Evolution of damped Lyman alpha kinematics and the effect of spatial resolution on 21-cm measurements

We have investigated the effect of spatial resolution on determining pencil-beam like velocity widths and column densities in galaxies. Three 21-cm datasets are used, the HIPASS galaxy catalogue, a subset of HIPASS galaxies with ATCA maps and a high-resolution image of the LMC. Velocity widths measured from 21-cm emission in local galaxies are compared with those measured in intermediate redshift Damped Lyman alpha (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 two. 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/dz from local low-resolution 21-cm measurements are expected to be overestimated by only ~10 per cent.