Predicted number counts and clustering of Hi galaxies from future radio surveys
Pith reviewed 2026-05-07 12:00 UTC · model grok-4.3
The pith
Predictions of HI galaxy number counts, redshift distributions, and clustering for SKA-MID using S3-SAX, GAEA, and IllustrisTNG simulations, with HOD fits for sample variance.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We present predictions for galaxy number counts as a function of sensitivity cut and redshift, and use these to forecast the cosmological performance of a proposed SKA-MID cosmological survey. Finally, we fit a halo occupation distribution model to low-redshift angular correlation functions to constrain clustering properties.
Load-bearing premise
The three simulations (S3-SAX, GAEA, IllustrisTNG) accurately capture the HI content, distribution, and clustering of real galaxies across the relevant redshifts and luminosities; any systematic mismatch would directly affect the number-count and cosmological forecasts.
read the original abstract
The 21cm emission line from neutral hydrogen (HI) contained within galaxies provides a way to make accurate spectroscopic redshift determinations in the radio part of the spectrum. Large radio arrays such as SKA-MID are coming online that will have the sensitivity and survey time required to catalogue hundreds of thousands to millions of HI galaxies, opening up the possibility of studying the cosmological large scale structure using this technique. The expected number counts and clustering properties of the galaxies are still quite poorly understood however. We use three different simulated galaxy catalogues to predict the properties of the HI galaxy distribution that SKA-MID will be able to observe, along with estimates of the error on these predictions due to modelling uncertainty. The simulations in question are from S$^3$-SAX (semi-analytic models based on the Millennium dark matter-only simulation); GAEA (an updated semi-analytic model partially calibrated on hydrodynamical simulations); and IllustrisTNG (a hydrodynamical simulation). We present predictions for galaxy number counts as a function of sensitivity cut and redshift, and use these to forecast the cosmological performance of a proposed SKA-MID cosmological survey. Finally, we fit a halo occupation distribution model to low-redshift angular correlation functions to constrain clustering properties of multiple sub-volumes of the simulations to gain insight into the expected variation (sample variance) over smaller survey areas.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript uses three simulated galaxy catalogs (S³-SAX semi-analytic, GAEA semi-analytic, and IllustrisTNG hydrodynamical) to predict HI galaxy number counts as a function of sensitivity cut and redshift for SKA-MID, forecasts the cosmological performance of a proposed SKA-MID survey, and fits halo occupation distribution (HOD) models to low-redshift angular correlation functions from simulation sub-volumes to assess clustering variations and sample variance.
Significance. If the simulated HI populations prove faithful to observations, the work would supply practical forecasts for number counts, survey optimization, and cosmological constraints from future SKA-MID HI galaxy surveys. The explicit use of three independent simulation suites to bracket modeling uncertainty is a clear strength, as it supplies a concrete (if incomplete) estimate of systematic spread in the headline predictions.
major comments (3)
- [Abstract and simulation description] Abstract and simulation description: The headline number-count predictions and SKA-MID cosmological forecasts rest entirely on the fidelity of the three input catalogs in reproducing the HI mass function, luminosity distribution, and bias across the relevant flux limits and redshifts. No quantitative comparison of the simulated HI number densities or mass functions against existing low-z observational constraints (e.g., ALFALFA or HIPASS) is presented, so the quoted inter-simulation spread cannot be shown to bound the true systematic error.
- [Cosmological performance forecast section] Cosmological performance forecast section: The mapping from predicted number counts to forecasted cosmological constraints does not include an error budget that propagates potential correlated biases in the HI prescriptions (gas cooling, feedback, low-mass halo resolution) across the three suites; the modeling uncertainty is therefore likely underestimated.
- [HOD fitting and clustering section] HOD fitting and clustering section: The HOD fits to low-redshift angular correlation functions are used to quantify sample variance in clustering, yet no goodness-of-fit metrics, parameter posteriors, or covariance information are reported, making it impossible to judge whether the derived variations are robust or dominated by fitting noise.
minor comments (2)
- [Figures] Figure captions and axis labels should explicitly state the flux or sensitivity cuts used for each curve to avoid ambiguity when comparing number-count panels.
- [Methods] The manuscript should clarify whether the three simulations share any common sub-grid physics or calibration data that could induce correlated systematics in the HI properties.
Simulated Author's Rebuttal
We thank the referee for the thoughtful and constructive report. We address each major comment point by point below and have revised the manuscript to incorporate the suggested improvements where appropriate.
read point-by-point responses
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Referee: [Abstract and simulation description] Abstract and simulation description: The headline number-count predictions and SKA-MID cosmological forecasts rest entirely on the fidelity of the three input catalogs in reproducing the HI mass function, luminosity distribution, and bias across the relevant flux limits and redshifts. No quantitative comparison of the simulated HI number densities or mass functions against existing low-z observational constraints (e.g., ALFALFA or HIPASS) is presented, so the quoted inter-simulation spread cannot be shown to bound the true systematic error.
Authors: We agree that a direct quantitative comparison to low-redshift observational constraints would provide important context and help readers evaluate whether the inter-simulation spread is representative of the true systematic uncertainty. Although the manuscript's primary emphasis is on differences arising from distinct modeling approaches, we will add a new subsection (and associated figure) in the simulation description that compares the HI mass functions and number densities from S³-SAX, GAEA, and IllustrisTNG against ALFALFA and HIPASS data at z ≈ 0. This addition will explicitly discuss how well each simulation reproduces the observed HI mass function and will clarify the limitations of using the simulation spread alone as an error estimate. revision: yes
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Referee: [Cosmological performance forecast section] Cosmological performance forecast section: The mapping from predicted number counts to forecasted cosmological constraints does not include an error budget that propagates potential correlated biases in the HI prescriptions (gas cooling, feedback, low-mass halo resolution) across the three suites; the modeling uncertainty is therefore likely underestimated.
Authors: We appreciate the referee's point on the propagation of correlated modeling biases. The three simulations employ substantially different HI modeling frameworks, which already captures a range of uncertainties in gas physics and halo resolution. Nevertheless, we acknowledge that certain systematic effects (e.g., feedback prescriptions or low-mass halo resolution limits) could be partially correlated. In the revised manuscript we will expand the discussion within the cosmological forecasts section to include a qualitative assessment of these potential correlations, explicitly stating that the quoted modeling uncertainty represents a lower bound. We will also add a corresponding caveat to the abstract and conclusions. revision: partial
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Referee: [HOD fitting and clustering section] HOD fitting and clustering section: The HOD fits to low-redshift angular correlation functions are used to quantify sample variance in clustering, yet no goodness-of-fit metrics, parameter posteriors, or covariance information are reported, making it impossible to judge whether the derived variations are robust or dominated by fitting noise.
Authors: We agree that reporting fit quality metrics and parameter information is necessary for readers to assess the robustness of the sample-variance estimates. In the revised manuscript we will augment the HOD section with reduced χ² (or equivalent goodness-of-fit) values for each sub-volume fit, the best-fit HOD parameters together with their uncertainties, and a brief description of the covariance matrix used in the fitting procedure. These additions will allow a clear evaluation of whether the reported clustering variations are dominated by fitting noise. revision: yes
Circularity Check
No significant circularity; predictions extracted from independent external simulations
full rationale
The paper derives its headline number counts, redshift distributions, and SKA-MID cosmological forecasts by directly processing galaxy catalogs from three pre-existing, independent simulation suites (S³-SAX, GAEA, IllustrisTNG). The only fitting step—HOD modeling of low-redshift angular correlation functions—is performed on sub-volumes of those same simulations solely to quantify internal sample variance, not to generate or redefine the primary counts or forecasts. No self-citations, self-definitional loops, fitted inputs renamed as predictions, or ansatzes imported from prior author work appear in the derivation chain. The results remain tied to external simulation outputs rather than reducing to quantities constructed from the paper's own fitted parameters.
Axiom & Free-Parameter Ledger
Forward citations
Cited by 1 Pith paper
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HI Simulations for Cosmology with the SKA Observatory
Overview of HI modeling methods finds consistency in cosmic HI density but systematic differences in HI-halo mass relation shape and redshift evolution.
discussion (0)
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