Stream fanning

streamfanning In my first year at Columbia I worked with grad student Sarah Pearson on an idea that Kathryn Johnston had while trying to find an orbit for Palomar 5 in a Law & Majewski potential. Wait what? Who’s Sarah, who’s Palomar 5, and what is a Law & Majewski potential?

Palomar 5 is a globular cluster in the halo of our Galaxy, the Milky Way. It is about 12 billion years old and consists of roughly 30,000 stars. The star cluster can be seen within the footprint of the Sloan Digital Sky Survey. But even more fascinating is that we can also see a stream, consisting of at least as many stars, stretching out from the cluster along its orbit. This stream – there are actually two, one in the leading direction and one in the trailing direction – spans about 23 degrees on the sky, while being on average half a degree wide. That’s about the size of 50 full moons! 

In the above figure, Palomar 5 can be seen in the blue density contours as a blob at roughly RA = 229 deg and Dec = 0 deg. The stream stretches from the upper left to the lower right. There’s lots of random fluctuations all over this density map, but we made sure that the stream is really only along this diagonal, and that the rest is just noise. Red points in the figure show the N-body model that comes closest to the observed shape of the stream. That’s not close AT ALL you may say, and you’re right. What Sarah used to generate this model is a very specific form of the mass distribution within the Milky Way and its dark matter halo. This form was proposed by David Law and Steve Majewski as they modeled the Sagittarius stream – a similar stream to the one of Palomar 5, but much further out in the halo and from a different kind of progenitor, a dwarf galaxy.

The Law & Majewski potential, as we call it, looks like an American football, and its oriented such that it causes all stars, star clusters and dwarf galaxies to be on weird orbits within the Galaxy halo. Sarah found that the orbits are in fact so weird that it’s impossible to find a thin and curved stream like the Palomar 5 stream within this potential. The streams don’t stay thin and long, but instead they fan out. From this we concluded that a) the Law & Majewski potential must be wrong within the inner part of the halo of the Galaxy, and b) that whenever we see a thin and curved stream in the sky, we can rule out certain classes of (weirdly shaped) potentials. As a consistency test, Sarah tried the same with a simple spherical dark matter halo – and it was super easy to get perfect models of Palomar 5.

Given the fact that this was Sarah’s first-year project, and that she was still taking lots of classes at the time, we’re all stoked that she got this substantial paper out so quickly. Go check out her website, she’s up to great things.


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