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15-01-2007
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Dark gas clouds exist?

Submitter: Tom Oosterloo
Description: Some models of galaxy formation predict the existence of "dark galaxies": small clouds of neutral gas, floating in intergalactic space, in which no stars have formed. Several candidates for such clouds have been found near our own Galaxy and perhaps also near our neighbours M31 and M33 (e.g. by Robert Braun). If these galactic clouds would really be dark galaxies, we would expect them to exist also near other galaxies. Many searches for such clouds in other galaxies have been made, but none were found, posing a problem for the interpretation of the clouds seen in our Galaxy. However, ultra-deep WSRT observations of the nearby galaxy NGC 891 now do show a few blobs of gas that are very good candidates for being such dark galaxies, suggesting that they really exist.

The figure shows a channel and a spectrum showing such a blob, containg about one million solar masses of gas. The rotation of the blob is opposite to that of the gas in the disk of NGC 891 and its halo, suggesting that it is not a "funny" cloud belonging to NGC 891, but that it is a separate entity, possibly a dark gas cloud.

Copyright: astron
 
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