Correlation between galactic HI and the cosmic microwave background

We revisit the issue of a correlation between the atomic hydrogen gas in our local galaxy and the cosmic microwave background, a detection of which has been claimed in some literature. We cross correlate the 21-cm emission of galactic atomic hydrogen as traced by the Leiden/Argentine/Bonn Galactic Hi survey with the 3-year cosmic microwave background data from the Wilkinson microwave anisotropy probe. We consider a number of angular scales, masks, and Hi velocity slices and find no statistically significant correlation.

Figure 1

The full velocity LAB map (above) and the foreground reduced V-band WMAP map (below), in galactic coordinates and Mollweide projection. The scales are logarithmic in column density and linear in temperature fluctuation.

Figure 2

The cross correlation of the full velocity LAB survey with WMAP V band, outside the Kp2 mask, divided by the power spectra and V-band beam. Also plotted are the 1-sigma contours calculated from 5000 simulations (green thick lines) and from Eq. (2) (red thin lines). The equivalent plots for the Q and W bands are essentially identical to this one.

Figure 3

The cross correlation of the 215 Hi map with the WMAP data, outside the Kp2 mask, over scales $\ell =160–200$ where the ${\chi }^2$ is found to be high at the 99.93% level. To compare results from the different bands [Q (solid line), V (dotted line), W (dashed line)] we have divided out by their respective beams. 1, 2 and 3-sigma results are also displayed (green solid lines).