Unveiling the atomic hydrogen-halo mass relation via spectral stacking

TL;DR

This study uses simulations to evaluate how well different observational techniques can recover the true HI-halo mass relation, revealing systematic biases and limitations in current methods due to contamination and measurement challenges.

Contribution

It demonstrates that existing stacking methods and halo mass estimates introduce biases, and highlights the potential of deep spectroscopic surveys to improve HI mass measurements in galaxy groups.

Findings

Systematic effects in halo mass estimates distort the inferred HIHM relation.

Contamination from interloping galaxies affects the measured HI mass, especially at Mvir~10^(12-12.5)Msol.

Deep spectroscopic surveys like WAVES can significantly improve HI mass recovery.

Abstract

Measuring the HI-halo mass scaling relation (HIHM) is fundamental to understanding the role of HI in galaxy formation and its connection to structure formation. While direct measurements of the HI mass in haloes are possible using HI-spectral stacking, the reported shape of the relation depends on the techniques used to measure it (e.g. monotonically increasing with mass versus flat, mass-independent). Using a simulated HI and optical survey produced with the SHARK semi-analytic galaxy formation model, we investigate how well different observational techniques can recover the intrinsic, theoretically predicted, HIHM relation. We run a galaxy group finder and mimic the HI stacking procedure adopted by different surveys and find we can reproduce their observationally derived HIHM relation. However, none of the adopted techniques recover the underlying HIHM relation predicted by the simulation. We find that systematic effects in halo mass estimates of galaxy groups modify the inferred shape of the HIHM relation from the intrinsic one in the simulation, while contamination by interloping galaxies, not associated with the groups, contribute to the inferred HI mass of a halo mass bin, when using large velocity windows for stacking. The effect of contamination is maximal at Mvir~10^(12-12.5)Msol. Stacking methods based on summing the HI emission spectra to infer the mean HI mass of galaxies of different properties belonging to a group suffer minimal contamination but are strongly limited by the use of optical counterparts, which miss the contribution of dwarf galaxies. Deep spectroscopic surveys will provide significant improvements by going deeper while maintaining high spectroscopic completeness; for example, the WAVES survey will recover ~52% of the total HI mass of the groups with Mvir~10^(14)Msol compared to ~21% in GAMA.