Many mechanisms have been proposed to explain the - very common - phenomenon of warps in gaseous disks (see Binney 1992), with perhaps the most reasonable explanation being that warps are simply the natural oscillatory response of the gaseous disk to small perturbations. As such, it is conceivable that the Sagittarius dwarf may provide a perturbing influence which helps to excite periodically the Galactic warp, especially since it seems that warps are transient phenomena and need to be maintained (Binney 1992).
Perhaps most importantly, our simulations show that it is necessary to include the perturbative effect of the Sagittarius dwarf to fully understand the star formation history and spiral structure of the outer Galactic disk. In a subsequent contribution, we will improve on the accuracy of the present simulations by including a live Galactic halo; without accounting for the effects of a real halo, it is probably premature to attempt a detailed comparison of the models to the observed H Idistribution.
An interesting prospect is that it may be possible to detect indirectly the presence of dwarf galaxies from their effect on the H Idisks of their giant neighbors. For instance, a relatively massive galaxy similar to the Sagittarius dwarf would be extremely hard to detect (due to the very low surface brightness) if it were orbiting an external galaxy at a distance beyond which its brightest giants could be resolved, whereas the effect caused on the structure of its neighbor's gaseous disk could be readily observable (given a sufficiently high mass). Also, recent numerical simulations (Johnston et al. 1996) suggest that matter disrupted from former dwarf galaxies can form very long-lived streams in the halo of their massive companions. H Idisks could also be affected by these structures, depending on their mass density and clumpiness. This may yield a means to quantify the present merging rate (see, e.g., Zaritsky & Rix 1997), a parameter of great value to galaxy formation theory.
© European Southern Observatory (ESO) 1998
Online publication: July 7, 1998