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 $z\sim3$ 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 $\sim0.7$ 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 $z\sim0$ 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.

Figures (17)

• Figure 1: The location of the final sample in space of $M_{\rm HI}$ or SFR versus $M_*$. The solid and dashed lines in the left panel are for the star forming main sequence (SFMS) and scatter Saintonge16, and those in the right are for the median relation and scatter of galaxies on the SFMS Catinella18. The squares, diamonds, and downward triangles plot the FEASTS sample for which the interferometric data are taken from the HALOGAS, the THINGS and other VLA surveys or archive, respectively. The observed FEASTS sample of 55 galaxies are also plotted as grey dots for reference. The upward triangles plot the MHONGOOSE sample; one MHONGOOSE galaxy (J1321-31) has $M_*=10^{4.7}\,M_{\odot}$ and $M_{\rm HI}=10^{7.6}\,M_{\odot}$, and is thus beyond the plotted range.
• Figure 2: An atlas of column density maps for galaxies in the FEASTS sample. For each galaxy, from left to right, we display the column density maps from the FAST data, the interferometric data, the combined full-${\rm H}{\textsc{i}}$ image, and the combined and deblended full-${\rm H}{\textsc{i}}$ image, respectively.
• Figure 3: An atlas of column density maps for galaxies in the MHONGOOSE sample. For each galaxy, we display the column density maps from the r10_t00 data and the combined full-${\rm H}{\textsc{i}}$ image.
• Figure 4: Properties of the main sample that are most relevant for $f(N_{\rm HI})$ constructions. Panels a to f show the number distributions of galaxies in properties including image physical resolution (beam major axis in unit of kpc), image ${\rm H}{\textsc{i}}$ column density limit, the number of beam elements for $\log(N_{\rm HI}/\rm{ cm^{-2} } )>18$, the number of beam elements for $\log(N_{\rm HI}/\rm{ cm^{-2} } )>20$, the ${\rm H}{\textsc{i}}$ mass, and the galactic axis ratio. The distributions of the main sample (MS) are plotted as open histograms with black outlines. Distributions of the FEASTS sample are plotted in red histograms with upward diagonal stripes, and those of the MHONGOOSE sample in green histograms with downward diagonal stripes.
• Figure 5: Systematic error estimation for $f(N_{\rm HI})$. Panel a: different errors as a function of $\log(N_{\rm HI}/\rm{ cm^{-2} } )$. Three types of errors are considered, due to the stochastic sampling of galaxies (blue upward triangles), due to stochastic inclusion of highly inclined galaxies (orange downward triangles), and distance errors (green crosses). The vertical line marks $\log(N_{\rm HI}/\rm{ cm^{-2} } )=21.3$, above which the errors increase dramatically . Panel b: the contributions of different variance components to the final variance of $f(N_{\rm HI})$. Three types of variances are considered: HIMF parameter error of Guo23 (brown circles), Poisson error from the counts of $\log(N_{\rm HI}/\rm{ cm^{-2} } )$ areas (pink squares), and the combined effects of distance errors and galaxy stochastic samplings (purple diamonds).