Chapter

ISLAND UNIVERSES

Part of the series Astrophysics and Space Science Proceedings pp 101-106

A DARK GALAXY IN THE VIRGO CLUSTER IMAGED AT 21-CM

  • R. MinchinAffiliated withArecibo ObservatorySchool of Physics and Astronomy, Cardiff University
  • , M. J. DisneyAffiliated withSchool of Physics and Astronomy, Cardiff University
  • , J. I. DaviesAffiliated withSchool of Physics and Astronomy, Cardiff University
  • , A. R. MarbleAffiliated withSteward Observatory, University of Arizona
  • , C. D. ImpeyAffiliated withSteward Observatory, University of Arizona
  • , P. J. BoyceAffiliated withSchool of Physics and Astronomy, Cardiff University
  • , D. A. GarciaAffiliated withSchool of Physics and Astronomy, Cardiff University
  • , M. GrossiAffiliated withSchool of Physics and Astronomy, Cardiff University
  • , C. A. JordanAffiliated withJodrell Bank Observatory, University of Manchester
    • , R. H. LangAffiliated withSchool of Physics and Astronomy, Cardiff University
    • , S. RobertsAffiliated withSchool of Physics and Astronomy, Cardiff University
    • , S. SabatiniAffiliated withOsservatorio Astronomico di Roma
    • , W. van DrielAffiliated withObservatoire de Paris

Abstract

Dark Matter supposedly dominates the extragalactic Universe (Peebles 1993; Peacock 1998; Moore et al. 1999; D’Onghi & Lake 2004), yet no dark structure of galactic proportions has ever been convincingly identified. Earlier (Minchin et al. 2005) we suggested that VIRGOHI 21, a 21-cm source we found in the Virgo Cluster at Jodrell Bank using single-dish observations (Davies et al. 2004), was probably such a 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 have found what appears to be a dark, edge-on, spinning disc with the mass and diameter of a typical spiral galaxy. Moreover the disc has unquestionably interacted with NGC 4254, a luminous spiral with an odd one-armed morphology, but lacking the massive interactor normally linked with such a feature. Published numerical models (Vollmer et al. 2005) of NGC 4254 call for a close interaction ~108 years ago with a perturber of ~1011 solar masses. This we take as further, independent evidence for the massive nature of VIRGOHI 21.