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

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.

Figures (11)

• Figure 1: Example of the molecular hydrogen fraction profile $f_\mathrm{mol}(r)$ for a GAEA model galaxy as a function of redshift. Upper panel:$f_\mathrm{mol}(r)$ as output by the GAEA delucia_2024b SAM, where the hydrogen partition is explicitly computed with the blitz_2006 prescription at every time step and saved on the fly for a predefined number of radial bins. Lower panel:$f_\mathrm{mol}(r)$ re-evaluated on an arbitrarily smoother scale in post-processing with the blitz_2006 prescription at each redshift snapshot.
• Figure 2: H i 21cm line profile of galaxy NGC 7819 measured in the ALFALFA survey haynes_2018 (magenta curve), and a manual fit of our model as an example (black curve), displaying the model line width parameters, namely $w_\mathrm{peak}$, $w_{50}$ and $w_{20}$.
• Figure 3: H i mass functions on $z=0$ snapshot outputs. Blue lines correspond to GAEA outputs, and green lines to L-Galaxies outputs. Results are displayed for outputs from the MSI (solid lines), and MSII (dashed lines). These are compared to observational data derived from the HIPASS and ALFALFA surveys.
• Figure 4: H i velocity width ($w_{50}$) functions on $z=0$ snapshot outputs. Blue lines correspond to GAEA outputs, and green lines to L-Galaxies outputs. Results are displayed for outputs from the MSI (solid lines), and MSII (dashed lines). These are compared to observational data derived from the (40% and 100% releases of the) ALFALFA survey.
• Figure 5: Generalised Tully-Fisher relations between edge-on peak line widths ($w_\mathrm{peak}$) and stellar, baryonic, and H i masses on $z=0$ snapshot outputs. Contours show the 67% and 95% confidence levels of sample distributions for each simulation, namely the GAEA (blue) MSI (solid), MSII (dashed), and L-Galaxies (green) MSI (solid), MSII (dashed). Magenta contours present distributions derived from CosmicFlows 4 baryonic Tully-Fisher catalogue's data kourkchi_2022.