Figure 5 shows a wide-area HI map of the Magallanic System. The most prominent feature is the Magellanic Stream, a 10° wide filament with ~ 2 × 108 M of neutral hydrogen that spans more than 100° across the sky (e.g., Westerlund 1997; Putman et al. 2003). It consists of gas that trails the Magellanic Clouds as they orbit the Milky Way. A less prominent leading gas component was recently discovered as well (Lu et al. 1998; Putman et al. 1998), the start of which is seen at the left in Figure 5.
Figure 5. HI column density map of the Magellanic System in galactic coordinates from Putman et al. (2003). The LMC and SMC are indicated, as well as the Magellanic Bridge between them, several individual gas clumps in the Magellanic Stream (MS#), and the start of the Leading Arm Feature (LAF). The orientation of the LMC on the page is similar to that in Figure 4.
Many detailed theoretical models have been constructed for the Magellanic Stream (e.g., Murai & Fujimoto 1980; Lin & Lynden-Bell 1982; Shuter 1992; Liu 1992; Heller & Rohlfs 1994; Gardiner, Sawa & Fujimoto 1994; Moore & Davis 1994; Lin, Jones & Klemola 1995; Gardiner & Noguchi 1996; Yoshizawa & Noguchi 2003; Mastropietro et al. 2004; Conners et al. 2004). Models in which tidal stripping is the dominating process have been particularly successful. The most sophisticated recent calculations in this class are those by Yoshizawa & Noguchi (2003) and Conners et al. (2004), both of which build on earlier work by Gardiner & Noguchi (1996). In these models the LMC and SMC form a gravitationally bound system that orbits the Milky Way. The Magellanic Stream and the Leading Arm represent material that was stripped from the SMC ~ 1.5 Gyr ago. This was the time of the previous perigalactic passage, which coincided with a close encounter between the Clouds. The models successfully reproduce many properties of the Magellanic Stream, including its position, morphology, width variation, and the velocity profile along the Stream. The models also explain the presence of the Leading Arm, and why it is less prominent than the trailing Stream.
Given the successes of tidal models, it has always been surprising that no population of stars associated with the Stream has ever been found (e.g., Irwin 1991; Guhathakurta & Reitzel 1998; Majewski et al. 2003). In a tidal model where the stripping is dominated by gravity, one might naively expect that both stars and gas are stripped equally. However, galaxies generally have HI gas disks that are more extended than the stellar distribution. Since material is preferentially stripped from the outskirts of a galaxy, this can explain why there may not be any stars associated with the Stream (Yoshizawa & Noguchi 2003). Alternatively, it has been argued that the lack of stars in the Stream may point to important contributions from other physical processes than tidal effects. For example, Moore & Davis (1994) and Mastropietro et al. (2004) suggest that the Stream consists of material which was ram-pressure stripped from the LMC during its last passage through a hot (~ 106 K) ionized halo component of the Milky Way. Another alternative was proposed by Heller & Rohlfs (1994), who suggested that hydrodynamical forces (rather than gravitational/tidal forces) during a recent LMC-SMC interaction are responsible for the existence of the Stream.
Models of the Magellanic Stream have traditionally used the properties of the Stream to estimate the orbit of the LMC, rather than to base the calculations on estimates of the LMC proper motion. An important characteristic of the orbit is the present-day tangential velocity in Galactocentric coordinates, for which values have been inferred that include vLMC, tan = 369 km s-1 (Lin & Lynden-Bell 1982), 355 km s-1 (Shuter 1992), 352 km s-1 (Heller & Rohlfs 1994), 339 km s-1 (Murai & Fujimoto 1980), 320 km s-1 (Liu 1992) and 285 km s-1 (Gardiner et al. 1994; Gardiner & Noguchi 1996), respectively. Proper motion measurements have improved significantly over time, and it is now known that vLMC, tan = 281 ± 41 km s-1 (as discussed in detail in Section 4). This is consistent with the lower range of the values predicted by models for the Stream. The data are most consistent with the models of Gardiner & Noguchi (1996) and their follow-ups, which also provide some of the best fits to many other properties of the Magellanic Stream. The observational error on vLMC, tan is almost small enough to start ruling out some of the other models. It is possible that future proper motion measurements may yield a much more accurate determination of the LMC orbit. Models of the Magellanic Stream then hold the promise of providing important constraints on the mass, shape, and radial density profile of the Milky Way's dark halo (e.g., Lin et al. 1995).