Tidal streams in an evolving dark matter halo


2014 has been a great scientific year for me, in which I had the opportunity to contribute to many exciting projects. I’m very fond of the paper that came out of my collaboration with Ana Bonaca and Marla Geha, which we started back in 2013 when I was at Yale. Ana, who is a PhD student at Yale and who got famous for discovering the Triangulum stream in the southern part of the Sloan Digital Sky Survey, put a lot of effort into this project, and I’ve learned a lot from working with her. 

The question we asked was simple: “What is the effect of an evolving galactic dark matter halo on the formation and evolution of tidal streams!?” – motivated by the fact that in all tidal stream research we simplify the galactic halo by assuming that it has a simple analytical form and that it does not evolve with time. It was obvious to everyone that this has to be an oversimplification, as dark matter halos are supposedly made of dark matter particles, which form bound substructures on all length scales. That is, a galactic dark matter halo contains hundred thousands of subhalos orbiting within the main halo, which themselves are substructured. But due to computational difficulties, investigations have either focussed on tidal streams or on the structure of dark matter halos, not on both at the same time.

Thus, answering our question was tricky, and it required us to run a >1 billion particle simulation of a live-forming dark matter halo. Together with Jürg Diemand from the University of Zurich, we re-computed the Via Lactea II simulation (one of the highest-resolution N-body simulations of a Milky Way-sized dark matter halo) from a snapshot 6 billion years in the past to the present day. While doing so, we simultaneously generated tidal streams from >10,000 “cluster particles”, which we randomly inserted into the simulation such that they covered a wide range of possible orbits within the main halo.

Over the course of 6 billion years of simulation, each of the cluster particles generated a streakline-ish tidal stream by constantly releasing stream particles into the galactic halo. The same we did for an analytic galaxy halo that did not evolve with time. Two examples of streams forming in the evolving and in the non-evolving galaxy halos are shown in the figure above. It is immediately apparent that the streams in the “lumpy & evolving” halo are much wider and more dispersed than in the “smooth & static” case.

Instead of asking what is causing these differences in detail, we asked ourselves, how this would affect our goal of measuring the weight of a galaxy by modeling the streams. Ana took 256 random streams from both data sets (evolving halo and analytic halo) and modeled each of them with our Fast-Forward method – attempting to recover the mass and shape of the dark matter halo. It turned out that, while this was easily possible for the analytic, static halo, it can be fatal for the evolving halo.

Streams have millions of encounters with the dark matter subhalos while they are orbiting through the main halo. These encounters can be weak and negligible, or they can be strong and even deflect the orbit of the whole stream. Moreover, the encounters can be disruptive, and punch holes into the dynamically cold streams, or merely puff them up a bit. All these effects together make our recovery approach prone to biases, i.e. errors in the interpretation of the underlying mass and shape of the galactic dark matter halo. We may overestimate the mass of the galaxy by up to 50%, Ana found!

However, there’s still hope for us! By combining several streams, we will be able to correct for all the mistakes we could possibly make. So there’s our main motivation for observing more streams on the sky, and finally modeling them all together. It seems to be the only way to accurately measure the weight of a galaxy like our Milky Way.


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