The Many Streams of the Magellanic Stream

Snezana Stanimirovic, Samantha Hoffman (UW-Madison), Carl Heiles, Kevin A. Douglas (UC-Berkeley), Mary Putman (Univ. of Michigan), Joshua E. G. Peek (UC-Berkeley)

[No Frames Version]

(This article also appeared in the December 2007 NAIC Newsletter.)

More than 40 years since its discovery, the Magellenic Stream still provides a lot of surprises. As a part of the ongoing GALFA survey we have obtained an H I image of a region located at the tip of the Magellanic Stream. Data from several GALFA projects, especially the `Turn On GALFA Spectrometer' (TOGS) survey, were combined to produce an image covering about 1000 square degrees on the sky. Figure 1 shows the velocity field of the tip of the Stream.

Figure 1. First moment image of the tip of the Magellanic Stream obtained with ALFA. Color represents the velocity centroids and brightness represents integrated intensity. The image has been smoothed slightly to emphasize weak features. Click on the image to obtain a larger version.

While previous studies suggested that the Stream is dying off around Dec  ~ 0 deg, the new GALFA image reveals four coherent large-scale filaments extending continiously up to DEC  ~ 25 deg. These spectacular filaments are accompanied by many small H I clumps. Comparison with numerical simulations by Connors et al. (2006) suggests that a 3-way splitting of the main Stream filament may have taken place due to gravitational interactions between the SMC, the LMC, and the Galaxy. One of the newly discovered "streams" has significantly different morphology and could be much younger than the other three. This statement is in agreement with Connors et al. (2006), who suggest the existence of younger filaments running out of the Magellanic Bridge.

The existence of numerous discrete H I clumps in Fig. 1 is striking. We have made a catalog of H I clumps and found that they have several important characteristics. The clumps seem to have a characteristic angular size of about 10 arcmin, and their center velocities suggest kinematic splitting into two groups: the main one with velocity of -350 km/s, and a smaller one with velocity of -405 km/s. Kinematic splitting has also been postulated by the tidal model of Connors et al. (2006). In addition, 20% of clumps show evidence for a two-phase medium, a colder core and a warmer envelope.

We have investigated several important mechanisms that could cause such clumpy morphology and concluded that thermal instability, operating over a timescale of  ~ 50 Myr, must have played a very important role in the evolution of warm gas originally drawn out of the Magellanic Clouds. The characteristic size scale is something thermal instability would naturally produce. The condensed small clumps are long-lived even though they are embedded in the hot Halo gas or a diffuse ionized medium surrounding the Stream, with a typical lifetime of  ~ 1 Gyr.

If the clumpy morphology is induced by thermal instability, then the characteristic cooling length can be related to the characteristic clump angular size to provide an estimate of distance to the Stream tip. Interestingly, this analysis favors a distance of 80 kpc, which agrees with tidal models. This is significantly smaller than the distance inferred by Besla et al. (2007), who used the most recent proper motion measurements and suggested that, contrary to all previous assumptions, the Clouds are on their first passage around the Galaxy.