curriculum vitae link:  Greg's CV


The Illustris Project: Mock Data and Morphology [Click to expand/contract]

Large-scale production and analysis of mock galaxy images.

I collaborated on The Illustris Project to create and analyze large-scale simulations of galaxy formation. We converted the simulated galaxies into synthetic telescope observations and analyzed their image morphology, finding that simulated galaxies follow many of the scaling relations obeyed by real galaxies:

In October 2016, I submitted a paper addressing the unexpected behavior of distant galaxy pair statistics:, which used the Illustris Public Data Release (Nelson et al. 2015) to create synthetic survey catalogs of distant galaxies.

Our first paper appeared in Nature in May 2014 (doi:10.1038/nature13316), and the accepted article is also available open-access (arXiv/1405.1418). 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.

HDST Simulations [Click to expand/contract]

Predicting ultra-high-resolution observations with cosmological simulations.

I collaborated with Dr. Jason Tumlinson to study synthetic images from high-resolution cosmological simulations by Ceverino et al. 2014, with the goal to motivate the galaxy formation science case for ambitious future space telescopes:

This work became a figure and back cover for the AURA HDST Report "From Cosmic Birth to Living Earths". We are actively working to create and study statistically relevant sets of these images, which you can explore using the zoom-in UDF viewer here or here (

Background [Click to expand/contract]

A complete picture of the evolution of the universe is beginning to emerge. Today, stars and planets form...

A complete picture of the evolution of the universe is beginning to emerge. Today, stars and planets form in clouds of gas and dust in galaxies, clumps of matter denser than the universal average by roughly 30 orders of magnitude. However, the universe did not begin in such a heterogeneous fashion. Instead, 13.7 billion years ago, the distribution of matter was nearly perfectly uniform, with only tiny fluctuations that grew under the influence of gravity to form the structure of galaxies we see today.

Analytical and observational work confirmed and expanded this picture over the past several decades. The majority of gravitating matter in the universe is invisible, the so-called "dark matter"; visible matter ("baryons") fell into haloes of dark matter that grew from seed perturbations in the matter distributions, and subsequently formed stars and thus luminous galaxies. Individual dark matter haloes and their subhaloes attract to other structures, eventually merging and forming more massive systems. This growth of structure from smaller pieces is known as the "heirarchical" model of galaxy formation.

Heirarchical structure growth is part of the highly successful paradigm known as the Lambda Cold Dark Matter (LCDM) universe model, whose success has been affirmed by careful comparisons between large-volume numerical simulations of dark matter halo growth and sophisticated experiments on the cosmic microwave background radiation as well as the large-scale distribution of galaxies. However, by itself the LCDM model does not explain the present appearance of galaxies. Roughly half are star-forming spiral galaxies like our Milky Way, while the rest are "bulge-dominted" or "elliptical" galaxies, spheroidal clumps of stars that contain little gas and thus are not actively forming stars.

Under the heirarchical story, this galaxy bimodality may arise naturally from differences in assembly history. However, since galaxy bimodality is connected tightly to baryon content (gas and stars), and hence the complex physical processes that act on them, a complete physical understanding of the implications of such differences remains elusive.

Summary [Click to expand/contract]

The galaxies we see today were built...

The galaxies we see today were built by the collection of smaller objects and the formation of stars in clouds of interstellar gas and dust. Sometimes, galaxy mergers are violent events that cause numerous changes to the galaxies, and different forces influence the final properties of the final galaxy. On one project, we have calculated the appearance of descendants of these collisions using the code Sunrise, which takes into account the complex effects of interstellar clouds on the emergence of starlight in three-dimensional models of galaxies. In another project, I worked to analyze images taken with the Hubble Space Telescope that targeted large groups or clusters of galaxies as they existed up to 10 billion years ago. Today, clusters of galaxies contain old elliptical galaxies that look very different than our own Milky Way, and we want to understand their history by watching what happened to their precursors, specifically how they grew from smaller pieces.

The Aftermath of Starburst Galaxies [Click to expand/contract]

It has been known for several decades that some galaxies undergo periods when they form large numbers of new stars, a so-called "starburst"...

It has been known for several decades that some galaxies undergo periods when they form large numbers of new stars, a so-called "starburst" phase. Our galaxy likely did not undergo such an event in the recent past, the last 5 billion years or so. However, we observe this happening to other galaxies: for instance, the collision between two galaxies with a lot of gas can cause a starburst.

It is possible to trace this starburst phase, even up to several billion years after it happens, owing to the signatures of the many newly-formed stars. However, when astronomers found large numbers of so called "post-starburst" galaxies, there seemed to be too few. Given the number of merging galaxies observed, we expected to see ten times more than were present, as each of those mergers was thought to produce a remnant that was detectable for a billion or more years. However, mergers produce a large variety of outcomes, and so we attempted to reconcile this difference to the expectation. To do so, we simulated a suite of galaxy mergers with the code Gadget, then followed with Sunrise in order to predict the light we might observe from the simulated galaxy.

After estimating the contribution of each of our simulated mergers to real galaxies, in this paper we revised the estimated number of post-starburst galaxies downward, because not all mergers lead to the same extreme level of a starburst. This leads to better agreement between the number of mergers observed and the number of their descendents at a later time.

Below is an example set of images produced by Sunrise, showing what we might see through a telescope if we could trace the progress of a galaxy merger.

During Obscured Starbursts [Click to expand/contract]

under construction...

under construction

Assembly of Galaxy Clusters [Click to expand/contract]

under construction...

under construction...