A squashed galaxy
Published on Sep 20, 2007 at 7:20 pm.
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Filed under galaxies.
The Milky Way Galaxy is not all alone. It has a group of satellites that are associated with it. These satellites are smaller galaxies. With the exception of the two largest satellites, the Magellanic Clouds that I blogged about yesterday, the rest of the satellite galaxies are very small compared with the Milky Way. These tiny galaxies are a collection of not just stars, but also dark matter. In fact, the stars in these dwarfs are moving about in a swarm of dark matter. This tends to cause these tiny galaxies to appear as a roughly spherical shape. In fact, in appearance, they look very much like large globular clusters. A few, though, are distorted by tidal interactions with the Milky Way. The Sagittarius Dwarf, for example, is being torn apart. Other rather nearby dwarf satellite galaxies are also somewhat distorted. But, the more distant ones are far enough away from the Milky Way that tidal forces don’t really distort them too much. That is, they are not distorted except for possibly one exception: a newly discovered dwarf galaxy in Hercules, on of five such dwarf galaxies reported by Belokurov, et al, in a paper in the Astrophysical Journal (the preprint is here.) Now, in a paper by Coleman, et al, this Hercules Dwarf appears to be rather squashed, and not at all spherical.
Belokurov did report that the Hercules dwarf had an “extended morphology,” but Coleman’s team report that this dwarf is most definitely elongated, and not just in appearance. The Hercules dwarf galaxy is about 450,000 lightyears away. At that distance, it should be roughly spherical. Tidal forces should not have disrupted it. There does not seem to be anything else that we’ve seen yet nearby that could have done this to it. In fact, this is so unusual that Coleman says in a press release from the Large Binocular Telescope Observatory that this is the only galaxy out of millions studied that is like this. Well, I think that perhaps his numbers are off, but I will grant that it is a very unusual finding.
Coleman’s interpretation of these findings is that the Hercules dwarf may have an extremely elliptical orbit. If so, then we may have just been catching it at the apapsis of its orbit (the farthest out that it gets). If so, then the periapsis (the nearest to the Milky Way that it gets) may be so close to the Milky Way itself that tidal forces have distorted the Hercules dwarf, and it has just remained distorted after its earlier pass nearby.
Sky and Telescope reports that they will have an article about this discovery in the December issue of their magazine. So, keep a look out for it there and in other sources. This should be an interesting thing to read about, and it will be a lot easier to read there than to wade through the research papers that I referenced above at the arxiv preprint site.
-Astroprof
Image Credit: Coleman, et al, ApJ Letters
