Category Archives: News

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.

New Paper: Mocking the First Galaxies for JWST

Yesterday, PhD candidate Kirk Barrow submitted his first (!) paper (arXiv), titled “First Light: Exploring the Spectra of High-Redshift Galaxies in the Renaissance Simulations”. Correlations between physical and observational properties of the first galaxies are imperative to determine before JWST launches in October 2018. We have examined two of the most massive galaxies in detail in mock spectra, imaging, and photometry and then searched for any trends in the ~1,600 galaxies in the sample. There is very high variability in the smallest galaxies (like the ultra faint dwarfs around the Milky Way), but the trends settle into various relationships above 1 million solar masses in stars when star formation occurs on a more regular basis. We also found that viewing angle can account for a 3-fold difference in emergent flux due to the absorption of intervening gas in the galaxy.

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.