VIRGOHI 21 was a dark galaxy candidate discovered in 2005 – it was found to contain no starlight down to a very low surface-brightness level, yet its velocity structure implied that it had a mass of 1010 – 1011 solar masses. This was too large to simply be the product of the neutral hydrogen in the system as only 2 × 108 solar messes of neutral hydrogen were detected.
VIRGOHI 21 was first observed in the 21-cm line of neutral hydrogen with the Lovell telescope at Jodrell Bank Observatory and the detection was confirmed using the Arecibo telescope (Davies et al. 2004). At the time of its discovery, it was thought that the only way to form long streams was through slow, tidal interactions. This could be ruled out in the case of VIRGOHI 21 due to the lack of an interactor. This lead to the claim by Minchin et al. (2005), following deep CCD imaging with the Isaac Newton Telescope, that it was a “dark halo that does not contain the expected bright galaxy” (down to a surface brightness of 27.5 B mag arcsec-2). This led to extensive media coverage (see below) of the ‘dark galaxy”.
Simulations by Bekki et al. (2005) challenged this interpretation, showing that it was possible to form a long stream via a hyperbolic interaction, where the galaxies passed each other at high speed, thus overcoming the problem that the interactor was not nearby. However, their definition of ‘dark’ left something to be desired, as it would include any galaxy with a luminosity of less than ≈108 times that of the Sun!
Radio synthesis imaging in 2005 with the Westerbork telescope showed that there was a bridge of neutral hydrogen between a nearby spiral galaxy, NGC 4254, and VIRGOHI 21, as well as revealing more of the structure of the source itself than was apparent in the Arecibo observations. These findings were announced at the ‘Island Universes’ conference in the Netherlands in July 2005 (Minchin et al. 2007a). NGC 4254 has an unusual, lopsided structure, with one very large spiral arm, the theory that this could have been caused by an interaction with VIRGOHI 21 was put forward by Minchin et al. (2007b), which also presented the Westerbork synthesis data and new Hubble Space Telescope imaging that showed no signs of a counterpart down to a surface brightness of 31 I mag arcsec-2, far below the limit set by Bekki et al.'s simulations. At about the same time, Haynes et al. (2007) published ALFALFA data (originally presented in the January 2006 AAS meeting) on the VIRGOHI 21 system. This showed that the neutral hydrogen stream extended to the north of VIRGOHI 21.
Duc & Bournard (2008) used both the Haynes et al. ALFALFA data and the Minchin et al. Westerbork data to model the system. They found that: “High-speed collisions, although current in clusters of galaxies, have long been neglected, as they are believed to cause little damages to galaxies except when they are repeated, a process called ‘harassment.’ In fact, they are able to produce faint but extended gaseous tails.” In other words, it was possible to explain VIRGOHI 21 as part of a tidal tail formed from a high-speed galaxy encounter. While this does not absolutely rule out the hypothesis that VIRGOHI 21 is a dark galaxy, it presents a less exotic alternative and is therefore the more likely explanation.
But there is now even stronger evidence against VIRGOHI 21 being a dark galaxy. Davies et al. (2006) predicted that, if VIRGOHI 21 were a dark galaxy, 23% of the sources in AGES would be dark galaxies. ALFALFA would only have 4% dark galaxies, but the huge size of that survey would make this over a thousand sources! But nothing like that has been seen. There are no confirmed dark galaxy candidates in AGES, and only around 50 possibilities in ALFALFA. If VIRGOHI 21 were a dark galaxy, these numbers would have to be a lot higher.