Curriculum Vitae

In mid-August 2011, I'm starting as an assistant professor at Georgia Tech in the School of Physics and Center for Relativistic Astrophysics!

My research interests are star and galaxy formation in the high-redshift universe, especially prior to reionization. I use massively-parallel computer simulations to explore problems in these topics. Part of our main focus is the radiative feedback from high-redshift star formation. To model this, we have developed a coupled radiation transport code, contained in the adaptive mesh refinement code, Enzo.

I'm happily married to Emily Alicea-Muñoz, and we were married on February 27, 2010 at the Arecibo Observatory! We live together with our two kitties, Ditto and Sampson (RIP Jeremy). I like to race my car in track events and watch Formula 1 to satisfy my "need for speed".

Previously I was a postdoc at NASA's Goddard Space Flight Center from 2007-2009 and graduated from Stanford University in 2007 under the advise of Tom Abel. Below you can find some descriptions, images, and movies of my work. To the right, you can find a list of my publications. Some of them have links to their own pages, which contain more information, images, and movies.

Radiative Feedback from Primordial Stars

Collaborators: Tom Abel, Greg L. Bryan, Peng Wang

Computational simulations have recently shown that the first generation of stars in the universe are massive (approx. 100 solar masses) and very luminous. They form during the first 400 million years of the Universe and in cosmological halos that have one million solar masses, or equivalently a very small dwarf galaxy. Their radiation ionizes and heats the surrounding medium up to 3 kiloparsecs.

For the first time, we have coupled a radiative transfer solver with hydrodynamics in a cosmological study, using the Eulerian adaptive mesh code, Enzo. The radiative transfer solver utilizes a novel technique, adaptive ray tracing, where rays split if more sampling is required. This allows us to study the dynamical effects of the star's radiation, which drives a 30 km/s shock through its host gas cloud. The shock velocity exceeds the escape velocity of the halo, and nearly all of the gas is expelled. This will delay subsequent star formation in the host dark matter halo.

We study both high-resolution individual and collective cases of primordial star radiative feedback. The former case allows us to explore the shock-front instabilities, shadowing, and inhomogeneities that form once the star shines. Radiative feedback from these stars are expected to have a grand impact on the assembly of the first galaxies and the reionization of the universe. We use larger simulation volumes to investigate the large-scale effects.

Metal Enrichment from Primordial Stars

Collaborators: Tom Abel

Primordial stars between 140 and 260 solar masses are expected to end their lives tremendously. Due to runaway oxygen burning and e^--e⁺ pair production in the core, the star completely disrupts itself and leaves no remnant. Over half of the star is converted into metals (i.e. heavier than helium). Depending on stellar mass, the explosion releases 10⁵¹ to 10⁵³ ergs (1-100x the sun's total output over its lifetime!) in a split-second.

These explosions pollute the universe with the first metals. Currently, all observed objects contain some metals. A fraction of these metals, especially distant from galaxies, contain the metals from the first stars. In our simulations, the star illuminates the surrounding region and creates an under-dense environment. Then the star goes supernovae, and we track the metals from their origin to both the intergalactic medium and into the next generation of stars.

Protogalactic Supermassive Black Hole Formation

Collaborators: Tom Abel, Matthew J. Turk, Roger D. Blandford

Supermassive black holes up to 10⁹ solar masses exist when the universe is only 1 billion years old. How do they form in such a cosmologically short time? We study the traditional galaxy formation scenario where the first galaxies form in halos with only hydrogen and helium and are approximately 10⁸ solar masses. We follow their collapse over 14 orders of magnitude in length scale and 25 orders of magnitude in density. The movie to the right illustrates a zoom of 10¹² in length. The resolution in these simulations are 0.01 of a solar radius. The range in scales is equivalent of resolving chromosomes while simulating the Earth!

Contrary to standard galaxy formation models, we find the center will collapse into a 10⁵ solar mass black hole before stars can form when studying the case with only hydrogen and helium. Even though the structure is rotating, we find gravitational bar instabilities can transport angular momentum outwards so that the collapse proceeds as it was non-rotating. The central black hole will dramatically affect the surrounding gas and its associated star formation. We will supplement these results while including additional physics, such as molecular hydrogen formation, primordial star formation, metal cooling, and black hole feedback.

Virialization of Cosmological Halos

Collaborators: Tom Abel

The concept of the virialization of halos are integral to galaxy formation. Many generalizations about baryonic properties are made as gas falls into the potential wells created by dark matter. One major assumption is that the gas is shock-heated to a temperature derived from the virial theorem while ignoring baryonic effects such as pressure forces and turbulent kinetic energy. Here we include such effects and attribute the halo's internal structure through the virial theorem and violent relaxation. Furthermore, we study the virialization of halos in cosmological 3D adaptive mesh refinement simulations. As the halo attempt to virialize, it translates potential energy into both kinetic (previously ignored) and internal energy. Thus, we find that virialization also causes turbulence as well as heating. By analyzing the data while considering different types of inflows through voids or filaments, we find that filaments can penetrate the virial shock and enter the halo unheated (i.e. cold flows). This augments the previous predictions of cold flows as a channel of feeding galaxy formation.

Angular Momentum Generation during Violent Relaxation

Collaborators: Tom Abel, Peng Wang

In computational simulations, a universal angular momentum distribution is witnessed. A physical explanation is not yet known for this distribution. We hypothesize that this distribution is created during the virialization of halos as the object violently relaxes. This relaxation is based in statistical mechanics, where the energies of dark matter and baryons are described by a Maxwellian distribution. We generalize this distribution to a rotating isothermal sphere and calculate the angular momentum distribution. Since angular momentum is conserved in the system, the same distribution should exist as the system evolves if we consider a closed system. We can apply this idea to a central collapse since total angular momentum of the system does not affect the collapse but the distribution. In a turbulent medium, gas with different angular momenta can segregate, and the gas with low angular momentum can preferentially sink to the center.

Galaxy Mergers with Adaptive Mesh Refinement

Collaborators: Ji-hoon Kim, Tom Abel

Galaxy collisions are an important process in galaxy formation as it occurs often during the assembly of galaxies. Previously, galaxy mergers have been restricted to N-body and SPH techniques. With adaptive mesh refinement simulations, we hope to resolve the fine structure and induced star formation during galaxy mergers. Plus it will be a good comparison study against differing computational methods in galaxy mergers.

Photorealistic Rendering of Astrophysical Simulation Data

Collaborators: Ralf Kähler, Tom Abel

The norm in computational astrophysical visualization is to choose a false-color scale that best represents your data. However it is useful to represent the data how it would appear through a telescope. We use the temperature in simulations to calculate a blackbody spectrum. Then we convolve it through common observational filters and project it to the appropriate RGB values. This method can be generalized to any number of filters or emission measures to mimic observational techniques. Additionally, if a standard colormap is used by the computational community, visual comparison of results would become more transparent. We also account for the absorption of intervening gas in the simulation, which can absorb and/or redden the structures in the same line of sight. These algorithms are embedded in an optimized volume-renderer that can process 10 frames a second by utilizing hardware acceleration on modern graphics cards.

Supernova Rates and Detectibility of Primordial Stars

Collaborators: Tom Abel

A metal-free star has never been observed in the universe. The only epoch when these can exist is before supernovae expelled sufficient metals into the universe. This corresponds to over 4 gigaparsecs (13 billion light years) away. Due to their extreme distances, the best chance to glimpse the first generation of stars is when they die in a supernova.

Metal-free stars are heavily depend on molecular hydrogen for cooling and collapsing into a protostar. However molecular hydrogren can be destroyed by (Lyman-Werner) radiation from very large distances. Primordial stellar formation can be delayed by such radiation backgrounds. Adaptive mesh refinement simulations explored the consequences of a background on primordial stars and found a critical minimum halo mass in which a star can form, given a radiation intensity. We use this result along with (1) dark matter halo abundances, (2) propagation of a comsological radiation background, and (3) the WMAP optical depth to electron scattering to self-consistently calculate a supernova rate on the night sky. We find that ~0.34 supernovae per square degree per year should occur, or equivalently 38 per day in the total night sky. We also calculate the detectibility of these supernovae with Spitzer and JWST, and find that JWST can easily detect these events.

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Previous Research

Variability of Associated Quasar Absorption Lines

Collaborators: Michael Eracleous, Jane C. Charlton, Rajib Ganguly

Associated quasar absorption lines are important to classify by intervening or intrinsic lines since they can appear similar but exist in totally different environments. Intervening material exists in galactic systems that are very distant from the quasar. However intrinsic systems are gas clouds in the near vicinity of the quasar. We identified associated lines by comparing observations from different times to search for variability, which is indicative of a proximate energetic source. We searched for variability in 15 quasars (for a total of 19 absorption systems and 230 lines) in the ultraviolet using HST. We found that 4 of 19 systems are intrinsic to the quasars studied in this survey.

Tomography of Spectra of Massive Close Binary Stars

Collaborators: Douglas R. Gies, Laura R. Penny

In close binaries, the details and outcomes of Roche lobe overflow are still in question. Here we study a massive close (unresolved) binary star by separating their individual spectra by tomography. By exploring and fitting each star, we accurately determined the masses and orbital parameters of this system. Both stars are overluminous and are not experiencing significant mass transfer. We suggested that this binary had recently emerged from extensive mass transfer.

Last updated -- 29 July 2011

Contact: jwise [AT] /// astro [DOT] princeton [DOT] edu

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