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

Abstract

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(NH i) 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 H i 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(NH i) can be fitted satisfactorily with a gamma distribution, a single power law is not a good fit at the highest column densities NH i > 1021 cm−2. The vast majority (≈81 per cent) of the H i gas in the local Universe resides in column densities above the classical DLA limit (NH i > 2 × 1020 cm−2), with NH i∼ 1021 cm−2 dominating the cosmic H i mass density.

Galaxy counterparts of metal-rich damped Lyα absorbers: the case of J205922.4−052842★

“…DLA galaxies are selected by their crosssection areas, because their detection rate depends on the probability that a QSO sight line intersects them. The cross-section area of a galaxy is known to scale locally with its luminosity to a given power (Wolfe et al 1986;Zwaan et al 2005). On the assumption that a similar relation was at play at higher redshifts, and combining this with the faint end slope of the luminosity function (Schechter 1976), one can conclude that DLA galaxies are mostly selected from this faint end.…”

The potential of tracing the star formation history with H I 21-cm in intervening absorption systems

“…This has a value relative to the critical density of Ω HI ≈ 0.5 × 10 −3 at z < ∼ 0.5 (Zwaan et al 2005;Lah et al 2007;Braun 2012;Delhaize et al 2013;Rhee et al 2013;Hoppmann et al 2015;Neeleman et al 2016), rising to Ω HI ≈ 1 × 10 −3 at z ∼ 0.5, where it remains nearly constant over the observed 0.5 < ∼ z < ∼ 5 (Rao & Turnshek 2000;Prochaska & Herbert-Fort 2004;Rao et al 2006;Curran 2010;Prochaska & Wolfe 2009;Noterdaeme et al 2012;Crighton et al 2015). Furthermore, the inflow of neutral gas, from within the galaxy or from the intergalactic medium, which may feed fuel to the star formation sites (Michałowski et al 2015), also exhibits a near constancy with redshift (Spring & Michałowski 2017).…”

On the mass–metallicity relation, velocity dispersion, and gravitational well depth of GRB host galaxies

“…This difference in paths changes the observed spectral lines in three different ways. First the HI column density of GRBDLAs is higher because of the known impact parameter vs HI column density anti correlation (Møller and Waren 1998;Zwaan et al 2005;Krogager et al 2012), second this will have an effect on the measured metallicity if the galaxies have metallicity gradients (van Zee et al 1998;Swinbank et al 2012), and third the sightlines will sample different paths through the dark matter gravitational well of the galaxy and they will therefore sample different depths of this gravitational well. We illustrate this in Figure 4.…”