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

TL;DR

This study analyzes low-redshift 21-cm intensity maps cross-correlated with galaxy surveys, revealing a significant lack of expected clustering of neutral hydrogen, especially around red galaxies, indicating a weaker than anticipated HI-galaxy correlation.

Contribution

It provides the first detailed measurement of HI clustering at low redshift through cross-correlation, highlighting a discrepancy with theoretical expectations and previous surveys.

Findings

Cross correlation detected at 5.18σ significance.

Cross power spectrum is over 6 times lower than expected at certain scales.

Red galaxies show weaker correlation with HI than blue galaxies.

Abstract

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.