Category Archives: News

Newly minted PhD: Kirk Barrow

A week ago, Kirk Barrow successfully defended his thesis, “Synthetic Observations using a Robust End-to-End Radiative Transfer Modeling Pipeline.” You can find most of his results in his two published papers (link 1, link 2). He was also awarded the Amelio Prize that “rewards excellence in research by a physics graduate student.” He will be starting as a Porat Fellow at KIPAC at the SLAC National Accelerator Laboratory in the Fall.

Newly Minted PhD: Qi Ge

Today, Qi Ge successfully defended his thesis, “On the Effect of Lyman Alpha Trapping during the Initial Collapse of Massive Black Hole Seeds”.  You can find most of his results in this paper. He’ll be starting as a software research scientist at a Chinese startup that specializes in self-driving cars.  In particular, he’ll be working on machine learning techniques there. Congratulations, Qi!

New Paper: Supermassive Star Formation in a Cosmological Context

Last week we submitted a paper (arXiv) on simulating the direct gaseous collapse into a massive black hole, led by Osaka postdoc Kazem Ardaneh. This work is a companion to an earlier paper led by Yang Luo, a postdoc at the University of Kentucky and Osaka.  It provides more realistic conditions (mass distribution, turbulence & rotational properties) to study this catastrophic collapse.

Here we used the same radiation hydrodynamics machinery to follow a cosmological halo collapsing to stellar densities. We study the complex interplay between the strong inflowing gas and the radiation coming from the photosphere of the nascent star. The luminosity of the central object strongly fluctuates around the Eddington luminosity (10^38 to 10^39 erg/s) and grows to 10 solar masses only within 35 years. This object is thought to be the seed of a supermassive star on the order of 1,000 to 100,000 solar masses that continues to grow from the strong inflows, even in the presence of radiative feedback.

New paper: Connection between early and local galaxies

Earlier this month, our paper (arXiv) on the chemical enrichment of the first galaxies and their connection to local ultra-faint dwarf galaxies was accepted to MNRAS. This paper was led by JHU postdoctoral researcher Lauren Corlies. We compared the average metallicities of both simulated and actual galaxies to determine how and when they were enriched, in particular if Population III (metal-free) stars imprinted on them. We also explored an analytical model of metal enrichment (i.e. blastwave evolution) that matched the simulation data to a reasonable degree. We found that the simulation produced metallicity distributions in single galaxies that are narrower than the data, and future work is needed to address this inconsistency in simulations of low-mass galaxies.

New paper: Supermassive star formation

Last month we submitted a paper (arXiv) on the direct collapse of a gaseous cloud into the beginnings of a supermassive star that will then further collapse into massive black hole around 10,000 to 100,000 solar masses.  This paper was led by postdoctoral research Yang Luo at Osaka University and University of Kentucky.  We used the simulation code Enzo to perform this work.  The unique and groundbreaking aspect of this study was that we used radiation transport to understand how radiation slowed any cooling of the gas and thus collapse.  This can change the collapse dynamics and could have impacts on the final properties of the star. We followed the collapse all the way down to the protostellar accretion with resolution

New Paper: Early massive black hole growth

Today we submitted a paper (arXiv) that follows the growth of a massive black hole seed in the early universe. We found that supernova explosions heat and stir up the gas inside the gas, preventing the central black hole to accrete and grow rapidly. Stars form at rates 10,000 to 100,000 times faster than the black hole grows. The main galaxy has a stellar mass of 1 billion solar masses, which totally outshines the central black hole. This could explain why there is a lack of faint AGN at high-redshift where these more “normal” massive black holes aren’t accreting very rapidly and cannot be detected with current telescopes.

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