Abstract We present the first z = 0 H I column density distribution function, f ( N HI ), extending down to log ( N HI / cm − 2 ) = 17.8 . This was derived from high-sensitivity 21 cm emission-line imaging at ∼1 kpc resolution. At high column densities (19.8 log ( N HI / cm − 2 ) 21.3), our results align with earlier z = 0 studies but benefit from 100 times greater sensitivity. Comparisons with z ∼ 3 quasar absorption-line studies reveal that f ( N HI ) at z = 0 is systematically lower by 0.1–0.4 dex for 19.2 log ( N HI / cm − 2 ) 21 . However, the distributions become comparable at 17.8 log ( N HI / cm − 2 ) 19.2 , suggesting weak evolution in this regime. Extrapolating the length incidence ( d N / d X ) for log ( N HI / cm − 2 ) > 17.5 implies a covering fraction ( f cov ) of ∼0.7 within 1 kpc-scale H I -detected pixels at z = 0. Notably, for 17.8 log ( N HI / cm −
王 et al. (Thu,) studied this question.