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06-11-2007
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The dark galaxy that isn't

Submitter: Tom Oosterloo
Description: About 2 years ago, the astronomical community was stirred up by a claim that an optically "dark galaxy" was discovered. Observations done with the Jodrell Bank and Arecibo telescopes had revealed a large, extragalactic cloud of neutral hydrogen in a location where no stars were seen, even on deep optical exposures. If this interpretation of the observations would be correct, it would be the first detection of such an object and could hint at the existence of an entirely new and unknown class of galaxies. Needless to say that this would have major implications for our understanding of galaxies and galaxy formation.

For many years, astronomers have discussed the possibility that galaxies without stars could exist. The standard picture is that, when a galaxy forms, things start by gas falling into the galaxy and a little while later stars form from this gas. However, some people think that, under certain extreme conditions, no stars manage to form and the galaxy remains (optically) dark and is only detectable through the hydrogen gas.

Unfortunately, recent observations with the WSRT analysed by Pierre-Alain Duc and Frederic Bournaud from Saclay in France have spoilt the fun. Because the WSRT is a larger telescope, it can make much sharper images than Jodrell Bank or Arecibo. The WSRT data now show that the gas cloud is part of a long arm of gas coming from a galaxy called NGC 4254. Such long arms have been seen before in many galaxies and are caused by the tidal effects of another galaxy flying by, tearing off some of the gas from, in this case, NGC 4254. So no "dark galaxy", just some wreckage left over from a brief, cosmic encounter.

On the left, the Arecibo spectra are shown together with a map of where these spectra were taken. On the right the WSRT image of the neutral hydrogen is shown (blue), on top of an optical image, showing the long tail of gas coming from NGC 4254 (see http://xxx.sissa.it/abs/0710.3867 for the full story)
Copyright: Pierre-Alain Duc and Frederic Bournaud (CEA- Saclay)
 
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