Reconciling the local galaxy population with damped Lyman a cross-sections and metal abundances

A comprehensive analysis of 355 high-quality Westerbork Synthesis Radio Telescope (WSRT) H I 21-cm line maps of nearby galaxies shows that the properties and incident rate of damped Lyman α absorption systems (DLAs) observed in the spectra of high-redshift QSOs are in good agreement with DLAs originating in gas discs of galaxies like those in the z ≈ 0 population. Comparison of low-z DLA statistics with the H I incidence rate and column density distribution f (N_HI) for the local galaxy sample shows no evidence for evolution in the integral 'cross-section density' 〈nσ〉 = l^-1 (l = mean free path between absorbers) below z ≈ 1.5, implying that there is no need for a hidden population of galaxies or Hi clouds to contribute significantly to the DLA cross-section. Compared with z ≈ 4, our data indicate evolution of a factor of 2 in the comoving density along a line of sight. We find that dN/dz(z = 0) = 0.045 ± 0.006. The idea that the local galaxy population can explain the DLAs is further strengthened by comparing the properties of DLAs and DLA galaxies with the expectations based on our analysis of local galaxies. The distribution of luminosities of DLA host galaxies, and of impact parameters between QSOs and the centres of DLA galaxies, is in good agreement with what is expected from local galaxies. Approximately 87 per cent of low-z DLA galaxies are expected to be fainter than L*, and 37 per cent have impact parameters less than 1 arcsec at z = 0.5. The analysis shows that some host galaxies with very low impact parameters and low luminosities are expected to be missed in optical follow-up surveys. The well-known metallicity-luminosity relation in galaxies, in combination with metallicity gradients in galaxy discs, causes the expected median metallicity of low-z DLAs to be low (∼1/7 solar), which is also in good agreement with observations of low-z DLAs. We find that f(N_HI) can be fitted satisfactorily with a gamma distribution, a single power law is not a good fit at the highest column densities N_HI > 10 ²¹ cm^-2. The vast majority (≈81 per cent) of the HI gas in the local Universe resides in column densities above the classical DLA limit (N_HI > 2 × 10²⁰ cm^-2), with N^HI ∼ 10²¹ cm^-2 dominating the cosmic HI mass density.