Thoughts on the Canis Major controversy

Submitted by Kevin Jardine on 24 February, 2010 - 09:45

In 2003, the Observatoire Astronomique de Strasbourg sent out a dramatic press release, Astronomers find nearest galaxy to the Milky Way. They announced a major new discovery, the Canis Major dwarf galaxy, a satellite galaxy in the process of being torn up and absorbed by the Milky Way.

Statements from the press release, as well as content taken from follow-up news stories and the related scientific papers, made their way into major Internet information sites, including Wikipedia, SEDS, David Darling and the Astronomy Picture of the Day.

The difficulty is, however, that a dwarf galaxy is only one possible explanation for the overdensity of M-class giant stars seen towards Canis Major, observed in the 2MASS data and discussed in the original 2004 paper.

The dwarf galaxy hypothesis caused a rapid response and rebuttal. First off the mark was a 2004 letter to Astronomy and Astrophysics by a group of Italian astronomers, attributing the overdensity to more local Milky Way structure (the outer galactic warp).

More detailed objections soon followed. I posted a list of 8 recent papers rejecting the dwarf galaxy hypothesis (or at least concluding that other explanations were more credible) to this Wikipedia discussion page. I think that it's fair to say that a Canis Major dwarf galaxy is currently best described as an interesting hypothesis rather than a confirmed object.

If so, then why do so many Internet-based astronomy information sources misrepresent the dwarf galaxy hypothesis as a confirmed object? Scientists often shake their heads sadly at the mass media distortion and misrepresentation of their work. In this case, however, I can only reluctantly conclude that the distortion and misrepresentation appears to have originated in the 2003 Observatoire Astronomique de Strasbourg press release, which claimed a level of certainty for their conclusions which just did not exist.

I draw three main conclusions from this sad story (and similar ones, for there are others):

  • Never believe press releases, or news stories based upon them, no matter how apparently credible the source.
  • Be very cautious in believing scientific papers trumpeting major new discoveries, at least at first.
  • A year or two after the publication of a paper, take a look at the list of citations conveniently provided by the Astrophysics Data System. This will give you a good idea of how the paper has been received by peers of the scientists publishing the original paper. In the case of the "dwarf galaxy" paper, the list of citations contains 195 references as of today!

I think that these conclusions might be useful for scientists as well as the general public. It is interesting to see that some of the earlier papers citing the dwarf galaxy paper also seem to assume that it is a confirmed object, and it is only more recently that it appears to be considered safer to mention the controversy and describe the dwarf galaxy as a hypothesis.

Lynds bright nebulae

Submitted by Kevin Jardine on 17 February, 2010 - 17:51

I've added the Lynds bright nebulae as an optional overlay to the false colour version of Douglas Finkbeiner's all sky hydrogen-alpha map in the Milky Way Explorer.

At some point soon, I'll add some commentary on the Lynds bright nebulae catalog. In short, it's a large catalog (1125 objects) covering the same area of sky as the BFS and Sharpless catalogs but in more detail. This sounds more interesting than it actually is, because in many cases the Lynds nebulae are just parts of nebulae already in the Sharpless catalog. However, it also contains designations for fainter and smaller nebulae not in the Sharpless catalog, so is useful in some cases.

NASA composite galactic centre image

Submitted by Kevin Jardine on 15 November, 2009 - 16:39

NASA released a new galactic centre image on 10 November that was supposedly constructed from three previous galactic centre images from Hubble, Spitzer and Chandra. I've now put it into the Milky Way Explorer and you can view it here.

Some of the mysteries around a few of the objects in this image have been discussed on the APOD discussion site. At least one of these objects does not appear on any of the three original images so it is a mystery where it came from.

Gamma ray data

Submitted by Kevin Jardine on 10 November, 2009 - 10:00

Gregory Dobler, Douglas Finkbeiner and colleagues have released a set of all-sky gamma ray images based upon the Fermi Large Area Telescope data release back in August. I've put a colour composite of three of these images in the Milky Way Explorer.

Normally I work with the full FITS format data released by astronomers, but in this case, the FITS files use the fairly obscure HEALPix projection which unfortunately is not yet supported by a number of key software tools. For example, Aladin hangs when I try to read these FITS files. So for the moment, I've used the black and white jpeg images also supplied to create the colour composite.

I experimented with a number of data combinations, and ended up using the smoothed versions (with no point sources removed) for these energy ranges: 0.5 to 1 GeV (red), 2 to 5 GeV (green) and 10 to 20 GeV (blue). Higher energy levels up to 300 GeV are available, but the higher energy gamma rays are not particularly constrained to the galactic plane. According to my reading of the accompanying paper, it appears that these higher energy photons are mostly from relatively local objects (within a few thousand parsecs).

The Fermi data is not very high resolution, but nevertheless, it is interesting to see major gamma ray concentrations in the directions of Orion, the Taurus and Ophiuchus dark cloud complexes, and the Large Magellanic Cloud.

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