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Paper: H I Throughout the Circum-galactic Environment of Andromeda
Volume: 331, Extra-Planar Gas
Page: 113
Authors: Thilker, D.A.; Braun, R.; Westmeier, T.
Abstract: Green Bank Telescope (GBT) λ21 cm observations have revealed a faint, yet extensive HI cloud population surrounding the Andromeda Galaxy (M31). The newfound objects are likely analogs of the high-velocity HI clouds (HVCs) seen around the Milky Way. This contribution reviews our initial discovery and presents new high-resolution WSRT interferometric observations of the M31 clouds, discussing their multi-phase substructure in addition to lower limits for the dynamical mass of detected compact cores. Using the GBT, twenty discrete HI features cleanly separated from disk (and intermediate velocity emission from the Milky Way) were discovered within 50 kpc of M31, often having radial velocities comparable to the sense of outer disk rotation. In addition, a filamentary "halo" component of at least 30 kpc extent appears concentrated at the M31 systemic velocity. Some of the discrete features are organized into elongated systems with velocity continuity, suggestive of tidal streams. The discrete population can be characterized by a steep power-law distribution of number versus HI mass in the range between 105 and 107 MSolar, although our new WSRT maps show that some of the GBT clouds are actually blends of smaller HVCs. The velocity line-width of discrete clouds is correlated with the cloud HI mass, such that if the clouds are gravitationally bound this implies a dark-to HI mass ratio of ~ 100:1. Possible origins for the discrete and "halo" M31 features include: a Local Group "cooling flow", tidal debris from recent mergers or interactions, and the gaseous counterparts of low mass dark-matter halos. We briefly touch on implications regarding prolonged thin disk formation, galactic chemical evolution (the G-dwarf problem) and QSO absorption line systems.
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