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S. C. Simonson (LLNL)
In a previous study (Simonson 1975, ApJ, 201, L103), the H I feature at l = 197, b = +2, with radial velocities extending from v = –30 to –87 km/s, has been attributed to a low-mass galaxy being tidally disrupted as it approaches the Milky Way. Although several local hypotheses have been advanced to explain the H I feature, quantitative support has been lacking for the energy requirements to account for the wide velocity range observed. On the other hand, the rotation of a gaseous disk in an external galaxy naturally displays such a velocity range. A simple dynamical model accounts for the velocity pattern not only of this feature but also of the stream of high-velocity H I clouds starting at this location and extending to l = 155, b = +8, crossing l = 180 at v = –90 km/s. In this tidal disruption model, calculations show that the distance to the approaching galaxy is directly related to the mass of the Milky Way. Given a current estimate of about 1.2e12 suns, the mass of the small galaxy is in the range 6-10e8 suns (mostly dark matter), and the distance modulus is about m – M = 20 mag. For a range of Milky Way mass estimates, plus uncertainties in matching the model to the observations, m – M could range from 19 to 21.5 mag. The absorption in the visual is estimated to be 3 – 4 mag. The lack of optical counterparts detected until now implies a Low Surface Brightness dwarf galaxy. It may currently be feasible to detect the stellar component of the inferred galaxy, using techniques similar to those employed in the case of the Sagittarius dwarf galaxy and tidal streams in the halo of the Milky Way. (Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48.)
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.