Animation of the neutral hydrogen data cube from Westerbork. Up indicates
increasing recessional velocity. VIRGOHI 21 is the structure in the
centre, connected by a bridge to NGC 4254 (near the top of the image).
NGC 4262 is the ring-like structure near the bottom of the image.
VIRGOHI 21 is the best dark-galaxy candidate to date – it contains
no starlight down to a very low surface-brightness level, yet its velocity
structure implies that it has a mass of 1010 –
1011 solar masses. This is too large to simply be the product
of the neutral hydrogen in the galaxy as only 2 × 108
solar messes of neutral hydrogen have been detected.
VIRGOHI 21 was first observed in the 21-cm line of neutral hydrogen
with the Lovell telescope at Jodrell Bank and the detection was confirmed
using the Arecibo telescope. Optical follow-up was carried out with the
Isaac Newton Telescope on La Palma and high-resolution interferometry
observations in the neutral hydrogen line were carried out with the
Westerbork Synthesis Radio Telescope in the Netherlands.
The discovery received extensive coverage across the world, a list
of all the online stories that I know of can be found
here - many of these are, however, simply
rehashes of the coverage on the newswires. A few good links are given
below.
Sloan Digital Sky Survey
image of VIRGOHI 21 from their Image of the Week gallery. The orginal
caption reads: Radio telescopes at Arecibo and Jodrell Bank Observatory
detect a large cloud of hydrogen gas at the center of the region of the
sky covered by this image, but no corresponding objects can be seen in it.
The rotation of this cloud indicates the presence of a significant mass of
dark matter (matter that we cannot currently detect directly) as well.
