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

Abstract

The FAST All Sky HI Survey (FASHI) conducted with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) has mapped $\sim 19500{\mathrm{deg}}^2$ of the sky north of DEC $=-14^{\circ }$, detecting $156411$ extragalactic HI sources at $z<0.09$ with a median sensitivity of $0.57\mathrm{mJy}{\mathrm{beam}}^{-1}$ at a velocity resolution of $6.4\mathrm{km}{\mathrm{s}}^{-1}$. 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 $M_{\mathrm{HI}}\sim 10^{6.2}{M}_{\odot }$. When systematic uncertainties are included, the HIMF is well described by a single-Schechter function with a characteristic mass $\log (M_{\ast }/h_{70}^{-2}{M}_{\odot })=9.89\pm 0.02$, low-mass end slope $\hat{I}\pm =-1.31\pm 0.02$, and amplitude $\ddot{I}{•}_{\ast }=(6.38\pm 0.49)\times 10^{-3}{h}_{70}^3{\mathrm{Mpc}}^{-3}{\mathrm{dex}}^{-1}$. The derived cosmic HI density is $\hat{I}{\text{c◯}}_{\mathrm{HI}}=(4.71\pm 0.03_{\mathrm{stat}}\pm 0.40_{\mathrm{sys}})\times 10^{-4}{h}_{70}^{-1}$. 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.

Accepted by SCPMA. The full FASHI DR2 catalog will be publicly available in August 2026 at https://zcp521.github.io/fashi