Galaxy Quest: Stellar Bars and Dark Halos

Model of Dark Matter Halos and Stellar Bars

A model of isodensity contours for the surface density of the stellar disks for three galaxy models. 

Image Credit
Steven Burrows/Madigan Lab

When it comes to galaxies in our universe, there is still much work to do. Part of this work is being done by JILA Fellow and Assistant Professor of Astrophysics, Ann-Marie Madigan, and postdoc Dr. Angela Collier. In a paper recently published in The Astrophysical Journal Collier and Madigan postulate that the evolution of a galaxy can be affected by dark matter interacting with the stars within the galaxy. Like everything else, galaxies evolve over billions of years, changing shape, speed of rotation, and other factors. Studying what affects galaxy evolution is important in answering questions from the foundation of our universe, how stars and planets are formed, to the origins of dark matter.

The Importance of Dark Matter Halos:

The type of dark matter being studied by Collier and Madigan is in the form of a dark matter halo. All galaxies live within these massive, invisible dark matter halos. While these halos have never been observed directly, their presence has been inferred from their gravitational effects on a spiral galaxy’s rotation curve. These dark matter halos are important as they provide clues to the origins of dark matter, and how it interacts with other matter.

According to Collier: “we are looking at how dark matter affects the material we can observe. By studying the interaction between the dark halo and the stars, we can look at other galaxies and see how the stars are moving and hopefully make inferences about what the dark matter is doing in those systems.” Dark matter halos can also hint at galaxy formation. Madigan explained that “what Angela's work is showing is that dark matter evolves in response to baryonic material (this is the material we can observe - stars and gas for example). It sometimes forms really interesting and unexpected structures that are not at the center of the galaxy. So, it gives us new places to look for evidence of dark matter.” In studying the effects of these dark matter halos, Collier and Madigan found that a fraction of this dark matter spun in an opposite rotation to the stellar bar. These opposite rotations would make it easier to see and study the dark matter, answering many important questions.

Stellar Bars and Galaxy Evolution:

In their paper, Collier and Madigan postulated that dark matter halos must be coupled with stellar bars. Stellar bars are clusters of stars that travel in a group, as if they’re rigidly connected. Collier explained how a stellar bar functioned by stating: “stellar bars are the drivers of galaxy evolution; they funnel angular momentum throughout the galaxy. The stellar bar rotates as a solid object and as it does that, it kind of hits the dark matter halo and slows down. In this way it can move angular momentum to the dark matter halo. But also, the stellar bar is this deep gravitational well that can trap dark matter which forms the dark matter bar. The strength of your dark bar can affect how much the [stellar] bar slows down over time.” Using numerical simulations, Collier and Madigan theorized that the dark matter halos and stellar bars interacted over billions of years to affect the formation of a galaxy’s structure and shape. This shift in shape could have affected how the galaxy evolved over these billions of years. This galaxy evolution was what Collier planned to focus on in the next phase of her work as she studies galaxies that are closer to home. “The next steps will be trying to simulate a more representative model of our galaxy. One that looks more like the Milky Way and then makes a prediction of what we think the dark matter bar at our galactic center looks like and what the observables of that would be.”

Collier’s and Madigan’s work will be tested when new galactic surveys are completed using the soon-to-be-launched James Webb Space Telescope. The team also postulates that using their theory of coupling dark halos and stellar bars could help explain the exotic structures that make up observed galaxies. Collier is looking forward to continuing to study these dark halos and stellar bars. She has received a prestigious NSF postdoctoral fellowship to fund this research.

This research was possible due to funding from the NSF

Written by Kenna Castleberry, JILA Science Communicator

Synopsis

When it comes to galaxies in our universe, there is still much work to do. Part of this work is being done by JILA Fellow and Assistant Professor of Astrophysics, Ann-Marie Madigan, and postdoc Dr. Angela Collier. In a paper recently published in The Astrophysical Journal, Collier and Madigan postulate that the evolution of a galaxy can be affected by dark matter interacting with the stars within the galaxy. Like everything else, galaxies evolve over billions of years, changing shape, speed of rotation, and other factors. Studying what affects galaxy evolution is important in answering questions from the foundation of our universe, how stars and planets are formed, to the origins of dark matter. 

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