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arxiv: 2606.31539 · v1 · pith:SPO5JLNZnew · submitted 2026-06-30 · 🌌 astro-ph.GA · astro-ph.SR

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

Chuan-Peng Zhang (NAOC) , Ming Zhu , Peng Jiang , Hong Guo , Jin-Long Xu , Xiao-Lan Liu , Nai-Ping Yu , Cheng Cheng
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Jing Wang Jie Wang FAST Collaboration
This is my paper

Pith reviewed 2026-07-01 04:50 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.SR
keywords HI mass functionFASHI surveycosmic HI densitySchechter functionextragalactic HI sourcesFAST telescoperadio astronomy surveyneutral hydrogen
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The pith

The FASHI survey measures the HI mass function down to 10^6.2 solar masses using over 109000 sources, fitting a single Schechter function with log M* = 9.89, alpha = -1.31, and Omega_HI = 4.71e-4.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper reports the second data release of the FAST All Sky HI Survey, which mapped 19500 square degrees and detected 156411 extragalactic HI sources at redshifts below 0.09. A detailed completeness analysis corrects for non-uniform sensitivity and line-width effects to build the HI mass function from more than 109000 sources. The resulting function is described by a single Schechter form with the quoted parameters once systematic uncertainties are folded in. The derived cosmic HI density supplies a benchmark value for local-universe gas content.

Core claim

The FASHI survey has mapped ~19500 deg^2 north of DEC = -14° and detected 156411 extragalactic HI sources, allowing construction of a robust HI mass function from a completeness-corrected sample of over 109000 sources. When systematic uncertainties are included, the HIMF is well described by a single-Schechter function with characteristic mass log (M* / h70^{-2} M_⊙) = 9.89±0.02, low-mass end slope α = -1.31±0.02, and amplitude φ* = (6.38±0.49)×10^{-3} h70^3 Mpc^{-3} dex^{-1}. The derived cosmic HI density is Ω_HI = (4.71±0.03_stat±0.40_sys)×10^{-4} h70^{-1}.

What carries the argument

The completeness-corrected HI mass function constructed from the survey catalog after accounting for non-uniform sensitivity and line-width dependence.

If this is right

  • The HIMF is robustly constrained down to M_HI ~ 10^{6.2} M_⊙.
  • FASHI supplies the most extensive and sensitive HI catalog to date for local-universe studies.
  • The measured cosmic HI density serves as a benchmark for gas accretion and galaxy evolution models.
  • The single-Schechter description holds once systematic uncertainties are included.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Cross-matching the catalog with optical or infrared surveys could reveal how HI mass correlates with galaxy type or environment.
  • The low-mass slope constrains the abundance of faint HI galaxies that may be missed by optical selections.
  • The Omega_HI value can be compared directly with outputs from hydrodynamic simulations to test gas retention and feedback prescriptions.

Load-bearing premise

The detailed completeness analysis that accounts for the survey's non-uniform sensitivity and line-width dependence produces an accurate correction for the sample of over 109000 sources.

What would settle it

An independent survey covering comparable area and depth that recovers a significantly different low-mass slope or characteristic mass for the HI mass function after its own completeness correction.

read the original 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 $\log (M_{*} / h_{70}^{-2}M_{\odot}) = 9.89\pm 0.02$, low-mass end slope $\alpha = -1.31\pm 0.02$, and amplitude $\phi_{*} = (6.38\pm 0.49)\times 10^{-3}\,h_{70}^{3}\,\mathrm{Mpc}^{-3}\,\mathrm{dex}^{-1}$. The derived cosmic HI density is $\Omega_{\mathrm{HI}} = (4.71\pm 0.03_{\mathrm{stat}}\pm 0.40_{\mathrm{sys}})\times 10^{-4}\,h_{70}^{-1}$. 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 1 minor

Summary. The paper presents the FAST All Sky HI Survey (FASHI) DR2, which mapped ~19500 deg² and detected 156411 extragalactic HI sources at z<0.09. From a completeness-corrected sample of >109000 sources, it constructs the HI mass function (HIMF) down to M_HI ~10^{6.2} M_⊙ and fits a single-Schechter function with log(M*/h70^{-2} M_⊙)=9.89±0.02, α=-1.31±0.02, φ*=(6.38±0.49)×10^{-3} h70^3 Mpc^{-3} dex^{-1}, yielding Ω_HI=(4.71±0.03_stat±0.40_sys)×10^{-4} h70^{-1}.

Significance. If validated, this is a significant advance as the largest and deepest single-dish HI survey to date, substantially increasing the sample size and depth for local HIMF measurements and providing a new benchmark for cosmic HI density with explicit statistical and systematic errors reported.

major comments (1)
  1. [Abstract and associated methods] Abstract (completeness analysis): The central claim that the HIMF parameters and Ω_HI are robust requires that the completeness correction for non-uniform sensitivity and line-width dependence accurately recovers the >109000 sources. The text does not present quantitative recovery statistics from end-to-end injection tests spanning the observed sensitivity map and line-width range; without these, the quoted uncertainties on α and the systematic floor on Ω_HI are under-constrained.
minor comments (1)
  1. Define all acronyms (FAST, FASHI, HIMF, Schechter function) at first use in the main text for clarity.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading and constructive comment on the completeness analysis. We address the point below and will revise the manuscript accordingly.

read point-by-point responses

  1. Referee: [Abstract and associated methods] Abstract (completeness analysis): The central claim that the HIMF parameters and Ω_HI are robust requires that the completeness correction for non-uniform sensitivity and line-width dependence accurately recovers the >109000 sources. The text does not present quantitative recovery statistics from end-to-end injection tests spanning the observed sensitivity map and line-width range; without these, the quoted uncertainties on α and the systematic floor on Ω_HI are under-constrained.

    Authors: We agree that explicit quantitative recovery statistics from end-to-end injection tests are needed to fully support the robustness claims. While the manuscript describes the completeness analysis (including its dependence on the non-uniform sensitivity map and line width), we acknowledge that tabulated or plotted recovery fractions as a function of M_HI, W_50, and local sensitivity were not presented in sufficient detail. In the revised version we will add a new subsection (or expanded figure) showing the recovery rates from our injection tests, which were performed across the full range of observed sensitivities and line widths. These statistics will be used to refine the quoted uncertainties on α and to justify the adopted systematic floor on Ω_HI. revision: yes

Circularity Check

0 steps flagged

No circularity: HIMF parameters obtained by direct fitting after standard completeness correction

full rationale

The derivation proceeds by performing a completeness analysis that accounts for non-uniform sensitivity and line-width dependence, applying the resulting corrections to a sample of >109000 sources, and then fitting a single-Schechter function to the corrected counts. No step reduces the reported parameters (M*, α, φ*, Ω_HI) to a prior fitted quantity or self-citation by construction; the output is an empirical fit to the corrected data rather than a renaming or tautological re-expression of the inputs. The chain is self-contained and does not invoke load-bearing self-citations or ansatzes smuggled from prior work.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on the accuracy of the completeness correction for non-uniform sensitivity; the Schechter parameters are fitted results rather than inputs.

free parameters (1)
  • Schechter parameters (M*, alpha, phi*)
    Fitted to the completeness-corrected sample; these are the output values reported in the abstract.
axioms (1)
  • domain assumption The completeness analysis correctly recovers the underlying HI source population despite non-uniform sensitivity and line-width effects.
    Invoked when constructing the HIMF from the >109000 source sample.

pith-pipeline@v0.9.1-grok · 5937 in / 1386 out tokens · 50595 ms · 2026-07-01T04:50:44.909105+00:00 · methodology

discussion (0)

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