Annu. Rev. Astron. Astrophys. 1992. 30: 51-74
Copyright © 1992 by . All rights reserved

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For as long as we have known that the outer Milky Way is warped, astronomers have speculated that warps are generated by the tidal fields of companions and intruders. In the case of the Milky Way the obvious culprits are the Magellanic Clouds (which are best treated as a single unit). However, as was already apparent to Burke (1957) and Kerr (1957), at their present distance, R appeq 55 kpc, the tidal field of the Clouds is unlikely to be large enough to distort the disk by gtapprox 2 kpc vertically: Clouds of mass appeq 1010 Msun would generate a warp of amplitude ltapprox 70 pc at a radius of 16 kpc (Hunter & Toomre, 1969).

When Hunter & Toomre used their normal mode machinery to analyze this problem, it seemed just possible that its resolution might be that the Clouds were two to three times more massive than one would otherwise suppose, and had pericenter near 20 kpc (e.g. Avner & King 1967). Subsequently, however Murai & Fujimoto (1980) and Lin & Lynden-Bell (1982) have shown that the Clouds are currently near pericenter - their pericentric distance is probably appeq 50 kpc. (4) If we accept this conclusion and persist in believing that the Clouds have generated the Galactic warp, we must multiply Hunter & Toomre's already uncomfortably large mass of the Clouds by a factor ~ 33/2 appeq 5.2: the Clouds must weigh over 1011 Msun.

Can we endow the Clouds with a sufficiently mighty dark halo that the whole system is that heavy? Perhaps. The only snag is that such massive Clouds will spiral extremely rapidly to the Galactic center - essentially in a dynamical time, ~ 1 Gyr.

Sancisi (1976) has emphasized that warps occur in apparently isolated galaxies. So if all warps are tidally generated, the generator must often be invisible. Nothing in our present understanding of galaxy formation rules out the possibility that many dark halos contain no embedded galaxy - indeed there are reasons for believing that galaxies that try to form in small dark halos frequently blow themselves, but not their host halos, to smithereens (Dekel & Silk, 1986). Any halo of this class which does not merge completely into a giant galaxy, could generate a warp by tidally disturbing its host galaxy.

4 This conclusion follows from the requirements that (a) the Small and Large Clouds remain together as one follows them into the past, and (b) a tidal tail be generated that resembles the Magellanic Stream. Back.

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