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The H I Column Density Distribution Function at z=0: the Connection to Damped Ly Statistics

The Há I Column Density Distribution Function at z=0: the Connection to Damped Ly$\alpha $ Statistics

Martin A. Zwaan , Marc A. W. Verheijen , Frank H. Briggs, PASA, 16 (1), 100
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Next Section: Introduction
Contents Page: Volume 16, Number 1

The Há I Column Density Distribution Function at z=0: the Connection to Damped Ly$\alpha $ Statistics

Martin A. Zwaan 1 - Marc A. W. Verheijen 2
Frank H. Briggs 1

1 Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands
zwaan, fbriggs@astro.rug.nl

2 National Radio Astronomy Observatory, P.O. Box 0, Socorro, NM 87801, U.S.A.
mverheij@aoc.nrao.edu

Abstract:

We present a measurement of the Há I column density distribution function [$f(N_{\rm HI})$] at the present epoch for column densities

$>10^{20}~\mbox{$\rm cm^{-2}$}$. These high column densities compare to those measured in damped Ly$\alpha $ lines seen in absorption against background quasars. Although observationally rare, it appears that the bulk of the neutral gas in the Universe is associated with these damped Ly$\alpha $ systems. In order to obtain a good anchor point at z=0 we determine $f(N_{\rm HI})$ in the local Universe by using 21cm synthesis observations of a complete sample of spiral galaxies. We show that $f(N_{\rm HI})$ for damped Ly$\alpha $ systems has changed significantly from high z to the present and that change is greatest for the highest column densities. The measurements indicate that low surface brightness galaxies make a minor contribution to the cross section for Há I, especially for

$\mbox{$N_{\rm HI}$}>10^{21}~\mbox{$\rm cm^{-2}$}$.

Keywords: galaxies: ISM -- galaxies: evolution -- quasars: absorption lines



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