2005ApJ...622L..21M

VIRGOHI 21 is an H I source detected in the Virgo Cluster survey of Davies et al. that has a neutral hydrogen mass of 10⁸ M_☉and a velocity width of ΔV₂₀=220 km/s. From the Tully-Fisher relation, a galaxy with this velocity width would be expected to be 12 mag or brighter; however, deep CCD imaging has failed to turn up a counterpart down to a surface brightness level of 27.5 B mag/arcsec2. The H I observations show that it is extended over at least 16 kpc, which, if the system is bound, gives it a minimum dynamical mass of ∼10¹¹ M_☉and a mass-to-light ratio of M_dyn/L_B>500 M_☉/L_☉. If it is tidal debris, then the putative parents have vanished; the remaining viable explanation is that VIRGOHI 21 is a dark halo that does not contain the expected bright galaxy. This object was found because of the low column density limit of our survey, a limit much lower than that achieved by all sky H I surveys such as those carried out at Parkes and Jodrell Bank. Further such sensitive surveys might turn up a significant number of the dark matter halos predicted by galaxy formation models.