Statistics Martin A. Zwaan , Marc A. W. Verheijen , Frank H. Briggs, PASA, 16 (1), in press.
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Contribution of Low Surface Brightness Galaxies
It has been argued in the literature that low surface brightness (LSB) galaxies might contribute a considerable HÒšI cross section. In particular, Linder (1998) explores a scenario in which the outskirts of galaxies are responsible for most of the cross section for low column density neutral gas ( 
). She concludes that Ly
absorber counts at low redshifts can be explained if LSB galaxies of moderate absolute luminosity with extended low density gas disks are included in the analysis. Contrary to this view, Chen et al. (1998) claim that extended disks of luminous galaxies can account for most of the observed Ly lines below
. The contribution of dwarf and LSB galaxies to the cross section for high column density HÒšI is also unclear. For instance, Rao & Turnshek (1998) show that there are no luminous spiral galaxies in the vicinity of the quasar OIÒš363 in which spectrum they identify two low-z DL systems.
Here we evaluate the contribution of LSB galaxies to the cross section for high column density gas at z=0. First we have to address the problem of completeness. The Ursa Major sample is essentially a magnitude limited sample. Selection effects against LSB galaxies are therefore to be expected. Tully & Verheijen (1997) discuss the completeness of the sample by plotting the observed central surface brightness against the exponential disk scale length. Theoretical approximations of the visibility limits seem to describe the boundaries of the observed sample satisfactorily. We apply the same visibility limits to the HÒšI selected galaxy sample of Zwaan et al. (1997) to estimate what fraction of the HÒšI mass density in the Ursa Major cluster could be missed in the present study. It appears that galaxies below the optical detection limits of our Ursa Major sample probably contain 10% of the total HÒšI density of the cluster. Not surprisingly, most of this missed HÒšI density resides in LSB galaxies. Following Tully & Verheijen (1997), the separation between LSB and HSB galaxies is made at an extrapolated central surface brightness of
in the K'-band, which roughly compares to
in the B-band.
FigureÒš3 illustrates the contribution of LSB galaxies to the cross section for high column density gas, relative to the total galaxy population. We have corrected for the incompleteness by adding extra cross section for the LSB galaxies, equally over all column densities, in such a way that the mass density in these galaxies increases by an amount equal to 10% of the total HÒšI density. The left panel shows the CDDF, the right panel shows the cosmological mass density of HÒšI as a function of column density. The full resolution data are used. LSB galaxies do not make a significant contribution to the cross section for column densities higher than
. Below that value they are responsible for approximately 25% of the cross section. The right panel shows that LSB galaxies make a minor contribution to the local neutral gas density, a conclusion very much in concordance with the results of Briggs (1997), Zwaan et al. (1997), Sprayberry (1998) and CÓÄtÓ? et al. (1998).
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