unsaturated, saturated, and damped regimes, as well as provides an accuracy not worse than 10% in the intermediate cases. With the use of this formula we have obtained a simple analytical approximation to the relationship between a rest-frame equivalent width distribution for Ly-alpha forest absorption lines, {cal N}(W), and an HI column density distribution of the observed cloud number, {cal N}(N). Assuming a simple power-law form for d {cal N}/d N~ N(-beta ) and that the velocity width b=const, it is shown that beta = 1.4 agrees fairly well with the observed form of {cal N}(W) in a broad range of column densities 10(13) cm(-2) lax N lax 10(17) cm(-2) , which corresponds to equivalent width range 0.06 Angstrom slax Wlax 0.9 Angstroms. Only this value of beta makes consistent the procedure of transforming {cal N}(W) into {cal N}(N), and vice versa. The above result has a number of important implications, of which worth mentioning is `the proximity effect' (Chernomordik & Ozernoy 1993, ApJ 404 L5).

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