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[0706.1703] Correlation between galactic HI and the Cosmic Microwave Background
Authors:Kate Land, Anze Slosar
Abstract:We revisit the issue of a correlation between the atomic hydrogen gas in our local Galaxy and the Cosmic Microwave Background (CMB), 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 CMB 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.
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Thomas Dent

Joined: 14 Nov 2006
Posts: 28
Affiliation: ITP Heidelberg

PostPosted: June 13 2007  Reply with quote

Seems to be a quick rebuttal of the Verschuur paper previously discussed here.


in contrast with [15] we do not observe
any systematic correlation between the HV, LV, IV, or
FV Hi maps and the CMB. Moreover, we do not observe
a significant correlation between Hi and the CMB in the
area of the sky defined by the Rectangular mask, as just
2 of the 89 maps find some correlation above the 99%
level, and for only one of the ℓ-ranges - consistent with a
chance occurrence.

We do, however, observe some correlation on degree
scales for a number of velocity slices, as seen in the third
row of Table I. The 15 maps that demonstrate a Kp2
masked V-band degree scale correlation above the 95%
level correspond to mean velocities of −405, −345, −325,
−315,−205, 105. . . 155, 215. . . 235, 345 km/s. Since this is
an a-posteriori observation and adjacent LAB slices are
highly correlated, it is impossible to ascertain the sta-
tistical significance of these. Nevertheless, we track the
correlation of 105. . . 155 km/s and 215. . . 235 km/s maps
to the same extended feature at (l, b)  (−50◦,−45◦)
(none of the remaining 15 maps are significant above the
99% level). In Figure 3 we examine the correlation of 215
km/s with the different frequency bands to ascertain if
the signal is due to some kind of contamination from the
presence of this extended Hi feature. However, the signal
remains very consistent between the CMB bands indicat-
ing that there is no obvious foreground contamination on
these scales. The correlation also appears quite random
in nature,

It sounds fairly decisive.

Can someone explain the y-axes on the two graphs? They are slightly beyond my technical level - how do you divide by a beam?

Also, is 'a posteriori' really the right expression for 'deciding what to analyze after eyeballing the data first'?

Bring back the crackpots!
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Kate Land

Joined: 27 Sep 2004
Posts: 29
Affiliation: Oxford University

PostPosted: June 14 2007  Reply with quote

Hi Thomas,
Thanks for the comments. For the Figures we wanted to plot our correlation term X_\ell but in such a way that it didn't depend on units. Therefore we divided out by the power spectrum at each \ell.

Regarding the beam, we took the beam transfer functions b_\ell from WMAP (Legendre transform of the radial beam profiles). These basically tell you how much smoothing you have on each scale, and were also used in our simulations. For the Figures we divided out by this function too so that the results for the Q,V,W bands can be compared, as the different levels of smoothing is removed.

And regarding, 'a posteriori', I think your definition is spot on! I mean, that is what I think the phrase means, i.e. 'after knowledge has been gained through experience'.

Cheers, Kate
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Thomas Dent

Joined: 14 Nov 2006
Posts: 28
Affiliation: ITP Heidelberg

PostPosted: June 15 2007  Reply with quote

OK, after momentary bewilderment I got what ought to have been obvious... this is not the 'Legendre transform' our grandfathers knew. Just CMB jargon for expansion in spherical harmonics.

At the risk of trivializing the question of who is allowed to post here or not, I'm passing on an email I just received. Its author is not, and never has been, a member of CosmoCoffee.


Your paper did not prove Big Bang. If you have sense, you would know that BB theory and
Einstein were the big gamble gunned at almost a century ago when people considered the world
were iceland universe. To save their face, the gamble went such a long way which would be
energized billions of more power than BB itself.

Did you return to normal sense event within one minute at the thought that the gamble might
lose???? Do you fill chlilly at the thought. The recent twenty yrs of observation tell you
that the gamble got big big trouble!!! You want to save your grant, funding??????


Compare this with the writings of unaffiliated CC members... which would you rather have?
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Richard Lieu

Joined: 27 Nov 2005
Posts: 11
Affiliation: University of Alabama, Huntsville

PostPosted: October 06 2007  Reply with quote

Kate: I am in agreement with you.

I performed an extensive study (with B. Z. Jiang, a PhD
student of Prof. S.N. Zhang at Tsinghua, Beijing) using wavelet analysis
of the WMAP and HI data, then cross comparing the number of close associations between the degree-scale wavelet hot spot centroids and HI clouds with that expected from simulated WMAP data where the hot spot
locations are by definition randomized.

The verdict is that we found no statistically significant associations
between the first acoustic peak hot spots and HI. We therefore
cannot support the claim of Verschuur.
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