Local Group Archaeology

These projects were carried out with collaborators at the Harvard-Smithsonian Center for Astrophysics, the University of Maryland, and the Max-Planck Institut for Astronomie in Heidelberg, Germany.

This talk at the University of California Santa Cruz's 2015 Galaxy Workshop presents an overview of our work modeling the orbits of Local Group galaxies.

Simulating the M31-M32 galactic collision

Signatures of the M31-M32 Galactic Collision
In collaboration with Avi Loeb and Laura Blecha.

The unusual morphologies of the Andromeda spiral galaxy (M31) and its dwarf companion M32 have been characterized observationally in great detail. The two galaxies' apparent proximity suggests that Andromeda's prominent star-forming ring as well as M32's compact elliptical structure may result from a recent collision. Is it possible to build a self-consistent model of the M31-M32 interaction to simultaneously reproduce observed positions, velocities, and morphologies for both galaxies? 

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The two videos below present the simulated time evolution of the Andromeda-M32 system. The collision occurs 1.2 Gyr after the start of the simulation, and the current observed configuration of the system is reached another 800 Myr later. 

In this April 2014 talk at Harvard's Institute for Theory and Computation, I present a 15-minute overview of the main findings of our simulation of the Andromeda-M32 system.

Local Group galaxy structure and dynamics

Constraining Local Group Dark Matter Using M33's Past Orbit

Senior thesis with Professor Avi Loeb and Dr. Laura Blecha.

This work was awarded the 2012 Thomas T. Hoopes Prize (Harvard Crimson link) and the Goldberg Prize for the best senior thesis in Astrophysics. 

We compare results derived from a newly developed semi-analytic model of Local Group orbits with output from GADGET-2 simulations. Existing constraints on the repartition of intergalactic dark matter in the Local Group are limited to the classic timing argument approach. Here we use M33's past orbit  as a probe for the diffuse dark matter distribution in the Local Group. Given the parameter space granted by uncertainties in M31's proper motion and diffuse intergalactic dark matter, our semi-analytic model finds plausible orbits for the Milky Way, Andromeda and Triangulum galaxies over the past 5 Gyr. The system is treated as a three-body problem of particles moving on a background potential of superimposed bulge, disk and Navarro, Frenk & White dark matter halo profiles. Using collisional and tidal stripping constraints, an upper limit of ~1 Milky Way  mass  is found for the amount of diffuse dark matter present in the Local Group. Previous studies have found evidence for a close M31-M33 past encounter in the tidal disruption of M33's disk. Using our models we evaluate the effect of such encounters on the simulated evolution of the Triangulum globular cluster population. Our results indicate that at most ~ 3*10^10 solar masses were stripped off of M33 due to interaction with M31 over the past 5 Gyr. Orbital solutions are then computed and confirmed with GADGET-2 and yield observable dynamic signatures in the globular cluster record. We show that some of M31's outer halo clusters may have been accreted from M33 and propose their identification as a diagnostic of past Local Group orbital history. 

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Observational Evidence from SDSS for a Merger Origin of the Milky Way's Thick Disk

Marion Dierickx, Rainer Klement, Hans-Walter Rix and Chao Liu.
The Astrophysical Journal Letters, 725:L186–L190, 2010 December 20
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