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[astro-ph/0606538] Point source power in three-year Wilkinson Microwave Anisotropy Probe data
 
Authors:K. M. Huffenberger, H. K. Eriksen, F. K. Hansen
Abstract:Using a set of multifrequency cross-spectra computed from the three year WMAP sky maps, we fit for the unresolved point source contribution. For a white noise power spectrum, we find a Q-band amplitude of A = 0.011 +/- 0.001 muK^2 sr (antenna temperature), significantly smaller than the value of 0.017 +/- 0.002 muK^2 sr used to correct the spectra in the WMAP release. Modifying the point source correction in this way largely resolves the discrepancy Eriksen et al. (2006) found between the WMAP V- and W-band power spectra. Correcting the co-added WMAP spectrum for both the low-l power excess due to residual foregrounds and the high-l power deficit due to over-subtracted point sources, we find that the net effect in terms of cosmological parameters is a ~ 0.7 sigma shift in n_s to larger values: For the combination of WMAP, BOOMERanG and Acbar data, we find n_s = 0.969 +/- 0.016, lowering the significance of n_s not equal to 1 from ~ 2.7 sigma to ~ 2.0 sigma.
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Anze Slosar



Joined: 24 Sep 2004
Posts: 205
Affiliation: Brookhaven National Laboratory

PostPosted: July 06 2006  Reply with quote

This is a quite interesting paper - I hoped that someone else will come up with discussion about it. They refit the residual point source spectrum in WMAP3 and show that ns moves by about 0.7 sigma: in general I wouldn't care about sub 1-sigma shifts, but this time it moves ns=1 from the tail of the distribution into a completely ok zone.
Few comments though:

  • I don't like 100−200 bin: it makes χ2 horrible and it cannot possibly be a noise fluctuation, unless errors are badly underestimated. I think it is also quite difficult to come up with something that changes things on such large scales.

  • β=−2 - is assuming this a good assumption? What happens if you put some sensible prior around β (say 2±0.5)? I would expect that faint sources are not the same population as sources so bright, that you can actually cut them out....

  • Poisson distribution: surely, there are some LSS effects in the unresolved sources; what authors do (I think) is to assume white noise for unresolved sources: instead you could do something like Limber between z=0 and 5, fit the amplitude on large scales and extrapolate to smaller scales - surely this would have been better than C = const?

  • Just as an asside - could you detected them via non-Guassianity? If there are not too many sources, you should seem them in bispectrum, I guess?
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Hans Kristian Eriksen



Joined: 25 Sep 2004
Posts: 58
Affiliation: ITA, University of Oslo

PostPosted: July 06 2006  Reply with quote

Anze Slosar wrote:

I don't like 100−200 bin: it makes χ2 horrible and it cannot possibly be a noise fluctuation, unless errors are badly underestimated. I think it is also quite difficult to come up with something that changes things on such large scales.


If it's any comfort, we don't like that bin either :-)

My best guess is that it's residual foregrounds leaking in from the Q-band. Diffuse foregrounds still have a significant impact at l~100, and the Q-band is not clean by any means. But one never knows. Another thing that may be worth checking out is the fact that the maps are cleaned with (smoothed) K-Ka as one of three templates. These are of course noisy, and since this noise contribution is the same for all channels in the yearly maps, it's not killed by the cross-correlation estimator. Don't know the magnitude of this effect yet, though..

Quote:

β=−2 - is assuming this a good assumption? What happens if you put some sensible prior around β (say 2±0.5)? I would expect that faint sources are not the same population as sources so bright, that you can actually cut them out....


Certainly, the fact that WMAP (and we) use the same spectral index for both the resolved and the unresolved point sources is a quite bold assumption. I think the best justification for doing so is that it seems to work reasonably OK. But of course, there is still a ~2σ difference between the V and W-bands, so there may be something there. As always, more work is needed..

Quote:

Poisson distribution: surely, there are some LSS effects in the unresolved sources; what authors do (I think) is to assume white noise for unresolved sources: instead you could do something like Limber between z=0 and 5, fit the amplitude on large scales and extrapolate to smaller scales - surely this would have been better than C = const?


Right. The justification for using a flat spectrum is simply that CMB experiments are rather insensitive, and therefore only pick up the brightest sources. In other words, the point sources are very sparsely sampled. And that pushes the effective spectrum towards a flat "white noise" spectrum. But in principle, there may certainly be clustering effects present, yes.


I think the main conclusion from all of this work is that current parameter constraints shouldn't be taken too literally, in the sense that 3σ detections really are 3σ detections. There is a reason why particle physicists operate with a 5σ criterion, and that's precisely because of unknown systematics. Of course, we like to think that CMB observations are both cleaner and simpler, but it's still a good idea to have these issues in mind, I think.
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