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13th June 2007
Confirmation that a mysterious cloud of hydrogen in the Virgo Cluster is a dark galaxy has been published by a team of astronomers
New evidence from NASA's Hubble Space Telescope and the Westerbork Synthesis Radio Telescope in the Netherlands has been used to rule out all but one of the ideas put forward to explain the existence of VIRGOHI 21, an intergalactic gas cloud 50 million light-years from the Earth. An international team of astronomers found that it was rotating like an ordinary galaxy but without any starlight shining out, making it a coveted dark galaxy. The lead author on the paper, Dr Robert Minchin of Arecibo Observatory, commented, “Not even the power of Hubble has been able to see any stars in it.”
But this is not all that the new data reveal. The results also answer a long-standing puzzle about another nearby galaxy, known as NGC 4254. This is a strangely lopsided galaxy, with one spiral arm much larger than the others. This is usually caused by the influence of a neighbouring galaxy, but no such galaxy could be found using optical telescopes As Professor Mike Disney of Cardiff University explains, “This is the smoking gun evidence – VIRGOHI 21 has been caught in the act of pulling gas from NGC 4254. It must be a massive galaxy.
The team compared the new observations to the predictions made by alternative explanations, such as that another galaxy could have passed close to NGC 4254, ripping off the gas to form VIRGOHI 21. None of the other theories were able to match the results of the study. Dr Jon Davies of Cardiff University said, “It was always going to be difficult convincing ourselves, let alone convincing the astronomical community, that something invisible really does exist. But the evidence here is so detailed that we've been able to decisively rule out every alternative idea.”
Dark galaxies are of great interest to astronomers as they are evidence for the existence of dark matter. Since the 1970s, dark matter has been invoked to explain how galaxies can spin at the rate they do without coming apart, but there are alternative explanations such as a subtle change in how the law of gravity works. In most galaxies, dark matter makes up around ten time more of the mass than the gas and stars, but in a dark galaxy there are no stars whatsoever and even the gas is present in much smaller quantities than normal. Only Dark Matter can account for the existence of such a system.
Are there more such dark galaxies out there? What can we learn by studying them? Ongoing neutral-hydrogen surveys with the 305-m Arecibo telescope, the largest radio telescope in the world, hope to answer these questions soon.
The dark galaxy VIRGOHI 21 has no starlight, but radio waves
from neutral hydrogen gas betray its existence. The contours superimposed on
this negative colour optical image from the Isaac Newton Telescope in the
Canary Islands indicate how much gas was detected with the Westerbork Synthesis
Radio Telescope.
Credit: R. Minchin / Arecibo Observatory / Cardiff University /
Isaac Newton Telescope / Westerbork Synthesis Radio Telescope
The dark galaxy VIRGOHI 21 has no starlight, but radio waves
from neutral hydrogen gas betray its existence. The contours superimposed on
this colour optical image from the Isaac Newton Telescope in the Canary Islands
indicate how much gas was detected with the Westerbork Synthesis Radio
Telescope.
Credit: R. Minchin / Arecibo Observatory / Cardiff University / Isaac Newton Telescope / Westerbork Synthesis Radio Telescope.
Neutral hydrogen gas streams between NGC 4254 (top left) and
the dark galaxy VIRGOH1 21 (centre right) in this image made from radio
telescope observations at a wavelength of 21 centimetres with the Westerbork
Synthesis Radio Telescope. This interaction explains the mystery of NGC 4254's
peculiar lopsided shape. To the bottom left, a ring of gas can
be seen around the galaxy NGC 4262.
R. Minchin / Arecibo Observatory / Cardiff University /
Westerbork Synthesis Radio Telescope.
Notes for Editors: