Disentangle the physical drivers of Na-absorber incidence evolution

Determine whether the observed low-redshift increase in intervening Na I absorber incidence is driven primarily by changes in host-galaxy abundance, Na-bearing gas cross-section, or covering fraction.

Background

The paper measures a factor of approximately 3.4 increase in the incidence of intervening Na I absorbers from redshift z approximately 0.25 to z approximately 0.03. Because absorber incidence is proportional to the product of host number density and effective Na-absorption cross-section, the observed evolution could reflect changes in either the abundance of host systems or the area and covering fraction of Na-bearing gas.

The authors emphasize that absorber statistics alone cannot distinguish these alternatives. They suggest that more complete host identifications, galaxy surveys, and measurements of associated ions could determine whether the evolution is caused mainly by changes in the host population or by changes in the Na-bearing gas around otherwise similar hosts.

References

Absorber statistics alone cannot separate these possibilities.

— The First Blind Survey of Intervening NaI Absorbers: Incidence and Cosmic Evolution with DESI  (2609.26794 - Anand et al., 22 Sep 2026) in Section 4.2, “Cosmic Evolution of Na at Low Redshifts”

The host-galaxy check above shows that at least some \ion{Na}{1}~absorbers are associated with foreground galaxies, but it does not yet identify the dominant physical origin of the population.

— The First Blind Survey of Intervening NaI Absorbers: Incidence and Cosmic Evolution with DESI  (2609.26794 - Anand et al., 22 Sep 2026) in Section 4.3, “An Exploratory Host-galaxy Check”