FEASTS and MHONGOOSE: HI Column Density Distribution at $z=0$ for $N_{\mathrm{HI}}>10^{17.8}{\mathrm{cm}}^{-2}$

Abstract

We present the first $z=0$ HI column density distribution function, $f(N_{\mathrm{HI}})$, extending down to $\log (N_{\mathrm{HI}}/{\mathrm{cm}}^{-2})=17.8$. This was derived from high-sensitivity 21-cm emission-line imaging at $\sim$1 kpc resolution. At high-column-densities (19.8$<\log (N_{\mathrm{HI}}/{\mathrm{cm}}^{-2})<$21.3), our results align with earlier $z=0$ studies but benefit from 100 times greater sensitivity. Comparisons with $z\sim 3$ quasar absorption-line studies reveal that $f(N_{\mathrm{HI}})$ at $z=0$ is systematically lower by 0.1-0.4 dex for $19.2<\log (N_{\mathrm{HI}}/{\mathrm{cm}}^{-2})<21$. However, the distributions become comparable at $17.8<\log (N_{\mathrm{HI}}/{\mathrm{cm}}^{-2})<19.2$, suggesting weak evolution in this regime. Extrapolating the length incidence ($\mathrm{d}N/\mathrm{d}X$) for $\log (N_{\mathrm{HI}}/{\mathrm{cm}}^{-2})>17.5$ implies a covering fraction ($f_{\mathrm{cov}}$) of $\sim 0.7$ within 1-kpc-scale HI-detected pixels at $z=0$. Notably, for $17.8<\log (N_{\mathrm{HI}}/{\mathrm{cm}}^{-2})<20$, impact parameters at a given $N_{\mathrm{HI}}$ are significantly lower than previous $z\sim 0$ 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 $\log (N_{\mathrm{HI}}/{\mathrm{cm}}^{-2})>17.8$ 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.

30 pages, 17 figures. ApJ in press

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