  Intensity mapping of the neutral hydrogen (HI) is a new observational tool
that can be used to efficiently map the large-scale structure of the Universe
over wide redshift ranges. The power spectrum of the intensity maps contains
cosmological information on the matter distribution and probes galaxy evolution
by tracing the HI content of galaxies at different redshifts and the
scale-dependence of HI clustering. The cross-correlation of intensity maps with
galaxy surveys is a robust measure of the power spectrum which diminishes
systematics caused by instrumental effects and foreground removal. We examine
the cross-correlation signature at redshift z=0.9 using a variant of the
semi-analytical galaxy formation model SAGE (Croton et al. 2016) applied to the
Millennium simulation in order to model the HI gas of galaxies as well as their
optical magnitudes based on their star-formation history. We determine the
clustering of the cross-correlation power for different types of galaxies
determined by their colours, acting as a proxy for their star-formation
activity. We find that the cross-correlation coefficient for red quiescent
galaxies falls off more quickly on smaller scales k>0.2h/Mpc than for blue
star-forming galaxies. Additionally, we create a mock catalogue of highly
star-forming galaxies using a selection function to mimic the WiggleZ survey,
and use this to predict existing and future cross-correlation measurements of
the GBT and Parkes telescope. We find that the cross-power of highly
star-forming galaxies shows a higher clustering on small scales than any other
galaxy type and that this significantly alters the power spectrum shape on
scales k>0.2h/Mpc. We show that the cross-correlation coefficient is not
negligible when interpreting the cosmological cross-power spectrum. On the
other hand, it contains information about the HI content of the optically
selected galaxies.
