  Much of our current understanding of neutral, atomic gas in galaxies comes
from radio observations of the nearby Universe. Until the next generation of
instruments allow us to push to much higher redshifts, we must rely mostly upon
theoretical models of galaxy formation to provide us with key insights into the
likely cosmic evolution of HI in the Universe, and its links to molecular
clouds and star formation. In this work, we present a new set of methods to
convert mock galaxy catalogues into synthetic data cubes containing model
galaxies with realistic spatial and spectral HI distributions over large
cosmological volumes. Such synthetic data products can be used to guide
observing and data handling/analysis strategies for forthcoming HI galaxy
surveys. As a demonstration of the potential use of our simulated products we
use them to conduct several mock HI stacking experiments for both low and
high-redshift galaxy samples. The stacked spectra can be accurately decomposed
into contributions from target and non-target galaxies, revealing in all
co-added spectra large fractions of contaminant mass due to source confusion.
Our results are consistent with similar estimates extrapolated from z=0
observational data. The amount of confused mass in a stacked spectrum grows
almost linearly with the size of the observational beam, suggesting potential
over-estimates of Omega_HI by some recent HI stacking experiments. Our
simulations will allow the study of subtle redshift-dependent effects in future
stacking analyses.
