The Magellanic Stream (MS) is an extended HI stream encircling the Milky Way (MW). It contains at most a handful of stars, but it has more than 10^8 solar masses of neutral hydrogen. First understood as a remnant of the Magellanic Clouds (MCs) some 30+ years ago (Wannier & Wirixon 1972; Mathewson et al. 1974), the Magellanic Steam has since been studied heavily, for its insights into extragalactic gas replenishment, hierarchical merging scenarios, as well as to understand its progenitor(s?) – the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). As observational techniques and telescope sensitivity have improved, the observed size of the MS has increased from ~100˚ to potentially ~200˚ (Nidever et al. 2010).
Today, the MS is understood in the context of the larger Magellanic system, which includes — in addition to the MS — the LMC, the SMC, a “Bridge” of gas connecting the two interacting galaxies, and a “Leading Arm” (LA) of diffuse HI clouds extending from the LMC/SMC in the opposite direction as the MS.
History of History: The Debate over the Formation
Until recently, the formation of the MS was understood as either one or a combination of two physical processed: tidal stripping of the LMC/SMC by the MW (e.g., Murai & Fujimori 1980; Gardiner & Noguchi 1996), or ram pressure stripping of the LMC/SMC pair as they plunged through the MW hot halo (e.g., Meurer et al. 1985; Mastropietro et al. 2005). The first of these processes necessarily implied that the LMC/SMC had long been kept in a tight orbit (~2-2.5 Gyr) around the MW, during which time there would have been time to strip the requisite amount of gas to form the Magellanic Stream. Even in the ram-pressure-dominated case, close proximity is required for the LMC-SMC pair to be disturbed by passing through the hot halo. If either of these formation models were correct, then the MS is a relatively young feature, formed ~1.5 Gyr ago in the temporal vicinity of its pericentric passage.
However, Hubble Space Telescope (HST) proper motion studies by Kallivayalil et al. (2006a, 2006b) have led to revised models for the past orbital motions of the LMC/SMC pair. Besla et al. (2007) used these new velocities (~ 80 km/s higher than previous measurements) as priors in a backward-integration orbital model to show that the Magellanic Clouds could at most have made one orbit about the MW, and that with a Lambda-CDM-motivated NFW dark matter profile, the MCs are on their first passage around the MW. Such an orbit would rule out the traditional formation models, so Besla et al. (2010) argue that the Magellanic Steam is a product of only LMC-SMC interaction. In an N-body+SPH simulation, they manage to reproduce a many key observational features of the Magellanic system, including the absence of stars from the MS, the projected location of the MS, and the asymmetry between the MS and the LA. HI column densities are qualitatively similar to observations, but they do no reproduce them faithfully. Line-of-sight velocities have a much larger spread than in reality. Besla et al. suggest that the inclusion metal cooling, ionization, and interactions with the hot halo may allow for a more realistic reproduction of HI density gradients across the stream.
Observations, meanwhile, continue to drive competing theories over the MS formation. Canonical values for the MW circular velocity are ~ 220 km/sec. However, recent astrometric parallax measurements suggest that the circular velocity of the MW ought to be revised upward to ~ 250 km/sec (e.g. Reid et al. 2009). Using this higher circular velocity, Diaz & Bekki (2011a) provide a model that reproduces many observational features of the MS, in which the LMC and SMC are independently bound to the MW for at least 5 Gyr, and only more recently became mutually bound. Like Besla et al., this model relies on the LMC tidally stripping SMC gas to form the MS, but it does so in a bound orbit. This model, however, relies on an unrealistic isothermal dark matter halo, which is used to artificially institute a flat circular velocity profile. A more cosmologically-motivated NFW profile, however, would likely not change the fundamental result, though, which is that a bound orbit still remains plausible, given the uncertainties even in the most recent observations. Moreover, Diaz & Bekki (2011b) introduce a semi-analytic term for the drag caused by hot halo, and they are able to better reproduce LA kinematics, while leaving the MS itself relatively unchanged. This hints at the fact that more realistic treatments of the hydrodynamic effects of the hot halo may indeed be able to account for the diffuse, filamentary, and cloud-like structure of the MS and the LA.
While the formation theories continue to disagree on fundamental points (such as whether the LMC and SMC spent the majority of a Hubble time bound or unbound to the MW), they increasingly also point to a consensus in which a combination of gravitational, hydrodynamic, and stellar feedback effects are responsible for the formation of the Magellanic Stream, and also for the particulars of the content of the MS’s interstellar medium. For example, Nidever et al. (2008) propose a different, but not contradictory model of formation based on observations of large outflows from supergiant shells in the LMC. In this scenario, SNe explosions push gaseous material to larger radii, allowing ram pressure and/or gravitational forces to do the remaining work of removing the gas into the MS and LA. Such a proposal can coexist with either the picture of Besla et al. or of Diaz & Bekki.
Streaming Forward: A Reservoir of Science and Cold Gas
The debate over the formation of the Magellanic Stream does not take place in a vacuum (pun intended). Understanding its history allows for a better understanding of its present and future. While the above theories and observations are primarily focused on dynamics of the HI stream/clouds in the context of interacting galaxies, many current avenues of research are concerned with the using analysis of the stream as a probe of the interstellar medium. Fox et al. (2010) present spectroscopic measurements of the MS in order to determine the metallicity and ionization of the gas, using background quasars as backlights. The most interesting results are the high levels of ionization in the gas which the authors argue can only be explained by a multi-phase plasma model, where the ionization is the result of collisions between clouds and the warm-hot medium. These high ionization levels suggest that the Magellanic Stream may not survive long enough to replenish the MW and thus allow for more star formation; instead, the MS filaments may be merely transitory features that subsequently dissolve into the coronal plasma. Meanwhile, metallicity measurements of [O/H] = –1.00, which is comparable to the metallicity of the SMC, provide additional evidence that that the the MS is formed from LMC stripping of the SMC, rather than MW stripping of both the LMC and SMC.
What Can be Learned?
The Magellanic Stream will obviously continue to have much to offer to both observational and theoretical astrophysicists in the upcoming decade. As the modeling discussed in the historical section above demonstrates, the addition of more realistic hydrodynamics will potentially allow for incredibly close replication of actual observables of the LMC/SMC dwarf galaxy pair. It is important, however, to remain cautious towards any tendency to attempt to over-predict the specifics of one system. Chaotic effects will render certain features difficult if not impossible to replicate, and in the attempt to reproduce them, it is possible to stray from understanding the basic physics to adapting unneeded prescriptions. Hopefully, instead, the Magellanic Stream will be a proof-of-concept for these types of mergers, in general. Another possibility is that with improved models, constraints will be able to be placed on the halo shape and symmetry, rather than relying on a profile as a prior. In the shorter term, however, it is more likely to remain a probe of the warm-hot halo, since predictions can be directly compared with other observations, unlike the absence of direct observations of the dark matter halo.