Simulations of the 21cm emission line for upcoming large-scale HI galaxy surveys

TL;DR

This paper introduces a modular simulation framework for modeling the 21cm emission line in large-scale HI galaxy surveys, aiding interpretation of upcoming SKA observations and cosmological studies.

Contribution

It develops a flexible, validated simulation framework based on semi-analytical models for predicting HI observables in future large-scale surveys.

Findings

Framework is consistent with local Universe observations.

Predicts redshift distributions for future SKA surveys.

Reproduces statistical properties of HI-selected galaxies.

Abstract

Upcoming galaxy surveys with the SKA Observatory will detect neutral hydrogen (HI) across unprecedented volumes, and their scientific return will crucially depend on predictive models for HI observables. In this work, we present a framework to simulate the neutral hydrogen 21cm emission line in such large-scale HI galaxy surveys. This framework is developed as a modular layer that builds on semi-analytical models. In particular we use as bases the Galaxy Evolution and Assembly (GAEA) and L-Galaxies semi-analytical models, coupled to merger trees from the Millennium Simulation suite. We validate our framework against local Universe observations, demonstrating consistency with velocity functions, and generalised Tully-Fisher relations. Predictions based on GAEA and L-Galaxies exhibit mutual consistency despite the distinct underlying physical prescriptions. We construct mock galaxy catalogues that incorporate forward-modelled selection functions, inclination effects, and redshift broadening, reproducing the statistical distributions of HI-selected galaxies in the ALFALFA survey. Finally, we present redshift distribution forecasts for future SKA Observatory HI galaxy surveys. This framework offers a flexible tool for interpreting upcoming large-scale radio surveys and studying HI line observables as cosmological probes.