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

Abstract

We report results from 21-cm intensity maps acquired from the Parkes radio telescope and cross-correlatedwith galaxy maps from the 2dF galaxy survey. The data span the redshift range 0.057 <z <0.098 and cover approximately 1300 deg(2) over two long fields. Cross-correlation is detected at a significance of 5.7 sigma. The amplitude of the cross-power spectrum is low relative to the expected dark matter power spectrum, assuming a neutral hydrogen (H-I) bias andmass density equal to measurements from the ALFALFA survey. The decrement is pronounced and statistically significant at small scales. At k similar to 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 sigma. 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 similar to 1.5 h Mpc(-1) scales, suggesting that H-I is more associated with blue star-forming galaxies and tends to avoid red galaxies.