arXiv subjects

Sunmyon Chon

Publications and source records attributed to Sunmyon Chon.

24 records · Page 2Linked to original sources

arXiv2019

Forming Pop III binaries in self-gravitating disks: how to keep the orbital angular momentum

The disk fragmentation is a possible process leading to the formation of Population III stellar binary systems. However, numerical simulations show diverse fates of the fragments; some evolve into stable binaries and others merge away with a central star. To clarify the physics behind such diversity, we perform a series of three dimensional hydrodynamics simulations in a controlled manner. We insert a point particle mimicking a fragment in a self-gravitating disk, where the initial mass and position are free parameters, and follow the orbital evolution for several tens of orbits. The results show great diversity even with such simple experiments. Some particles shortly merge away after migrating inward, but others survive as the migration stalls with the gap-opening in the disk. We find that our results are well interpreted postulating that the orbital angular momentum is extracted by (i) the gravitational torque from the disk spiral structure, and (ii) tidal disruption of a gravitationally-bound envelope around the particle. Our analytic evaluations show the processes (i) and (ii) are effective in an outer and inner part of the disk respectively. There is a window of the gap-opening in the middle, if the envelope mass is sufficiently large. These all agree with our numerical results. We further show that the binaries, which appear for the "survival" cases, gradually expand while accreting the disk gas. Our theoretical framework is freely scalable to be applied for the present-day star and planet formation.

astro-ph.GA↗

arXiv2019

Titans of the Early Universe: The Prato Statement on the Origin of the First Supermassive Black Holes

In recent years, the discovery of massive quasars at z~7 has provided a striking challenge to our understanding of the origin and growth of supermassive black holes in the early Universe. Mounting observational and theoretical evidence indicates the viability of massive seeds, formed by the collapse of supermassive stars, as a progenitor model for such early, massive accreting black holes. Although considerable progress has been made in our theoretical understanding, many questions remain regarding how (and how often) such objects may form, how they live and die, and how next generation observatories may yield new insight into the origin of these primordial titans. This review focusses on our present understanding of this remarkable formation scenario, based on discussions held at the Monash Prato Centre from November 20--24, 2017, during the workshop "Titans of the Early Universe: The Origin of the First Supermassive Black Holes."

astro-ph.GA↗

arXiv2017

Radiation hydrodynamics simulations of the formation of direct-collapse supermassive stellar systems

Formation of supermassive stars (SMSs) with mass ~10^4 Msun is a promising pathway to seed the formation of supermassive black holes in the early universe. The so-called direct-collapse (DC) model postulates that such an SMS forms in a hot gas cloud irradiated by a nearby star-forming galaxy. We study the DC SMS formation in a fully cosmological context using three-dimensional radiation hydrodynamics simulations. We initialize our simulations using the outputs of the cosmological simulation of Chon et al. (2016), where two DC gas clouds are identified. The long-term evolution over a hundred thousand years is followed from the formation of embryo protostars through their growth to SMSs. We show that the strength of the tidal force by a nearby galaxy determines the multiplicity of the formed stars and affects the protostellar growth. In one case, where a collapsing cloud is significantly stretched by strong tidal force, multiple star-disk systems are formed via filament fragmentation. Small-scale fragmentation occurs in each circumstellar disk, and more than 10 stars with masses of a few times 10^3 Msun are finally formed. Interestingly, about a half of them are found as massive binary stars. In the other case, the gas cloud collapses nearly spherically under a relatively weak tidal field, and a single star-disk system is formed. Only a few SMSs with masses ~ 10^4 Msun are found already after evolution of a hundred thousand years, and the SMSs are expected to grow further by gas accretion and to leave massive blackholes at the end of their lives.

astro-ph.GA↗

arXiv2017

The impact of ionizing radiation on the formation of a supermassive star in the early Universe

A massive primordial halo near an intensely star-forming galaxy may collapse into a supermassive star (SMS) and leave a massive black hole seed of about $10^5~M_{\odot}$. To investigate the impact of ionizing radiation on the formation of an SMS from a nearby galaxy, we perform three-dimensional radiation hydrodynamical simulations by selecting a pair of massive dark matter halos forming at $z >10$. We find that rich structures such as clumps and filaments around the source galaxy shield the cloud from ionizing radiation. In fact, in some cases, cloud collapse is accelerated under ionizing radiation. This fact suggests that the ionization of the cloud's surroundings helps its collapse. Only strong radiation at the early stage of structure formation can halt the cloud collapse, but this is much stronger than observationally allowed value. We also explored the effect of ionizing radiation on a sample of 68 halos by employing an analytical model and found that increase in the mean density of the gas between the SMS forming cloud and the source galaxy protects the gas cloud from ionizing radiation as they approach each other. Thus, we conclude that ionizing radiation does not prevent the formation of an SMS in most of the cases.

astro-ph.GA↗

arXiv2016

Cosmological Simulations of Early Blackhole Formation: Halo Mergers, Tidal Disruption, and the Conditions for Direct Collapse

Gravitational collapse of a massive primordial gas cloud is thought to be a promising path for the formation of supermassive blackholes in the early universe. We study conditions for the so-called direct collapse (DC) blackhole formation in a fully cosmological context. We combine a semianalytic model of early galaxy formation with halo merger trees constructed from dark matter $N$-body simulations. We locate a total of 68 possible DC sites in a volume of $20\;h^{-1}\;\mathrm{Mpc}$ on a side. We then perform hydrodynamics simulations for 42 selected halos to study in detail the evolution of the massive clouds within them. We find only two successful cases where the gas clouds rapidly collapse to form stars. In the other cases, gravitational collapse is prevented by the tidal force exerted by a nearby massive halo, which otherwise should serve as a radiation source necessary for DC. Ram pressure stripping disturbs the cloud approaching the source. In many cases, a DC halo and its nearby light source halo merge before the onset of cloud collapse. Only when the DC halo is assembled through major mergers, the gas density increases rapidly to trigger gravitational instability. Based on our cosmological simulations, we conclude that the event rate of DC is an order of magnitude smaller than reported in previous studies, although the absolute rate is still poorly constrained. It is necessary to follow the dynamical evolution of a DC cloud and its nearby halo(s) in order to determine the critical radiation flux for DC.

astro-ph.GA↗

arXiv2013

On de-Sitter Geometry in Cosmic Void Statistics

Starting from the geometrical concept of a 4-dimensional de-Sitter configuration of spheres in Euclidean 3-space and modelling voids in the Universe as spheres, we show that a uniform distribution over this configuration space implies a power-law for the void number density which is consistent with results from the excursion set formalism and with data, for an intermediate range of void volumes. The scaling dimension of the large scale structure can be estimated as well. We also discuss the effect of restricting the survey geometry on the void statistics. This work is a new application of de-Sitter geometry to cosmology and also provides a new geometrical perspective on self-similarity in cosmology.

astro-ph.CO↗