Category Archives: News

Postdoc opening (Deadline Dec 15th)

Applications are invited for a postdoctoral research position at the Center for Relativistic Astrophysics (CRA; in the Georgia Institute of Technology. The successful candidate will work in conjunction with Dr. John Wise on simulations of the first stars and galaxies and their observability with JWST. The position is for two years; renewal for a third year is subject to performance. Expertise in running and analyzing cosmological simulations is highly desired, and experience in parallel code development is preferred. Continue reading Postdoc opening (Deadline Dec 15th)

New Paper: Connecting Simulations with Galaxy Models

This week submitted a paper (arXiv) that compares, validates, and calibrates semi-analytic models of high-redshift galaxy formation to our previous simulations of the first stars and galaxies.  This paper was led by Benoit Côté, a postdoc at the Research Centre for Astronomy and Earth Sciences in Budapest. Semi-analytic models of galaxy formation are a great, computation-lite method to explore which physical processes are important during galaxy formation. Here we compared the most massive galaxy in the “Birth of Galaxy” simulations to GAMMA. We determined the best model that accurately tracks the amount of gas transferred from the star-forming regions to the circumgalactic medium (CGM). This gas lifecycle is a key component to understand how galaxies form, and we found it to be much more rapid during the initial assembly of galaxies. In particular, supernova eject creates a inhomogeneous distribution of metals that is well captured by imposing a spread of 0.2 dex in metallicities in the semi-analytic models. This adjustment results in a good match in the metallicity distribution function of stars in dwarf galaxies, which is a key indicator of the assembly history of the smallest building blocks of the galaxies we see around us.

New Paper: Population III observational signatures in the First Galaxies?

Earlier this week, we submitted a new paper (arXiv), led by Kirk Barrow, on possible Population III observational signatures in the first galaxies, working off the Renaissance Simulations. While most metal-free stars might be tens of solar masses, intermediate-mass stars (1-8 solar masses) are still possible.  Alone, they wouldn’t contribute significantly to the overall spectrum of the galaxy. However if they are part of a binary system, its material might overflow onto a companion black hole as it enters the giant phase. This is called a high-mass X-ray binary and might have observational consequences as they could occur more frequently in Population III. We look at a few case studies in simulation data, while also providing statistics on the overall population of early galaxies. For more information, take a look at the paper!

New Paper: Lyman-alpha trapping in massive BH formation

Last week, Qi Ge and I submitted a new paper (arXiv) on the importance of Lyman-alpha radiation trapping during the initial phases of massive black hole (BH) formation. Here we used a Monte Carlo method to trace photons in a pre-galactic cloud to see how they can suppress cooling that aids the gravitational collapse. We found that gas temporarily heats to 50,000 K from 10,000 K as it collapses, only to cool back down to its original 10,000 K temperature. This heating could change the characteristic mass scales of massive BH seeds and perhaps the initial BH spin. Continue reading New Paper: Lyman-alpha trapping in massive BH formation

New Paper: Stochastic Reionization from the Smallest Galaxies

Also last week, we submitted a paper, led by Pengfei Chen (now in the finance sector), on the stochastic nature of reionization during its initial phases. Here we used the galactic properties from the Renaissance Simulations as the source model in a reionization simulation. We find that before a redshift of 10 (age of the universe = 500 million years), small galaxies dominate, whose ionized regions are stochastically flicker because of the burstiness of the first galaxies. After that time, larger galaxies that have more steady star formation dominate the ionizing photon budget and contribute the majority of the photons to reionization.

New Paper: Turbulent accretion onto black holes

Last week, our paper (arXiv), led by KwangHo Park, on the behavior of accretion flows onto intermediate mass black holes was submitted to the Astrophysical Journal. Here we used 3D radiation hydrodynamics simulations to study how the surrounding environment is affected by radiation feedback that originates from the gas around the black hole. We found that the accretion rates are oscillatory in nature, agreeing with previous 1D and 2D simulations, but these bursts induce turbulence, which can enhance black hole growth rates during quiescent phases. Although turbulent energy does not dominate the energy budget, it plays a key role in the regulation of black hole fueling.

Newly minted PhD: Daegene Koh

Today, Daegene Koh successfully defended his PhD thesis! Congratulations! He is the first PhD student to defend out of my group. He’s written two papers so far on the growth of magnetic fields around the first stars (arXiv) and extending reionization models to include the first stars and first galaxies (arXiv). He’ll be starting at KIPAC at Stanford University in the Fall as a postdoctoral researcher.

New Paper: Massive Black Hole Formation

Yesterday, our paper (free arXiv link), led by John Regan at the Dublin City University, on the formation of massive black holes in the early universe was published in Nature Astronomy. We investigated the “close-pair scenario” where a nearby nascent galaxy shines on a pre-galactic cloud, which destroys most of its molecular hydrogen that is a crucial ingredient in forming stars. Without this catalyst, the gas cloud cannot form stars, but it proceeds to collapse into a single massive black hole without fragmenting into stars.  Its mass is on the order of 100,000 times the mass of the Sun.

We simulated both this radiation source and the collapsing gas cloud to find the necessary conditions for such an object to form. We performed tens of simulations investigating various separations, orbital parameters, and galaxy luminosities. We found that a particular set of conditions, such as distance and synchronization of the onset of the formation of the first stars and galaxies, are needed to prompt this pathway toward forming supermassive black holes (one billion times the mass of our Sun!) observed only a billion years after the Big Bang.

New Paper: X-ray Binaries during Reionization

Last week, we published a paper, led by Arpan Das at the Scuola Normale Superiore in Pisa, Italy, on high mass X-ray binaries and their impact on the 21-cm signal during cosmic reionization. Here we used the “Birth of a Galaxy” simulations to calculate how the host galaxy attenuates UV and X-ray radiation from these objects. This effect is important in determining the exact imprint these high-energy photons have on the surrounding intergalactic medium, heating and partially ionizing it in the process, which will be detectable with future 21-cm experiments, such as SKA and HERA. Image credit: ESO.