New Results Confirm Dark Galaxy Existence

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.

Negative colour optical image of VIRGOHI 21 with 
neutral hydrogen contours superimposed

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

Colour optical image of VIRGOHI 21 with neutral
hydrogen contours superimposed

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.

Radio view of VIRGOHI 21 and surrounds

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:

  1. Arecibo Observatory is part of the National Astronomy and Ionosphere Center, which is operated by Cornell University under a cooperative agreement with the National Science Foundation
  2. The Westerbork Synthesis Radio Telescope is operated by ASTRON (Netherlands Foundation for Research in Astronomy) with support from the Netherlands Foundation for Scientific Research (NWO).
  3. Observations made with the NASA/ESA Hubble Space Telescope were obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
  4. The scientific paper “21-cm synthesis observations of VIRGOHI 21 – a possible dark galaxy in the Virgo Cluster” has been accepted for publication in the Astrophysical Journal, a pre-print is available at http://arxiv.org/abs/0706.1586. Authors are Robert Minchin (Arecibo Observatory), Jon Davies (Cardiff University), Mike Disney (Cardiff University), Marco Grossi (Arcetri Observatory), Sabina Sabatini (Rome Observatory), Peter Boyce (Cardiff University), Diego Garcia (University of Bonn), Chris Impey (Steward Observatory), Christine Jordan (Jodrell Bank Observatory), Robert Lang (Cardiff University), Andrew Marble (Steward Observatory), Sarah Roberts (Cardiff University and Wim van Driel (Paris Observatory).
  5. This paper is a follow-up to earlier papers Davies, Minchin, Sabatini, et al. 2004 (Monthtly Notices of the Royal Astronomical Society, vol. 349, p. 922) which presented the results of the VIRGOHI survey carried out with the Lovell telescope at Jodrell Bank and first identified VIRGOHI 21, and Minchin, Davies, Disney, et al. 2005 (Astrophysical Journal, vol. 622, p. L21) which presented observations of VIRGOHI 21 with the 305-m Arecibo telescope and with the Isaac Newton Telescope and first suggested it could be a dark galaxy.
  6. For more on the neutral hydrogen surveys being carried out at Arecibo, please see