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Session 61 - Formation and Structure of the Milky Way.
Oral session, Tuesday, January 14
Harbour B,

[61.02] The Formation of the Local Group and the High Velocity Clouds

D. N. Spergel (Princeton U.), L. Blitz (UC, Berkeley), P. J. Teuben (U. Maryland, College Park), D. Hartmann (Harvard-Smithsonian CfA), B. Burton (U. Leiden, Netherlands)

We simulate the formation and evolution of the Local Group. The dynamics of the Local Group is governed primarily by the its two largest members, Andromeda (M31) and the Galaxy (M0) and seconarily by the tidal effects of neighboring galaxies. In the simulation, a long filament forms which contains M31 and M0. While the gas near M31 and M0 is likely shock heated, we expect that much of the gas in the filament is cold. The kinematics of this gas in the simulation is remarkably similar to the kinematics of the High Velocity Clouds (HVCs). This similarity suggests reinterpreting the HVCs as primarily extragalactic.

In this model, the HVCs are similar to the Lyman alpha clouds. Recent work (Hernquist et al. 996) suggests that the Lyman alpha clouds are primarily condensations in the filaments between galaxies. We suggest a similar picture for most of the HVCs: they are gravitationally confined, rather than pressure confined, clouds infalling into the Local Group and are likely associated with a substantial amount of dark matter. In this picture, the two phase structure seen in some of the HVCs (Wakker amp; Schwarz 1991) would be due to self shielding that arises in gas clouds ionized by external UV (Murakami amp; Ikeuchi 1990). This model suggests that there is a substantial amount of gas in the HVCs: \sim 1 \times 10^10 M_ødot. This gas is and was a reservoir of relatively unprocessed gas for both M31 and our Galaxy and likely plays an important role in the evolution of both galaxies.

Hernquist, L, Katz, N., Weinberg, D. amp; Miralda-Escude, J. 1996, ApJ L 457, 51

Murakami, I. amp; Ikeuchi, S. 1990 PASJ, 41 , L11.

Wakker, B.P. amp; Schwarz, U.J. 1991 A amp; A, 250, 48.

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