New Insights on the Mystery of an Invisible Dark-Matter Galaxy

Evidence was discovered by an international team led by astronomers from the National Science Foundation’s Arecibo Observatory and from Cardiff University in the United Kingdom 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. Their results not only indicate the presence of a dark galaxy but also explain the long-standing mystery of its strangely stretched neighbour.

Skeptics of the dark-matter interpretation argue that VIRGOHI21 is simply a tidal tail of the nearby galaxy NGC 4254.

The observations, made with the Westerbork Synthesis Radio Telescope in the Netherlands, show that the hydrogen gas in VIRGOHI 21 appears to be rotating, suggesting 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.

The results may also solve a long-standing puzzle about nearby galaxy NGC 4254 -a lopsided object, 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.

“The Dark Galaxy theory explains both the observations of VIRGOHI 21 and the mystery of NGC 4254,” says Dr. Robert Minchin of the Arecibo Observatory.






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 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.”

“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!”

The image top of page shows contours of HI column density obtained from the ALFALFA observations of the field around VirgoHI21 and NGC 4254 superposed on an optical image. The HI tail extends from NGC 4254 (visible in the lower left) more than 250 kpc to the north (assuming it lies at the Virgo distance of 16.7 Mpc)

Cardiff University & egg.astro.cornell.edu


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