image: Dark Galaxy VIRGOHI 21 has no starlight but radio waves from neutral hydrogen betray its existence. The contours superimposed on this optical image indicate how much gas was detected. This material was presented to the American Astronomical Society meeting in Washington, D. C. on January 12, 2006.

New evidence that VIRGOHI 21, a mysterious cloud of hydrogen in the Virgo Cluster 50 million light-years from the Earth, is a Dark Galaxy, emitting no starlight, was presented today at the American Astronomical Society meeting in Washington, D.C. by an international team led by astronomers from the National Science Foundation’s Arecibo Observatory and from Cardiff University in the United Kingdom. Their results not only indicate the presence of a dark galaxy but also explain the long-standing mystery of its strangely stretched neighbour.

The new observations, made with the Westerbork Synthesis Radio Telescope in the Netherlands, show that the hydrogen gas in VIRGOHI 21 appears to be rotating, implying a dark galaxy with over ten billion times the mass of the Sun. Only one percent of this mass has been detected as neutral hydrogen — the rest appears to be dark matter.

But this is not all that the new data reveal. The results may also solve a long-standing puzzle about another nearby galaxy. NGC 4254 is lopsided, with one spiral arm much larger than the rest. This is usually caused by the influence of a companion galaxy, but none could be found until now — the team thinks VIRGOHI 21 is the culprit. Dr. Robert Minchin of Arecibo Observatory says: “The Dark Galaxy theory explains both the observations of VIRGOHI 21 and the mystery of NGC 4254.”

Gas from NGC 4254 is being torn away by the dark galaxy, forming a temporary link between the two and stretching the arm of the spiral galaxy. As the VIRGOH1 21 moves on, the two will separate and NGC 4254’s unusual arm will relax back to match its partner.

The team have looked at many other possible explanations, but have found that only the Dark Galaxy theory can explain all of the observations. As Professor Mike Disney of Cardiff University puts it, “The new observations make it even harder to escape the conclusion that VIRGOHI 21 is a Dark Galaxy.”

The team hope that this will be the first of many such finds. “We’re going to be searching for more Dark Galaxies with the new ALFA instrument at Arecibo Observatory,” explains Dr. Jon Davies of Cardiff University. “We hope to find many more over the next few years — this is a very exciting time!”

NOTES FOR EDITORS:

Results published in 2005 first suggested that VIRGOH1 21 is a dark matter galaxy. For more information see press release at      http://www.pparc.ac.uk/Nw/dark_galaxy.asp

1. The following images will be available on the web in a variety of sizes up to 6 inches at 300 dpi from http://www2.naic.edu/~rminchin/press/ as soon as the embargo expires on Thursday January 12, 2006 9:20 AM EST or click on the small versions below for a higher resolution version.

2. 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

3. 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).

4. The scientific paper “High resolution HI imaging of VIRGOHI 21 — a dark galaxy in the Virgo Cluster” was presented on 12th January 2006 at the 207th meeting of the American Astronomical Society held in Washington, D. C. Authors are Robert Minchin (Arecibo Observatory), Jon Davies, Mike Disney (Cardiff University), Andy Marble, Chris Impey (Steward Observatory), Peter Boyce, Diego Garcia, Marco Grossi (Cardiff University), Chris Jordan (Jodrell Bank Observatory), Hugh Lang, Sarah Roberts (Cardiff University), Sabina Sabatini (Osservatorio Astronomico di Roma), and Wim van Driel (Observatoire de Paris).

5. For an interview or further technical comment please contact:

Dr. Robert Minchin, Arecibo Observatory
Tel: ++1-787-878-2612 ext. 283 (contact through AAS during the meeting)

Dr. Jon Davies, Cardiff University
Tel: ++44-(0)29-20-875255

Julia Maddock, PPARC Press Office
Tel +44 1793 442094

About PPARC,      http://www.pparc.ac.uk/ap/intro.asp

IMAGE CAPTIONS:

[Image 1: http://www.pparc.ac.uk/Nw/Image_1_300dpi.jpg (480KB)] Dark Galaxy VIRGOHI 21 has no starlight but radio waves from neutral hydrogen betray its existence. The contours superimposed on this optical image indicate how much gas was detected. This material was presented to the American Astronomical Society meeting in Washington, D. C. on January 12, 2006.

CREDIT: Arecibo Observatory / Cardiff University / Isaac Newton Telescope / Westerbork Synthesis Radio Telescope.

[Image 2: http://www.pparc.ac.uk/Nw/Image_2_300dpi.jpg (369KB)] 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. This interaction could explain 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. This material was presented to the American Astronomical Society meeting in Washington, D. C. on January 12, 2006.

CREDIT: Arecibo Observatory / Cardiff University / Westerbork Synthesis Radio Telescope.