21 cm Synthesis Observations of VIRGOHI 21—A Possible Dark Galaxy in the Virgo Cluster

Many observations indicate that dark matter dominates the extragalactic universe, yet no totally dark structure of galactic proportions has ever been convincingly identified. Previously, we have suggested that VIRGOHI 21, a 21 cm source we found in the Virgo Cluster using Jodrell Bank, was a possible dark galaxy because of its broad line width (~200 km s-1) unaccompanied by any visible gravitational source to account for it. We have now imaged VIRGOHI 21 in the neutral hydrogen line and find what could be a dark, edge-on, spinning disk with the mass and diameter of a typical spiral galaxy. Moreover, VIRGOHI 21 has unquestionably been involved in an interaction with NGC 4254, a luminous spiral with an odd one-armed morphology, but lacking the massive interactor normally linked with such a feature. Numerical models of NGC 4254 call for a close interaction ~108 yr ago with a perturber of ~1011 M☉. This we take as additional evidence for the massive nature of VIRGOHI 21, as there does not appear to be any other viable candidate. We have also used the Hubble Space Telescope to search for stars associated with the H I and find none down to an I-band surface brightness limit of 31.1 ± 0.2 mag arcsec-2.