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Session 61 - Formation and Structure of the Milky Way.
Oral session, Tuesday, January 14
Harbour B,
We re-evaluate the evidence for the nature of the High Velocity HI clouds (HVCs) and show that they are most plausibly explained as members of the Local Group of galaxies. We examine the three large scale HI surveys performed to date and show that a large and important subset of the HVCs is centered on the barycenter of the Local Group. If the Magellanic Stream is deleted from the sample, The velocity centroid of the entire HVC cloud ensemble has the same kinematic radial velocity centroid as the Local Group. Furthermore, only clouds gravitationally bound to the Local Group are found in the surveys. We show further that if the clouds are stable entities, with formation and destruction timescales long compared to their crossing times, that upper and lower distance limits placed by gravitational boundedness and tidal stability place the clouds, on average, at distances consistent with membership in the Local Group. We show also that the clouds exhibit a relation between their angular sizes and their velocities relative to the Local Group Standard of Rest, with clouds inferred to be closer to the Milky Way having larger angular sizes. Three individual clouds are presented in detail. One, in the plane of the Milky Way, is shown to have a distance greater than about 40 kpc. Two others are positionally associated with the most massive memebers of the local group, M31 and M33. The observed metallicities of the clouds from HST observations are consistent with the Local Group hypothesis, but not with a galactic fountain hypothesis. No other explanation can account for all of these observed properties. In the accompanying paper, we present a dynamical simulation that successfully reproduces the spatial and velocity distribution of HVCs on the sky.
