2005MNRAS.364L..51R

We have investigated the effect of spatial resolution on determining pencil-beam-like velocity widths and column densities in galaxies. Three 21-cm data sets are used, the Hi Parkes All Sky Survey (HIPASS) galaxy catalogue, a subset of HIPASS galaxies with Australia Telescope Compact Array maps and a high-resolution image of the Large Magellanic Cloud. Velocity widths measured from 21-cm emission in local galaxies are compared with those measured in intermediate-redshift damped Lyman α (DLA) absorbers. We conclude that spatial resolution has a severe effect on measuring pencil-beam like velocity widths in galaxies. Spatial smoothing by a factor of 240 is shown to increase the median velocity width by a factor of 2. Thus any difference between velocity widths measured from global profiles or low spatial resolution 21-cm maps at z= 0 and DLAs at z > 1 cannot unambiguously be attributed to galaxy evolution. The effect on column density measurements is less severe and the values of dN_DLA/dz from local low-resolution 21-cm measurements are expected to be overestimated by only ∼10 per cent.