Ecliptic Anomalies

Once a week the small band of cosmologists at Cardiff University have a little discussion group during which we look at an interesting and topical subject. Today my PhD student Rockhee chose an interesting paper by Diego et al entitled “WMAP anomalous signal in the ecliptic plane”. I thought I’d mention it here because it relates to an ongoing theme of mine, and I’ll refrain from commenting on the poor grammatical construction of the title.

The WMAP referred to is of course the Wilkinson Microwave Anisotropy Probe and I’ve blogged before about the tantalising evidence it suggests of some departures from the standard cosmological theory. These authors do something very simple and the result is extremely interesting.

In order to isolate the cosmic microwave background from foreground radiation produced in our own Galaxy, the WMAP satellite is equipped with receivers working at different frequencies. Galactic dust and free-free emission dominate the microwave sky temperature at high frequencies and Galactic synchotron takes over at low frequencies. The cosmic microwave background has the same temperature at all frequencies (i.e. it has a thermal spectrum) so it should be what’s left when the frequency-dependent bits are cleaned out.

What Diego et al. did was to make a map by combining the cleaned sky maps obtained at different frequencies obtained by WMAP in such a way as to try to eliminate the thermal (CMB) component. What is left when this is done should be just residual noise, as it should contain neither known foreground or CMB. The map they get is shown here.ecliptic

What is interesting is that the residual map doesn’t look like noise that is uniformly distributed over the sky: there are two distinct peaks close to the Ecliptic plane delineated by the black tramlines. Why the residuals look like this is a mystery. The peaks are both very far from the Galactic plane so it doesn’t look like they are produced by Galactic foregrounds.

One suggestion is that the anomalous signal is like an infra-red extension of the Zodiacal light (which is produced inside the Solar System and therefore is too local to be confined to the Galactic plane). The authors show, however, that a straightforward extrapolation of the known Zodiacal emission (primarily measured by the IRAS satellite) does not account for the signal seen in WMAP. If this is the explanation, then, there has to be a new source of Zodiacal emission that is not seen by IRAS but kicks in at WMAP frequencies.

Another possibility is that it is extragalactic. This is difficult to exclude, but is disfavoured in my mind because there is no a priori reason why it should be concentrated in the Ecliptic plane. Coincidences like this make me a bit uncomfortable. Some turn out to be real coincidences, but more often than not they are clues to something important. Agatha Christie would have agreed:

“Any coincidence,” said Miss Marple to herself, “is always worth noting. You can throw it away later if it is only a coincidence.”

On the other hand, the dipole asymmetry of the CMB (thought to be caused by our motion through a frame in which it is isotropic) is also lined up in roughly the same direction:

The dipole has a hot region and a cold region in places where the residual map has two hot regions and anyway it’s also a very large scale feature so the chances of it lining up by accident with the ecliptic plane to the accuracy seen is actually not small. Coincidences definitely do happen, and the rougher they are the more commonly they occur.

Obviously, I don’t know what’s going on, but  I will mention another explanation that might fit. As I have already blogged, the WMAP satellite scans the sky in a way that is oriented exactly at right angles to the Ecliptic plane. If there is an as yet unknown systematic error in the WMAP measurements, which is related in some way to the motion of the satellite, it could at least in principle produce an effect with a definite orientation with respect to the Ecliptic.

The only way we can rule out this (admittedly rather dull) explanation is by making a map using a different experiment. It’s good, then, that the Planck satellite is going to be launched in only a few weeks’ time (April 16th 2009). Fingers crossed that we can solve this riddle soon.

10 Responses to “Ecliptic Anomalies”

  1. Anton Garrett Says:

    I don’t suppose the satellite could be tilted a bit and enough observations taken to see if the effect shifts too?


  2. Unfortunately it’s not steerable. It’s just spinning autonomously after being set going when it was deployed.

    It could be tilted if we could persuade the Russians to crash something into it, but unfortunately it’s also at L2 which is way too far for ordinary satellites…

  3. Anton Garrett Says:

    Any chance of searching the satellite’s calibration data for evidence of an anomaly?

  4. Just Learning Says:

    Since we really don’t have a good model of dark matter, if there is some DM concentrated locally in the ecliptic, how would we filter that out?

  5. Just Learning Says:

    To make my point more clear, although dark matter is generally defined by not interacting via EM spectrum, there is an whole lot of the stuff out there and it seems it is likely to be colliding and decaying, and some of the products must appear as ordinary matter and photons.

  6. telescoper Says:

    If the dark matter decays we would expect the products to be rather more energetic than this, emitting in gamma rays rather than microwaves.

    But whether or not it is decaying, there is no reason for dark matter (which is assumed to be essentially collisionless) to be concentrated in the ecliptic plane. Since it can’t radiate directly and is collisionless it can’t form very flattened structures lin the way that baryonic gas does in order to make galaxy disks or even our solar system.

  7. Just Learning Says:

    I certainly am not familiar enough with all the relevant literature to make any claims contrary to what you say, and I agree with you that we would expect a high energy signature from any decays, however a quick search led me to two papers I thought were interesting enough to look at.

  8. telescoper Says:

    I’m aware of those papers; there is nothing in them that suggests an effect that aligns with the ecliptic plane. Although it remains possible that dark matter may be somewhat clumpy, any alignment with our solar system would just be accidental.

  9. Anton,

    In principle yes one could look in the calibration data, but only the final maps are in the public domain. The WMAP team has all the time-ordered data still, though, and they say there’s no evidence of systematics in it. But absence of evidence isn’t the same as evidence of absence…


  10. […] temperature fluctuations in the cosmic microwave background (CMB). See my other posts here, here, here, here and here for related […]

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