Kavli Affiliate: Luis Ho
| Summary:
We present the first $z=0$ HI column density distribution function, $f(N_mathrmHI)$, extending down to $log (N_mathrmHI/mathrmcm^-2)=17.8$. This was derived from high-sensitivity 21-cm emission-line imaging at $sim$1 kpc resolution. At high-column-densities (19.8$< log (N_mathrmHI/mathrmcm^-2) <$21.3), our results align with earlier $z=0$ studies but benefit from 100 times greater sensitivity. Comparisons with $zsim3$ quasar absorption-line studies reveal that $f(N_mathrmHI)$ at $z=0$ is systematically lower by 0.1-0.4 dex for $19.2< log (N_mathrmHI/mathrmcm^-2) <21$. However, the distributions become comparable at $17.8< log (N_mathrmHI/mathrmcm^-2) <19.2$, suggesting weak evolution in this regime. Extrapolating the length incidence ($mathrmdN/mathrmdX$) for $log (N_mathrmHI/mathrmcm^-2) >17.5$ implies a covering fraction ($f_mathrmcov$) of $sim0.7$ within 1-kpc-scale HI-detected pixels at $z=0$. Notably, for $17.8< log (N_mathrmHI/mathrmcm^-2) <20$, impact parameters at a given $N_mathrmHI$ are significantly lower than previous $zsim0$ absorption-line results and TNG50 simulation predictions. This discrepancy indicates challenges in identifying galaxy counterparts for absorbers and in recovering low-column-density HI within cosmological simulations. Finally, we derive a covering fraction of 0.006 for $log (N_mathrmHI/mathrmcm^-2) >17.8$ gas within the virial radius around Milky-Way-like galaxies. These findings provide new constraints on the baryonic flows and gaseous dynamics governing galaxy evolution.
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