GALAXIES
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    dark galaxy

    Region of space occupied by suspected dark matter galaxy VIRGOHI21
    A galaxy composed almost exclusively of dark matter and in which virtually no star star formation has taken place. It is thought that dark galaxies should be very common, particularly since theories of large-scale structure work much better if the existence of these dark matter galaxies is assumed. Also, surveys have revealed fewer bright galaxies in the universe than expected, suggesting that many galaxies, for reasons yet to be understood, fail to trigger star birth.


    VIRGOHI21

    In 2005 astronomers in the United Kingdom announced the discovery of what may be the first dark galaxy, VIRGOHI21, lying about 50 million light-years away in the Virgo Cluster. Radio observations of the rotation of hydrogen gas in VIRGOHI21 revealed there must be at least 100 times as much dark matter as hydrogen in the galaxy and that its total mass is about one-tenth that of the Milky Way.

    NGC 4254
    Neutral hydrogen gas streams extends from NGC 4254 (top left) to the dark galaxy VIRGOH121 (center right) in this radio image. NGC 4262 is at bottom left. Image credit: R. Minchin/Arecibo Observatory/Cardiff U/Isaac Newton Telescope/WSRT)
    According to a counter theory, VIRGOHI21 may simply be the result of a close encounter between two other galaxies – NGC 4254, which lies nearby, and NGC 4262, which ago hurtled past NGC 4254 at 900 km/s millions of years. A prominent arm of stars stretching out from NGC 4254 and bending toward VIRGOHI21 lends some support to this idea. However, if VIRGOHI21 did originate in material pulled out of NGC 4254 then it ought to contain stars in addition to its several hundred million solar masses of hydrogen. In 2007, observations of a patch of sky 50,000 by 50,000 light-years across, centered on the hydrogen cloud's position, found only 119 red giant stars, which is only the number expected in a typical region of the same size in intergalactic space. One possibility is that NGC 4254's long stellar arm has been drawn out by the gravity of VIRGOHI21 itself.

    If VIRGOHI21 really is a dark galaxy it poses astronomers a challenge. Its ratio of dark matter to normal matter appears to be at least 10 times greater than the ratio found in other galaxies.


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