  We report results from 21-cm intensity maps acquired from the Parkes radio
telescope and cross-correlated with galaxy maps from the 2dF galaxy survey. The
data span the redshift range $0.057<z<0.098$ and cover approximately 1,300
square degrees over two long fields. Cross correlation is detected at a
significance of $5.18\sigma$. The amplitude of the cross-power spectrum is low
relative to the expected dark matter power spectrum, assuming a neutral
hydrogen (HI) bias and mass density equal to measurements from the ALFALFA
survey. The decrement is pronounced and statistically significant at small
scales. At $k\sim1.5$ $ h \mathrm{Mpc^{-1}}$, the cross power spectrum is more
than a factor of 6 lower than expected, with a significance of $14.8\,\sigma$.
This decrement indicates either a lack of clustering of neutral hydrogen (HI),
a small correlation coefficient between optical galaxies and HI, or some
combination of the two. Separating 2dF into red and blue galaxies, we find that
red galaxies are much more weakly correlated with HI on $k\sim1.5$ $h
\mathrm{Mpc^{-1}}$ scales, suggesting that HI is more associated with blue
star-forming galaxies and tends to avoid red galaxies.

Cosmology and Nongalactic Astrophysics