An Extragalactic H i Cloud with No Optical Counterpart ?

We report the discovery, from the H i Parkes All-Sky Survey (HIPASS), of an isolated cloud of neutral hydrogen which we believe to be extragalactic. TheH i mass of the cloud (HIPASS J1712-64) is very low, 1.7×10M⊙, using an estimated distance of ∼ 3.2 Mpc. Most significantly, we have found no optical companion to this object to very faint limits (μ(B) ∼ 27 mag arcsec). HIPASS J1712-64 appears to be a binary system similar to, but much less massive than, HI 1225+01 (the Virgo H i Cloud) and has a size of at least 15 kpc. The mean velocity dispersion, measured with the Australia Telescope Compact University of Melbourne, School of Physics, Parkville, Victoria 3052, Australia Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping, NSW 2121, Australia Anglo-Australian Observatory, P.O. Box 296, Epping, NSW 2121, Australia University of Wales, Cardiff, Department of Physics & Astronomy, P.O. Box 913, Cardiff CF2 3YB, U.K. University of Western Sydney Macarthur, Department of Physics, P.O. Box 555, Campbelltown, NSW 2560, Australia Research School of Astronomy and Astrophysics, ANU, Weston Creek P.O., Weston, ACT 2611, Australia. Center for Astrophysics and Space Astronomy, University of Colorado, Boulder CO, 80309-0389, USA. University of New Mexico, Department of Physics & Astronomy, 800 Yale Blvd. NE, Albuquerque, NM 87131, USA Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA Istituto di Fisica Cosmica, via Bassini 15, I-20133, Milano, Italy Joint Astronomy Center, 660 North Aohoku Place, Hilo, Hi 96720, USA University of Sydney, Astrophysics Department, School of Physics, A28, Sydney, NSW 2006, Australia [email protected] Visiting Astronomer, Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.