document: the range of hi  column densities typically seen in routine 21-cm emission line observations of the neutral gas disks in nearby galaxies is very similar to those that characterise the damped lyman-@xmath6 systems or dlas with @xmath7 . 
   an attractive experiment would therefore be to map the hi  gas of dla absorbing systems in 21-cm emission , and measure the dlas total gas mass , the extent of the gas disks and their dynamics . 
   this would provide a direct observational link between dlas and local galaxies , but unfortunately such studies are impossible with present technology ( see e.g. , kanekar et al . 
   the transition probability of the hyperfine splitting that causes the 21-cm line is extremely small , resulting in a weak line that can only be observed in emission in the very local ( @xmath8 ) universe , with present technology . on the other hand , 
   the identification of dlas as absorbers in background qso spectra is , to first order , not distance dependent because the detection efficiency depends mostly on the brightness of the background source , not on the redshift of the absorber itself . 
   in fact , the lowest redshift ( @xmath9 ) lyman-@xmath6 absorbers can not be observed from the ground because the earth s atmosphere is opaque to the uv wavelength range in which these are to be found . 
   furthermore , due to the expansion of the universe the redshift number density of dlas decreases rapidly toward lower redshifts . consequently , there are not many dlas known whose 21-cm emission would be within the reach of present - day radio telescopes . 
   so , we are left with a wealth of information on the cold gas properties in local galaxies , which has been collected over the last half century , and several hundreds dla absorption profiles at intermediate and high redshift , but little possibility to bridge these two sets of information . obviously , most observers resort to the optical wavelengths to study dlas but attempts to directly image their host galaxies have been notably unsuccessful ( see e.g. , warren et al . 
   2001 and mller et al . 
   2002 for reviews ) . 
   a few positive identifications do exist , mostly the result of hst imaging . 
   although the absolute number of dlas at low @xmath3 is small , the success rate for finding low-@xmath3 host galaxies is better for obvious reasons : the host galaxies are expected to be brighter and the separation on the sky between the bright qso and the dla galaxy is likely larger . 
   early surveys for low-@xmath3 dla host galaxies consisted of broad band imaging and lacked spectroscopic follow - up ( e.g. , le brun et al.1997 ) . 
   later studies aimed at measuring redshifts to determine the association of optically identified galaxies with dlas , either spectroscopically ( e.g. , rao et al . 2003 ) , or using photometric redshifts ( chen & lanzetta 2003 ) . all together , there are now @xmath10 dla galaxies known at @xmath11 . 
   the galaxies span a wide range in galaxy properties , ranging from inconspicuous lsb dwarfs to giant spirals and even early type galaxies . 
   obviously , it is not just the luminous , high surface brightness spiral galaxies that contribute to the hi  cross section above the dla threshold . as explained above 
   , we can not study these galaxies in the 21-cm line on a case - by - case basis , but we can do a study of a statistical nature to see if the properties of dlas and dla galaxies agree with our knowledge of hi  in the local universe . 
   blind 21-cm emission line surveys in the local universe with single dish radio telescopes such as parkes or arecibo have resulted in an accurate measurement of @xmath12 , which can be used as a reference point for higher redshift dla studies . 
   @xmath13 is simply calculated by integrating over the hi  mass function of galaxies , which is measured with surveys such as hipass ( zwaan et al . 
   however , due to the large beam widths of the singe dish instruments , these surveys at best only barely resolve the detected galaxies and are therefore not very useful in constraining the column density distribution function of @xmath0 hi . 
   hence , for this purpose we use the high resolution 21-cm maps of a large sample of local galaxies that have been observed with the westerbork synthesis radio telescope . 
   this sample is known as whisp ( van der hulst et al . 2001 ) and consists of 355 galaxies spanning a large range in hi  mass and optical luminosity . 
   the total number of independent column density measurements above the dla limit is @xmath14 , which implies that the data volume of our present study is the equivalent of @xmath14 dlas at @xmath1 !    each galaxy in the sample 
   is weighted according to the hi  mass function of galaxies . 
   we can now calculate the column density distribution function , @xmath15 where @xmath16 is the area function that describes for galaxies with hi  mass  the area in @xmath17 corresponding to a column density in the range  to @xmath18 , and @xmath19 is the hi  mass function . 
   @xmath20 converts the number of systems per mpc to that per unit redshift . 
   figure  [ whispfn2.fig ] shows the resulting  on the left , and the derived hi  mass density per decade of  on the right . for comparison with higher redshift observations 
   , we also plot the results from two other studies . 
   the proux ( 2005 ) measurements of  below the dla limit are the result of their new uves survey for `` sub - dlas '' . 
   the intermediate redshift points from rao et al . 
   ( 2005 ) are based on mgii - selected dla systems . 
   the surprising result from this figure is that there appears to be only very mild evolution in the intersection cross section of hi  from redshift @xmath21 to the present . from this figure 
   we can determine the redshift number density of @xmath22 gas and find that @xmath23 , in good agreement with earlier measurements at @xmath1 . 
   compared to the most recent measurements of @xmath24 at intermediate and high @xmath3 , this implies that the comoving number density ( or the `` space density times cross section '' ) of dlas does not evolve after @xmath4 . in other words 
   , the local galaxy population explains the incidence rate of low and intermediate @xmath3 dlas and there is no need for a population of hidden very low surface brightness ( lsb ) galaxies or isolated hi  clouds ( dark galaxies ) . 
   the right hand panel shows that at @xmath1 most of the hi  atoms are in column densities around @xmath25 . 
   this also seems to be the case at higher redshifts , although the distribution might flatten somewhat . 
   the one point that clearly deviates is the highest  point from rao et al . 
   ( 2005 ) at @xmath26 . 
   the figure very clearly demonstrates that this point dominates the  measurement at intermediate redshifts . 
   it is therefore important to understand whether the mgii - based results really indicate that high column densities ( @xmath27 ) are rare at high and low redshift , but much more ubiquitous at intermediate redshifts , or whether the mgii  selection introduces currently unidentified biases . 
   now that we have accurate cross section measurement of all galaxies in our sample , and know what the space density of our galaxies is , we can calculate the cross - section weighted probability distribution functions of various galaxy parameters . 
   figure  [ pdfs.fig ] shows two examples . 
   the left panel shows the @xmath28-band absolute magnitude distribution of cross - section selected galaxies above four different hi  column density cut - offs . 
   87% of the dla cross - section appears to be in galaxies that are fainter than @xmath29 , and 45% is in galaxies with @xmath30 . 
   these numbers agree very well with the luminosity distribution of @xmath11 dla host galaxies . 
   taking into account the non - detections of dla host galaxies and assuming that these are @xmath31 , we find that 80% of the @xmath11 dla galaxies are sub-@xmath29 . 
   the median absolute magnitude of a @xmath1 dla galaxy is expected to be @xmath32 ( @xmath33 ) . 
   the conclusion to draw from this is that we should not be surprised to find that identifying dla host galaxies is difficult . 
   most of them ( some 87% ) are expected to be sub-@xmath29 and many are dwarfs .    using similar techniques , we find that the expected median impact parameter of @xmath34 systems is 7.8 kpc , whereas the median impact parameter of identified @xmath11 dla galaxies is 8.3 kpc . assuming no evolution in the properties of galaxies gas disks , these numbers imply that 37% of the impact parameters are expected to be less than @xmath35 for systems at @xmath36 . 
   this illustrates that very high spatial resolution imaging programs are required to successfully identify a typical dla galaxy at intermediate redshifts . 
   the right panel in figure  [ pdfs.fig ] shows the probability distribution of oxygen abundance in @xmath1 dlas . 
   we constructed this diagram by assigning to every hi  pixel in our 21-cm maps an oxygen abundance , based on the assumption that the galaxies in our sample follow the local metallicity  luminosity ( @xmath37 ) relation ( e.g. garnett 2002 ) , and that each disk shows an abundance gradient of [ o / h ] of @xmath38 ( e.g. , ferguson et al . 
   1998 ) along the major axis . 
   the solid lines correspond to these assumptions , the dotted lines are for varying metallicity gradients in disks of different absolute brightness . 
   the main conclusion is that the metallicity distribution for hi  column densities @xmath34 peaks around [ o / h]=@xmath39 to @xmath40 , much lower than the mean value of an @xmath29 galaxy of [ o / h]@xmath41 . 
   the reason for this being that _ 1 ) _ much of the dla cross section is in sub-@xmath29 galaxies , which mostly have sub - solar metallicities , and _ 2 ) _ for the more luminous , larger galaxies , the highest interception probability is at larger impact parameters from the centre , where the metallicity is lower . 
   interestingly , this number is very close to the @xmath1 extrapolations of metallicity measurements in dlas at higher @xmath3 from prochaska et al . 
   ( 2003 ) and kulkarni et al . 
   for the mean mass - weighted metallicity of hi  gas with @xmath34 at @xmath1 we find the value of @xmath42 , also consistent with the @xmath1 extrapolation of -weighted metallicities in dlas , although we note this extrapolation has large uncertainties given the poor statistics from dlas at @xmath43 . 
   these results are in good agreement with the hypothesis that dlas arise in the hi  disks of galaxies . 
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summary: we calculate in detail the expected properties of low redshift dlas under the assumption that they arise in the gaseous disks of galaxies like those in the @xmath0 population . 
   a sample of 355 nearby galaxies is analysed , for which high quality hi  21-cm emission line maps are available as part of an extensive survey with the westerbork telescope ( whisp ) . we find that expected luminosities , impact parameters between quasars and dla host galaxies , and metal abundances are in good agreement with the observed properties of dlas and dla galaxies . 
   the measured redshift number density of @xmath1 gas above the dla limit is @xmath2 , which compared to higher @xmath3 measurements implies that there is no evolution in the comoving density of dlas along a line of sight between @xmath4 and @xmath1 , and a decrease of only a factor of two from @xmath5 to the present time . 
   we conclude that the local galaxy population can explain all properties of low redshift dlas . 
   galaxies : ism ; ( galaxies : ) quasars : absorption lines ; galaxies : evolution