Astron. J., 120, 1342-1350 (2000/September-0)
An extragalactic H I cloud with no optical counterpart?
KILBORN V.A., STAVELEY-SMITH L., MARQUARDING M., WEBSTER R.L., MALIN D.F., BANKS G.D., BHATAL R., DE BLOK W.J.G., BOYCE P.J., DISNEY M.J., DRINKWATER M.J., EKERS R.D., FREEMAN K.C., GIBSON B.K., HENNING P.A., JERJEN H., KNEZEK P.M., KORIBALSKI B., MINCHIN R.F., MOULD J.R., OOSTERLOO T., PRICE R.M., PUTMAN M.E., RYDER S.D., SADLER E.M., STEWART I., STOOTMAN F. and WRIGHT A.E.
Abstract (from CDS):
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. The H I mass of the cloud (HIPASS J1712-64) is very low, 1.7x107 M☉, 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–2]. HIPASS J1712-64 appears to be a binary system similar to, but much less massive than, H I 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 Array (ATCA) is only 4 km.s–1 for the main component and, because of the weak or nonexistent star formation, possibly reflects the thermal line width (T<2000 K) rather than bulk motion or turbulence. The peak column density for HIPASS J1712-64, from the combined Parkes and ATCA data, is only 3.5x1019 cm–2, which is estimated to be a factor of 2 below the critical threshold for star formation. Apart from its significantly higher velocity, the properties of HIPASS J1712-64 are similar to the recently recognized class of compact high-velocity clouds. We therefore consider the evidence for a Local Group or Galactic origin, although a more plausible alternative is that HIPASS J1712-64 was ejected from the interacting Magellanic Cloud-Galaxy system at perigalacticon ∼2x108 yr ago.
Galaxies: Formation - Galaxies: Irregular - Radio Emission Lines
[KSM2000] HIPASS JHHMM+DD N=1, [KSM2000] HIPASS JHHMM+DD WW (Nos NE, SW).
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