Category Archives: News

New Paper: AGORA Comparison Project, Isolated Galaxies

The AGORA Collaboration, primarily led by Ji-hoon Kim, aims to compare various computational astrophysics simulation code in several different tests. In our second paper (arXiv), we use the isolated disk galaxy as a test bed, comparing nine codes. Differences between the final results are small and are more dependent on the input physics instead of the underlying numerical methods. This work verifies the use of past, current, and future galaxy simulations as an accurate tool to understand the astrophysical processes governing star and galaxy formation through cosmic time.


New Paper: First stars and galaxies in reionization models

My graduate student, Daegene Koh, and I have just submitted a new paper (arXiv) on extending semi-numerical reionization models to include ionizing photons from the first stars and galaxies. These objects are often neglected because they may contribute a small fraction of the photon budget to reionization. However, we show that they do play a role during the start of reionization and alter the topology, i.e. biasing toward smaller ionized bubbles, at redshifts greater than ~10. With this new extension, we can explore the effects on reionization from the smallest star-forming dark matter halos and perhaps make predictions for upcoming 21cm detectors that will directly probe the Epoch of Reionization.

See the movie below to visually inspect the differences, where the left panel includes low-mass halos, whereas the right does not. The box size is 100 comoving Mpc, and the movie runs from redshift 25 to 6.

Solar Superstorms Video

During my visit to the NSF and US Senate, I was interviewed by the NSF for an educational video on solar storms and their effects. Take a look!

In a flash, all of your electronics could be gone, courtesy of a solar storm. However, a collaboration of National Science Foundation-funded scientists has created groundbreaking visualizations to help scientists and non-scientists alike understand these massive cosmic eruptions and develop ways to mitigate the disasters they could cause. These visualizations are available at no cost to educational institutions.

Original air date: July 18, 2016.

Credit: NSF

New Paper: Early X-ray Background

About a month ago, we submitted a new paper on the build-up of the X-ray radiation background. Here we used the Renaissance Simulations to estimate the number density of high-mass X-ray binaries from the first metal-free (Population III) stars. We found that these binaries produce about 6 eV of energy in the X-rays per hydrogen atom, potentially pre-heating the intergalactic medium to about 1000 K. These effects are important to consider when deciphering future 21-cm observations of the early universe that can tell us the last cosmic phase transition, reionization.

New Paper: Early Magnetic Field Amplification

My graduate student, Daegene Koh, submitted his first paper to MNRAS two months ago, titled “Amplification of Magnetic Fields in a Primordial HII Region and Supernova”. Late last week, we resubmitted the paper after making revisions after a favorable referee report, and posted it on arXiv today. Here we run a suite of three simulations, focusing on the amplification of magnetic fields during the generation of the ionized region and in the aftermath of its supernova. We find that through small-scale dynamo action the magnetic field grows primarily when the supernova blastwave can cool efficiently, fragments, and becomes turbulent. On average, the fields are amplified by a factor of 100 in the remnant shell and up to a factor of 106 within the shock. These strengthened fields will propagate into the first generations of galaxies, possibly affecting the nature of their star formation.

Congratulations, DK, on your first paper!

Presentation at the US Senate

On May 25th, I joined a group of people involved with the planetarium show Solar Superstorms to discuss impacts of space weather on Earth and the importance of computational science and visualization.  This event was organized by the NSF in support for a bill on space weather monitoring sponsored by Sen. Bill Peters (D-MI).  It was a totally new and pleasurable experience to speak in front of Senate staffers and interact with the Senator.  I spoke for 7 minutes on the importance of further supporting computational science and how visualization plays an essential role in the scientific process.  The event was standing room only with over 100 people in attendance, and from later feedback, it was very well received.  I hope in the future I can be involved in more events that have an impact on science policy.

Read more about the event in the Georgia Tech story.

Continue reading Presentation at the US Senate

New Paper: First Galaxy Properties and Reionization

Yesterday, we submitted a new paper (arXiv) focusing on the physical properties of the first generations of galaxies in the universe and their role during cosmic reionization.  This effort was led by Hao Xu at UC – San Diego. We analyzed all of the metal-enriched galaxies in the Renaissance Simulations in three different large-scale environments. We found that the galaxy properties are broadly independent on environment and redshift, given that they form during the epoch of reionization. These low-mass galaxies are largely responsible for providing the bulk of ionizing photons to reionization with the fraction of UV photons escaping into the intergalactic medium increasing with decreasing halo mass (~5% at 10^8 solar masses and 40-60% at 10^7 solar masses).  However these small galaxies are vulnerable to feedback suppressing star formation for large periods of time.  Here we quantify this “galaxy-halo occupation fraction” and find that it’s a increasing value (from 10% at 10^7 solar masses to 100% at 3 x 10^8 solar masses) as halos can cool through atomic transitions around 10^8 solar masses.

Congratulations Austin and Collin!

Two of my undergraduate researchers have secured positions that will further their careers.

  • Collin Cunningham, an applied math major, is graduating this semester, and he will be going to the Technical University of Denmark to receive his Masters in mathematical modeling. He has been working on increasing the accuracy of the ray tracing module in Enzo by creating an exact overlap model of the rays on a Cartesian grid.
  • Austin Gilbert, a 2nd year physics major, is going to the National Center for Supercomputing Applications (NCSA) this summer as an REU student, working in Matthew Turk’s Data Exploration Lab. He has been investigating the importance of molecular hydrogen cooling in the first stars and galaxies and has been developing a GUI for yt.

New Paper: Late Primordial Star Formation

Last week, our paper (arXiv) on the late formation of metal-free (Population III) massive stars was accepted to the Astrophysical Journal.  It was led by Hao Xu at UC – San Diego.  We used a suite of high-resolution simulations, named the Renaissance Simulations, that focuses on the formation of these primordial stars and the first generation of galaxies.  We searched for any living primordial stars in the simulation at a redshift of 7.6, approximately 700 million years after the Big Bang.  We found 14 Population III “galaxies” in the simulation suite, forming in a variety of environments, with the largest metal-free galaxies having just over 1000 solar masses in massive stars. Continue reading New Paper: Late Primordial Star Formation