Stellar Streams

A Sagittarius-like dwarf galaxy being torn apart by a Milky Way-like galaxy.

Stellar streams are the remnants of dwarf galaxies and globular clusters that have been/are being eaten by some host galaxy.  They are formed by tidal forces ripping apart some progenitor object as it orbits the host galaxy.  The featured image above is the ‘Field of Streams’ found in SDSS.  The featured image above was made by V. Belokurov and shows a number of the discovered streams.  You can read all about it here.

The first stellar stream was discovered by Ibata et al. (1997).  Since then, many more of these objects have been found.  Not only are these living fossils of the Milky Way’s formation, they also can tell us about the dark matter halo of the galaxy.  Most of the time in astronomy we only see a snapshot of where some object is at the current moment.  But streams tell us where the progenitor was and where it is going.  It’s possible to use that orbit information to constrain the shape and structure of our Galaxy’s dark matter halo!

During my Ph.D., I worked with Prof. Widrow at Queen’s University to attempt to find the shape of the Milky Way’s dark matter halo using stellar streams.  For our first attempt, we used the Sagittarius stream and a suite of other constraints to find the shape of the halo.  Ultimately, we found a shape that is kind of like a hamburger turned on its side.  For a more detailed read you can read our paper called “The Sagittarius stream and halo triaxiality“.


However, we soon found that our first attempt was flawed in a variety of key ways.  We had treated the stream like an orbit, but there are some systematic errors that are associated with this method (see Sanders & Binney (2013) for more details).  Secondly, we assumed that the symmetry axes of the halo were aligned with the disk, which doesn’t need to be true.  It turns out that the alignment assumption can cause the inferred shape to be completely different from the actual shape if the halo is misaligned.

We looked at all this in our paper “Incorporating streams into Milky Way models“.  At first this might seem a little bit depressing because it seems like we’ve stepped back from our initial results.  But one of the really exciting things that we found is that fitting multiple streams simultaneously is just as good as using a single long stream.  So the story is really that streams are powerful constraints on the potential of the Milky Way, but it is important to explore all the available models, to use as many constraints as possible, and to be careful to use an appropriate stream modeling method.

Since that time, I’ve explored a bit of the other stream modeling methods, but haven’t really focused on this area extensively.  But stellar streams are an area that I am eager to return to as these objects promise to be some of the most informative about the structure of not only our home galaxy, but other galaxies as well.  The dark matter halo of a galaxy is a bit like an invisible animal.  We know it’s there and that it’s large.  But to figure out if it’s an invisible rhino or elephant we need to spray it with water and look at how it drips off the animal.  Stellar streams are the sprays of water revealing whether dark matter halos are shaped like hamburgers or cigars!

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