The FAST All Sky HI Survey DR2: the FASHI Catalog and the HI Mass Function

The FAST All Sky HI Survey (FASHI) conducted with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) has mapped $\sim 19500{\mathrm{d}\mathrm{e}\mathrm{g}}^2$ of the sky north of DEC $=-{14}^{\circ }$, detecting $156411$ extragalactic HI sources at $z<0.09$ with a median sensitivity of $0.57\mathrm{m}\mathrm{J}\mathrm{y}{\mathrm{b}\mathrm{e}\mathrm{a}\mathrm{m}}^{-1}$ at a velocity resolution of $6.4\mathrm{k}\mathrm{m}{\mathrm{s}}^{-1}$. The survey achieves unprecedented depth and area coverage, significantly improving upon previous single-dish surveys. Through a detailed completeness analysis that accounts for the survey’s non-uniform sensitivity and line-width dependence, we construct a robust HI mass function (HIMF) using a completeness-corrected sample of over $109000$ sources. The HIMF is robustly constrained down to $M_{\mathrm{H}\mathrm{I}}\sim {10}^{6.2}{M}_{\odot }$. When systematic uncertainties are included, the HIMF is well described by a single-Schechter function with a characteristic mass $\log (M_{\ast }\mathrm{/}{h}_{70}^{-2}{M}_{\odot })=9.89\pm 0.02$, low-mass end slope $\alpha =-1.31\pm 0.02$, and amplitude ${\varphi }_{\ast }=(6.38\pm 0.49)\times {10}^{-3}{h}_{70}^3{\mathrm{M}\mathrm{p}\mathrm{c}}^{-3}{\mathrm{d}\mathrm{e}\mathrm{x}}^{-1}$. The derived cosmic HI density is ${\mathrm{\Omega }}_{\mathrm{H}\mathrm{I}}=(4.71\pm {0.03}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}}\pm {0.40}_{\mathrm{s}\mathrm{y}\mathrm{s}})\times {10}^{-4}{h}_{70}^{-1}$. FASHI provides the most extensive and sensitive HI catalog to date, establishing an important benchmark for studies of gas accretion, galaxy evolution, and large-scale structure in the local universe.