I study stellar and gas dynamics near massive black holes, with applications to our Galactic center, the Andromeda (M31) nucleus and post-starburst galaxies. I'm also interested in the strange orbits of icy bodies in the outer solar system, and exoplanets!

In 2016, I joined the Department of Astrophysical and Planetary Sciences at CU Boulder as an Assistant Professor of Astrophysics, and JILA as an associate member. Before this, I was a NASA Einstein & Theoretical Astrophysics Center Postdoctoral Fellow at UC Berkeley . I received my Masters and PhD (2012) in Astronomy at Leiden Observatory in the Netherlands, and my undergraduate degree in Physics & Astronomy at the National University of Ireland, Galway .

Recent Research

Instability in the outer solar system

The orbits of icy minor planets beyond Neptune are doing something very strange: they all tilt and pitch in similar ways. In a recent paper , my collaborator Mike McCourt and I show that when gravitational forces between minor planets on eccentric orbits are included in N-body simulations, the orbits incline rapidly off the disk plane, and tilt and pitch in exactly the same way. This gravitational instability is something that a disk of eccentric orbits does on its own -- we don't need to invoke an external reason (like a planet 9 ) for it. We do, however, predict a lot of minor planets in the outer Solar System, a "new Kuiper Belt" (with an order of magnitude more mass!), to defend the orbital clustering against the precessing effects of the giant planets.

Eccentric stellar disks orbiting massive black holes

The double nucleus of the Andromeda galaxy has been a puzzle since its discovery by balloon-borne experiments in the early 1970s. It is best modeled as a single eccentric disk of stars, which orbits a massive black hole. But how is such an old (~Gyr) disk stable? The orbits should differentially precess with respect to one another on Myr timescales! In collaboration with Andrew Halle and Mackenzie Moody, I've discovered that the stability mechanism works like a pendulum - perturbed orbits oscillate about the mean body of the disk due to gravitational torques which changes their eccentriticies and hence their precession rates. Many orbits reached very high eccentricities and the stars are vulnerable to being tidally disrupted by the massive black hole. We are currently writing up our results!

Gas clouds plunging through an accretion disk

The physics of how gas accretes onto supermassive black holes is hugely important in astrophysics. It is a difficult topic of research however, involving three-dimensional, hot, magnetic plasmas. In a recent study , we used the G1 and G2 gas clouds in the Galactic center as probes of the accretion flow around SgrA*. As they plunge through the background gas, their orbits change due to dynamical friction. By analyzing these changes, we showed that the rotation axis of the accretion flow points along that of the Galaxy and of a putative jet! We also found that the pericenter passage date of G2 was delayed, due to precession of its eccentricity vector. It likely occured in August 2014, several months after the G2 observing campaign ended. Interestingly, this coincided with an increase in both the rate and luminosity of X-ray flares from SgrA*.


Origin and evolution of planetary systems

Syllabus for graduate course, Fall 2016.

Students can access all course materials on D2L.

Fall 2017:
I will be teaching the (3710) undergraduate course Solar System Formation and Dynamics

Spring 2018:
I will be teaching the (1030) undergraduate course Accelerated Introduction to Astronomy

Group Members & Collaborators


Graduate Students

Undergraduate Students

  • Andrew Halle (UC Berkeley)
  • Jacob Fleisig (CU Boulder)
  • Aidan Jared (CU Boulder)
  • Sean Moss (CU Boulder)