curriculum vitae link:  Greg's CV


I am a Giacconi Fellow at STScI, where I collaborate with staff to unite simulations with surveys to explore galaxy evolution. I earned my PhD with Lars Hernquist's computational cosmology lab at Harvard Astronomy in 2013, and my undergraduate degree in 2008 at Princeton's Department of Astrophysical Sciences. I am interested in learning how galaxies assembled and which physics matter to their evolution, which I study by performing numerical simulations on supercomputers such as the Odyssey, Comet, and Pleiades clusters, as well as by analyzing data from the Hubble Space Telescope. I am actively involved in predicting what the James Webb Space Telescope might teach us about the origins of galaxy structure, and how to optimize science by combining simulations with data from the Large Synoptic Survey Telescope and Wide Field Infrared Survey Telescope.

Visual demos of some of my recent work are here and here (, and an overview of my work can be found here.


In November 2017, STScI announced the JWST Early Release Science program. I am thrilled to be a collaborator on the Cosmic Evolution Early Release Survey (PI Finkelstein), working together with a team of over 100 scientists. My contribution was to simulate data motivating the survey with JWST's NIRCAM and MIRI instruments, including a figure that has been used in a press release by the team: You can also find a wider version of these simulated data in this image on the first page of the JWST Pocket Guide produced by STScI:

In 2017, I published a paper reconciling predicted galaxy merger rates with data, showing for at least one model that the unexpected flat behavior of distant galaxy pair fractions results from very rapid galaxy growth: Data products related to this, my Illustris Mock Deep Fields FITS images and source catalogs, are now hosted at MAST! Download cosmologically simulated galaxy survey images:

In July 2015 I gave a talk at Mocking the Universe, an STScI Mini-Workshop. Videos are online here. My talk, Observing Galaxy Assembly in Simulations, is available here.

In October 2014 I gave a talk in the STScI Public Lecture Series: Studying Virtual Universes with Supercomputer Simulations, embedded link below:

As a postdoc at STScI, I create and analyze synthetic data from simulations of galaxy formation, treating them as if they were observed by the Hubble Space Telescope and in the future the James Webb Space Telescope. A recent paper analyzing the shapes of distant galaxies in very detailed simulations was posted in September 2014: arXiv:astro-ph/1409.1583

I am also involved in a collaboration (The Illustris Project) to create and analyze large-scale simulations of galaxy formation. I summarized my perspective on this project in a short post for STScI's research blog An Eye on the Universe: Observing Galaxy Formation in Simulations. Part of my contribution was to generate a side-by-side comparison of the Illustris Simulation with the Hubble Space Telescope Ultra Deep Field (XDF data release):

On the left is a 2.8 by 2.8 arcminute image of the UDF at three wavelengths, and on the right is an image of the same volume from a random sight-line through the Illustris Simulation. The mock image is in the same units as the real one, and was processed to closely mimic the effects of telescope resolution and noise. Broadly speaking, we find a mix of galaxy shapes and colors reminiscient of the real universe. Only recently have hydrodynamic simulations of galaxy formation reached the size and accuracy required to make such a direct comparison.