• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.36.1-36.1
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• 2016

This is a presentation of the paper published as Kimm et al. 2016, ApJ, 823, 52. We investigate the formation of metal-poor globular clusters (GCs) at the center of two dark matter halos with $Mhalo{\sim}4{\times}107Msun$ at z>10 using cosmological radiation-hydrodynamics simulations. We find that very compact (${\leq}1$ pc) and massive (${\sim}6{\times}105Msun$) clusters form rapidly when pristine gas collapses isothermally with the aid of efficient $Ly{\alpha}$ emission during the transition from molecular-cooling halos to atomic-cooling halos. Because the local free-fall time of dense star-forming gas is very short (${\ll}1Myr$), a large fraction of the collapsed gas is turned into stars before stellar feedback processes blow out the gas and shut down star formation. Although the early stage of star formation is limited to a small region of the central star-forming disk, we find that the disk quickly fragments due to metal enrichment from supernovae. Sub-clusters formed in the fragmented clouds eventually merge with the main cluster at the center. The simulated clusters closely resemble the local GCs in mass and size but show a metallicity spread that is much wider than found in the local GCs. We discuss a role of pre-enrichment by Pop III and II stars as a potential solution to the latter issue. Although not without shortcomings, it is encouraging that a naive blind (not tuned) cosmological simulation presents a possible channel for the formation of at least some massive GCs.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.321-325
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• 2015

Determining the absolute neutrino mass scale and the neutrino mass hierarchy are central goals in particle physics, with important implications for the Standard Model. However, the final answer may come from cosmology, as laboratory experiments provide measurements for two of the squared mass differences and a stringent lower bound on the total neutrino mass - but the upper bound is still poorly constrained, even when considering forecasted results from future probes. Cosmological tracers are very sensitive to neutrino properties and their total mass, because massive neutrinos produce a specific redshift-and scale-dependent signature in the power spectrum of the matter and galaxy distributions. Stringent upper limits on ${\sum}m_v$ will be essential for understanding the neutrino sector, and will nicely complement particle physics results. To this end, we describe here a series of cosmological hydrodynamical simulations which include massive neutrinos, specifically designed to meet the requirements of the Baryon Acoustic Spectroscopic Survey (BOSS) and focused on the Lyman-${\alpha}$ ($Ly{\alpha}$) forest - also a useful theoretical ground for upcoming surveys such as SDSS-IV/eBOSS and DESI. We then briefly highlight the remarkable constraining power of the $Ly{\alpha}$ forest in terms of the total neutrino mass, when combined with other state-of-the-art cosmological probes, leaving to a stringent upper bound on ${\sum}m_v$.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.33.2-33.2
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• 2019

Horizon Run 5 is the most massive cosmological hydrodynamic simulation ever performed until now. Owing to the large spatial volume ($717{\times}80{\times}80[cMpc/h]^3$) and the high resolution down to 1 kpc, we may study the cosmological effects on star and galaxy formations over a wide range of mass scales from the dwarf to the cluster. We have modified the public available Ramses code to harness the power of the OpenMP parallelism, which is necessary for running simulations in such a huge KISTI supercomputer called Nurion. We have reached z=2.3 from z=200 for a given simulation period of 50 days using 2500 computing nodes of Nurion. During the simulation run, we have saved snapshot data at 97 redshifts and two light cone space data, which will be used later for the study of various research fields in galaxy formation and cosmology. We will close this talk by listing possible research topics that will play a crucial role in helping us take lead in those areas.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.37.3-38
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• 2020

Ever since thick disk was proposed to explain the vertical distribution of the Milky Way disk stars, its origin has been a recurrent question. We aim to answer this question by inspecting 19 disk galaxies with stellar mass greater than 10^10 solar mass in recent cosmological high-resolution (>34 pc) zoom-in simulations: Galactica and New Horizon. The thin and thick disks are reproduced by the simulations with scale heights and luminosity ratios that are in reasonable agreement with observations. When we spatially classify the disk stars into thin and thick disks by their heights from the galactic plane, the "thick" disk stars are older, less metal-rich, kinematically hotter, and higher in accreted star fraction than the "thin" disk counterparts. However, we found that the the thick disk stars were spatially and kinematically thinner when they were born. Indeed, a large fraction of thick disk stars was born near the galactic plane at earlier times and get heated with time, eventually occupying high altitudes and exhibiting different population properties compared to the thin-disk stars. In conclusion, from our simulations, the thin and thick disk components are not entirely distinct at birth, but rather a result of the time evolution of the stars born in the main disk of the galaxy. (excerpted from the abstract of the upcoming paper submitted to Astrophysical Journal: Park, M.-J., Yi, S.K. et al. 2020)

• Journal of The Korean Astronomical Society
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• v.38 no.2
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• pp.129-133
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• 2005

The temperature (T) and entropy (S) fields of baryonic gas, or intergalactic medium (IGM), in the ACDM cosmology are analyzed using simulation samples produced by a hybrid cosmological hydrodynamic/N-body code based on the weighted essentially non-oscillatory scheme. We demonstrate that, in the nonlinear regime, the dynamical similarity between the IGM and dark matter will be broken in the presence of strong shocks in the IGM. The heating and entropy production by the shocks breaks the IGM into multiple phases. The multiphase and non-Gaussianity of the IGM field would be helpful to account for the high-temperature and high-entropy gas observed in groups and clusters with low-temperature IGM observed by Ly$\alpha$ forest lines and the intermittency observed by the spikes of quasi-stellar object's absorption spectrum.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.1
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• pp.43.1-43.1
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• 2012

We have performed our first cosmological hydrodynamic simulation using the recently developed SPH+GOPTM code that includes radiative cooling/heating, star formation, and supernova feedback. Here we present preliminary results from the simulation $3.4{\times}10^4M_{\odot}$, thus sub-galactic structures, such as satellite galaxies and globular clusters around a host galaxy, can be resolved with more than hundred particles. We follow formation and evolution of the sub-galactic structures in view of their star formation history, merging/accretion rate, and origins.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.76.1-76.1
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• 2017

In cluster environments, most of the galaxies are found to be red and dead, but the origin of these passive galaxies is not yet clearly understood. Using a set of cosmological hydrodynamic zoom-in simulations, we study gas depletion process in and outside clusters. Our results are consistent with previous studies showing rapid stripping of a galactic cold gas reservoir during the first infall to the cluster center. Moreover, we found a fraction of galaxies that were already in the gas deficient state before reaching the cluster (i.e., pre-processed galaxies) is non-negligible. These findings lead to the idea that a complete understanding of passive galaxy population in clusters can not be achieved without a detailed understanding of gas stripping process in group size halos prior to the cluster infall.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.72.3-73
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• 2017

The spatial distribution of sub-halos in a large host halo is usually described as isotropic in the ${\Lambda}CDM$ cosmology. Recent observations, however, show that satellite galaxies around massive galaxies are often located within a preferred plane. In order to understand the origin of such planar alignment, we investigate the spatial distribution of sub-halos around their hosts by using the hydrodynamic cosmological simulation, Illustris. In particular, we analyze the systems resembling the Milky Way (MW) and its satellites, i.e. consisting of MW-sized central galaxy and its at least 11 satellites. The result shows that ~10 % of MW-like systems have the anisotropic satellite galaxy distribution at z = 0. The satellites that are accreted more recently tend to form a flattened structure more frequently, indicating a link of satellite distribution to the surrounding environment. We discuss the physical origin of the anisotropic satellite distribution from the viewpoint of the ${\Lambda}CDM$ paradigm.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.2
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• pp.36.3-37
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• 2018

By utilizing large-scale graph analytic tools in the modern Big Data platform, Apache Spark, we investigate the topological structures of five different multiverses produced by cosmological n-body simulations with various cosmological initial conditions: (1) one standard universe, (2) two different dark energy states, and (3) two different dark matter densities. For the Big Data calculations, we use a custom build of stand-alone Spark cluster at KIAS and Dataproc Compute Engine in Google Cloud Platform with the sample sizes ranging from 7 millions to 200 millions. Among many graph statistics, we find that three simple graph measurements, denoted by (1) $n_\k$, (2) $\tau_\Delta$, and (3) $n_{S\ge5}$, can efficiently discern different topology in discrete point distributions. We denote this set of three graph diagnostics by kT5+. These kT5+ statistics provide a quick look of various orders of n-points correlation functions in a computationally cheap way: (1) $n = 2$ by $n_k$, (2) $n = 3$ by $\tau_\Delta$, and (3) $n \ge 5$ by $n_{S\ge5}$.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.48.1-48.1
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• 2020

The cosmological constant is accountable for the accelerated expansion of our Universe. Observational data have provided a tight constraint on the cosmic star formation history from z = 8 to the present. What happens to the star formation rate beyond z=0? I will discuss the star formation rates, along with the properties of the intergalactic mediumfrom our suite of simulations into the future. Since Lambda becomes dominant in the future of our universe, I further simulate counter-factual universes to assign anthropic weights to each universe within the multiverse setting. I will argue that using the asymptotic star formation efficiency as weights, we almost double previous estimates of observers living in universes similar to ours. The expected value of the energy density of Lambda is also closer to the observed value. I will also discuss potential future works to improve the applicability of the anthropic reasoning of the cosmological constant.