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Astron. Astrophys. 336, 130-136 (1998)

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1. Introduction

The observed warps of spiral galaxies are commonly believed to be either caused by the torque due to a misalignment of the outer halo and the disk, or to be the natural oscillatory response of the gas layer to small perturbations (Binney 1992). However, recent numerical simulations have shown that, in models with realistic disk to halo mass ratios, disks placed inside oblate halos align themselves with the plane of symmetry of the halo within a few orbital periods (Dubinski & Kuijken 1995). So in both scenarios it seems likely that perturbers are required to maintain the warps.

The influence of specific perturbers on the disk of the Milky Way has been considered. Weinberg (1995) put forward a model where disk modes are being excited by the joint torque from the Magellanic Clouds and the halo. Another possible perturber of the Galactic disk is the Sagittarius dwarf galaxy (Ibata et al. 1994; Ibata 1994), which likely collides with the Galactic disk every [FORMULA] (Ibata et al. 1997, hereafter IWGIS). This intriguing possibility was brought to attention by Lin (1996), who realized that the position where one of the likely orbits (given the initial kinematic data of Ibata et al. 1994) last passed through the Galactic disk is coincident with the position where the maximum displacement of the H Iwarp would have been at that time.

Lin modeled the Galactic H Igas with 4000 massless and collisionless tracer particles placed in circular orbits in a fixed Galactic potential. The response of these particles to the passage of a (collisionless) point-mass particle on the orbit deduced for the Sagittarius dwarf was then analyzed. He found that, if the Sagittarius dwarf has a mass of at least [FORMULA], and moves on an orbit which fit best the then extant data, then the resulting perturbation to the modeled disk, when evolved up to the present time, looked similar to the observed warped distribution of H Iin the outer Galactic disk.

However, by choosing massless particles as tracers of the Galactic H Idisk, the Lin (1996) study did not model the nature of that Galactic component, which of course is both self-gravitating and gaseous. Furthermore, an analysis of recent data (IWGIS) supports orbits with a longer period than that adopted by Lin (1996); the currently best-fit orbit, implies that the Sagittarius dwarf did not collide with the Galactic disk [FORMULA] years ago, as is required in Lin's model.

In this paper we present hydrodynamical simulations to analyze the effect that a single passage of the Sagittarius dwarf could have on the gaseous outer disk of the Milky Way. By including in the simulations pressure forces and self-gravity of the gas, and adopting the orbit determined from recent observations, we model this interaction much more accurately. More than [FORMULA] collisions with the Galactic disk are predicted to have occurred over the [FORMULA] Gyr lifetime of the dwarf (IWGIS); however, due to the effect of dynamical friction during tidal disruption, one cannot estimate accurately the position and velocity, or even the peri- and apo-Galactic distances, that the Sagittarius dwarf had 10 Gyr ago. We therefore restrict ourselves in the present contribution to the consideration of the effect of a single encounter with the Galactic disk; for concreteness, we will study the aftermath of the collision due to occur [FORMULA] Myr from the present time on the opposite side of the Milky Way from the Sun.

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© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998