On the Uncertainties of Results Derived from HI Spectral Line Stacking Experiments

We present the results of a set of mock experiments aimed at quantifying the accuracy of results derived from HI spectral line stacking experiments. We focus on the effects of spatial and spectral aperture sizes and redshift uncertainties on co-added HI spectra, and by implication on the usefulness of results from HI line spectral stacking experiments. Using large spatial apertures to extract constituent galaxy spectra yields co-added spectra with high levels of contamination and with relatively low signal-to-noise ratios. These properties are also affected by the size of the spectral aperture as well as HI redshift uncertainties of galaxies. When redshift uncertainties are high, S/N decreases while the contamination level remains roughly constant. Using small spectral apertures in the presence of large HI redshift uncertainties can yield significant decreases in S/N without the expected decrease in amount of contaminant flux. Our simulations show that a co-added spectrum rarely yields an accurate measure of the total HI mass of a galaxy sample. Total mass is generally over/under-estimated for large/small spatial apertures, regardless of spectral aperture size. Our findings strongly suggest that any co-added HI galaxy spectrum needs to be fully modelled in the ways presented in this paper in order to apply accurate corrections for flux contamination and derive realistic uncertainties in total HI galaxy mass. Failing to do so will result in unreliable inferences of galaxy and cosmological parameters.