As a National Science Foundation Postdoctoral Fellow at Caltech I spend most of my time using computer simulations to model how galaxies form and evolve over the age of the universe, but I also perform research in other areas of astronomy. I have used the Arecibo radio telescope to observe fuel for star formation in other galaxies, and I have investigated how explosions occur on the surface of the Moon. I contribute code to the Enzo hydrodynamics simulation code, the yt visualization and analysis suite, and the Trident synthetic spectral generation tool.
I am very involved in Astronomy Outreach, working previously at Columbia University as Director of Outreach, and having helped build up their program to be one of the premier astronomy public education programs in the country.
Outside of astronomy, I enjoy experiencing new cultures, long-distance bicycle tours, backpacking, learning new languages, and racing in triathlons and other endurance sports.
My Research Topics
Computational Galaxy Evolution
I investigate how galaxies evolve over cosmological timescales and the forces responsible for this evolution. My research goals focus on understanding the nature of star formation and stellar feedback in galaxies, the primary mechanisms by which gas, energy, and metals are injected into the intergalactic medium. These processes are crucial to the dynamical and chemical evolution of galactic systems, yet they are still not currently understood in computational and observational contexts.
One new avenue for comparing observations and theoretical predictions of galaxies is the circumgalactic medium, the vast reservoir of tenuous gas surrounding each galaxy out to several hundred kiloparsecs. Observations are now being made of the state of this gas, and they are providing a very useful tool for understanding the flow of material and energy into and out of galaxies. I was awarded a Hubble Space Telescope Theory Grant to investigate the nature of the CGM using computer simulations to better understand its origin, evolution, and to explain some of its peculiar observational signatures. Please visit this page for more information on my recent results as well as downloadable data.
Trident is a tool for generating synthetic spectra from hydrodynamical simulations enabling direct comparison between theory and observational data of the CGM and IGM. Britton Smith, Devin Silvia, and I are the developers for this new utility that will be made available very soon.
AGORA Simulation Comparison
I am involved in the AGORA Galaxy Simulation Comparison Project an effort to directly compare results from galaxy simulations produced by the leading astrophysical hydrodynamics codes. The goal of the project is to identify what characteristics and behaviors of simulated galaxies are real and which are simply products of the hydrodynamical methods employed in various codes. I am the leader of the working group investigating the characteristics of L* galaxies in cosmological environments, which will produce synthetic observations to directly compare against each other and real observations. This study should provide insight into why galaxies behave the way they do and what physical processes (e.g. what star formation and feedback prescriptions) are most responsible for producing realistic galaxy analogs.
I develop and use the adaptive mesh refinement hydrodynamics code, Enzo, to perform cosmological simulations following the formation and evolution of individual disk galaxies to present redshift. I am investigating new subgrid models in these simulations to better prescribe the detailed physics on small scales, specifically star formation and efficient stellar feedback. With better models for these two processes, we may be able to avoid dynamical pitfalls like the angular momentum problem and produce galaxies consistent with observations.
I am one of the core developers for the yt project, a software suite for visualization and analysis of computational hydrodynamical datasets. I'm contribute to the project in many ways, but my focus is on halo mergers and tracking through cosmological volumes, volume rendering and visualization, and developing methods for building realistic synthetic observations from simulation outputs for direct comparison against observations.
I am a collaborator and observer on the Galex Arecibo SDSS Survey (GASS), a multiwavelength project survey that targeted 1000 massive nearby galaxies. GASS was the first statistically significant sample of massive transition galaxies with homogeneously measured stellar masses, star formation rates and gas properties. It provides a means of understanding how galaxies react to their environments and their cold gas content, and why the bimodality in the galaxy color-magnitude diagram exists.
Transient Lunar Phenomena
Lastly, I had a brief foray into planetary science a few years ago, investigating how eruptions of gas out of the interior of the Moon would impact its surface. These models predicted sub-surface lunar ice with similar observational characteristics to ice discovered later that year by NASA mission scientists.
This is a fly-around of an output from an Enzo hydrodynamics simulation which followed the evolution of a Milky-Way-like galaxy from just after the Big Bang to present (z=99 to z=0). This output occurred at z=1.
Initially, you see the full cosmological volume of the simulation of 30 Megaparsecs (~100 million light years) on a side. You're looking at isodensity contours of the gas, so brighter and greener regions imply higher gas densities there. You can see the filaments of material which have formed over time by gravitational collapse of matter in the Universe.
After a few rotations, the camera zooms in on the target galaxy, a 10^12 solar mass disk galaxy, similar to our own Milky Way. You can see it has a disk and some spiral structure in the density contours. The movie switches to showing just column density to better display the galaxy's structure.
Lastly, the movie changes modes to displaying the stellar component of the galaxy. Red stars represent old stellar populations, whereas blue stars represent younger stars. Each particle is about 10,000 stars in the simulation. From this, one can see that younger stars tend to be clustered in the disk region as is true in our own Milky Way.
For more information about the science involved in running this simulation, see adsabs.harvard.edu/abs/2012ApJ...749..140H . This movie was produced using the yt analysis suite.
An illustration of how absorption spectra are generated by light passing through gas. The left panel is a two-dimensional slice taken from a hydrodynamics simulation of a spiral galaxy where we are viewing the gas number density and a light ray passing through that gas distribution. Overplot on the gas density are the velocity vectors of the gas.
The ride side shows the density of silicon-bearing gas encountered by the light ray as well as the line of sight velocity of that gas. In the bottom right panel, we see the spectrum generated by the light ray as it passes along the sightline zoomed in on the Si II 1260 Angstrom feature. The green line represents the "pure spectrum" and the black channel maps have added noise (S/N = 10) as well as a background quasar and Milky Way foreground added.
Details: Simulation output is for a sub-L* galaxy at z=0.2 produced using the Enzo AMR simulation code (Bryan et al. 2014). Spectra were generated using the Trident synthetic spectral generation code and visualized using yt (Turk et al. 2010).
A simulation following the evolution of a large representative volume of the Universe from just after the Big Bang to the present day. We're looking at the projected gas density from the side of the cube, and our view follows the reference frame of the expanding Universe over this time.
As you can see, initially the Universe starts out dense and uniform, but over time slight overdensities in the gas are amplified by the attractive effects of gravity to form the walls, filaments, and clusters seen here. Also note the axes scales change over time as the Universe expands by a factor of 100 between z=100 and z=0, and observe the significant drop in the general column density over that period.
Details: This is a 100 comoving Megaparsec box of a cosmological hydrodynamics simulation incorporating metal cooling, star formation (Cen & Ostriker), stellar feedback (Hummels & Bryan 2012), and a metagalactic UV background (Haardt & Madau 2012) using WMAP9 cosmological parameters. The simulation was conducted with the Enzo AMR hydrodynamics code (Bryan et al. 2014) visualized using the yt analysis suite (Turk et al. 2010).
Feel free to use these movies for personal and educational purposes, but please cite me as the source of the movie.
One of our duties as scientists is to share our knowledge of nature with our communities. I was previously very involved in Public Outreach in the Astronomy Department of Columbia University, having been the director of the program for six years, and helping to build it up to one of the premier astronomy public education programs in the country.
I organized, lectured, and volunteered at most of the Columbia Astronomy Outreach events including our biweekly public lecture series and stargazing, Harlem Sidewalk Astronomy, Science Fiction vs Science Fact Film Series, Family Astro Events, and many school group visitations. In addition, I am one of the founding members of the Rooftop Variables scientific mentoring program, whereby graduate students mentor local high school science teachers and help them in designing astronomy curricula and in starting up astronomy clubs at their respective schools around New York City.
During the International Year of Astronomy (2009), I was awarded the position of NASA Student Ambassador to New York State & City. As part of this role, I organized an outdoor astrophotography exhibition in the middle of Columbia's campus, which brought more than 10,000 attendees from around the city and state. I also helped to design and record several educational podcasts as part of the 365 Days of Astronomy project.
I've been featured in numerous media discussing astronomy and education including: National Public Radio, Science Careers Magazine, The Village Voice, and the Brian Lehrer Show. It is my hope that by continuing to bring the beauty of science to a larger audience, we will not only touch individual lives, but aid in improving society as a whole.
When I am not doing astronomy, I like to keep active mentally and physically. I have conducted several long-distance bicycle tours, traveling from New York City to Niagara Falls and biking along the American West Coast. I regularly train and participate in endurance sports like cycling races, marathons, and triathlons. In 2011 and 2013, I was ranked as an "All American" duathlete/triathlete for having finished top-ten nationally in my age group, and I qualified to represent Team USA at the world championships. In 2013, I completed my first Ironman.
I love international travel since it means I get to meet new people, experience new cultures, and learn new languages. In preparing for these travels, I have tried my hand at numerous languages both formally and informally. I find that it's always best to have some language preparation prior to traveling to a particular region, as it enables so many other opportunities and provides more insight into a culture. I have tried to photodocument my travels and post the results online in a single location, but as of yet, they are strewn between Facebook albums, Google Plus albums and an old blog.
In line with my love of astrophysics, travel, public education and challenging experiences, I have applied to NASA to become an astronaut. I hope that one day I will have the opportunity to aid in humanity's exploration and understanding of our little neighborhood in this vast Universe.