Low-amplitude clustering in low-redshift 21-cm intensity maps cross-correlated with 2dF galaxy densities

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 1300 deg² over two long fields. Cross-correlation is detected at a significance of 5.7 σ. The amplitude of the cross-power spectrum is low relative to the expected dark matter power spectrum, assuming a neutral hydrogen (H I) bias and mass density equal to measurements from the ALFALFA survey. The decrement is pronounced and statistically significant at small scales. At k ∼ 1.5 h Mpc^-1, the cross-power spectrum is more than a factor of 6 lower than expected, with a significance of 15.3 σ. This decrement indicates a lack of clustering of neutral hydrogen (H I), a small correlation coefficient between optical galaxies and H I, or some combination of the two. Separating 2dF into red and blue galaxies, we find that red galaxies are much more weakly correlated with H I on k ∼ 1.5 h Mpc^-1 scales, suggesting that H I is more associated with blue star-forming galaxies and tends to avoid red galaxies.