• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.42.1-42.1
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• 2014

Neutrino science has received a boost of attention quite recently in cosmology, since the outstanding discovery in particle physics over the last decade that neutrinos are massive: pinpointing the neutrino masses is one of the greatest challenges in science today, at the cross-road between particle-physics, astrophysics, and cosmology. Cosmology offers a unique 'laboratory' with the best sensitivity to the neutrino mass, as primordial massive neutrinos comprise a small portion of the dark matter and are known to significantly alter structure formation. I will first introduce a new suite of state-of-the-art hydrodynamical simulations with cold dark matter, baryons and massive neutrinos, specifically targeted for modeling the low-density regions of the intergalactic medium as probed by the Lyman-Alpha forest at high-redshift. I will then present and discuss how these simulations are used to constrain the parameters of the LCDM cosmological model in presence of massive neutrinos, in combination with BOSS data and other cosmological probes, leading to the strongest bound to date on the total neutrino mass.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.60.1-60.1
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• 2016

To obtain a reliable estimate of the cold dark matter (CDM) substructure mass function in a dense cluster environment, one needs to understand how long a merged halo can survive within the host halo. Measuring disruption time scale of merged halos in a dense cluster environment, we attempt to construct the realistic CDM mass function that can be compared with stellar mass functions to get a stellar-to-halo mass ratio. For this, we performed a set of high-resolution simulations of cold dark matter halos with properties similar to the Virgo cluster. Field halos outside the main halo are detected using a Friend-of-Friend algorithm with a linking length of 0.02. To trace the sub-halo structures even after the merging with the main halo, we use their core structures that are defined to be the most 10% bound particles.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.74.1-74.1
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• 2017

The two-halo conformity is that if a central galaxy in a dark matter halo is quenched in star formation, the central galaxies in other neighboring halos (within ~ 4 Mpc) even with no causal contact seem conformed to be quenched. The galactic similarity ranging far beyond the virial radius of each dark matter halo cannot be explained by known environmental effects (ram pressure, tidal interaction, etc.). Here, using a cosmological hydrodynamic simulation, we put forward new physical origins for the phenomenon; the back-splash galaxies scenario and the halo assembly bias scenario. We discuss the relative importance of the two explanations on a quantitative basis.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.51.1-51.1
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• 2017

The galaxy cluster Abell 115 shows ongoing merger features, which suggest that it might be in an intermediate phase of dynamical evolution. As merging clusters often show, the characteristic hints of A115's merging activities include radio relics, double X-ray peaks, and large offsets between the cluster member galaxies and the X-ray distributions. To constrain the exact stage of the merger, it is necessary to obtain its dark matter distribution. In this study, we carry out a precision weak lensing study of this interesting system based on Subaru images. We present our mass reconstruction together with descriptions on our core procedure of the analysis: Subaru data reduction, galaxy shape measurement, and source selection. We find that Abell 115 consists of two massive dark matter clumps, which closely follow the cluster galaxies. Our weak lensing mass estimate is a few factors lower than the published dynamical mass obtained from velocity dispersion. This large mass discrepancy may be attributed to a significant departure from dynamical equilibrium.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.51.2-51.2
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• 2011

We have studied the topology of volume-limited galaxy sample selected from the very luminous red galaxies (LRGs) in the completed Sloan Digital Sky Survey. LRGs are predominantly massive elliptical galaxies and tend to reside in massive dark matter halos. We compared the observed genus statistics with predictions from perturbation theory and mock LRG surveys constructed from dark matter halos in a Lambda CDM model. To compare with the observational data, we made 129 mock surveys in the past light cone space by using three different size CDM simulations: 41203 particle 6592 Mpc/h, 60003 particle 7200 Mpc/h, and $7210^3$ particle 10815 Mpc/h.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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• pp.67.2-67.2
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• 2016

SPT-CL J2106-5844 is known to be one of the most massive galaxy clusters ($M_{200}{\sim}1.27{\times}10^{15}M_{sun}$) ever found at z > 1. Given its redshift (z ~ 1.132), the mass of this cluster estimated by Sunyaev-Zel'dovich effect and X-ray observation is too large compared with the current ${\Lambda}CDM$ cosmology prediction. Mass estimation from these methods can be biased because they require assumptions on hydrostatic equilibrium, which are not guaranteed to hold at such high redshift (about 40% of the current age of the Universe). Thus, we need to verify the mass of this interesting cluster using gravitational lensing, which does not require such assumptions. In this work, we present our preliminary result of dark matter mass and its spatial mass distribution of SPT-CL J2106-5844 using weak-lensing analysis based on HST optical/NIR deep imaging data. We compare mass estimates from different sources and discuss cosmological implications.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.381-383
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• 2015

MOdified Newtonian Dynamics (MOND) is an alternative to the dark matter paradigm. MOND asserts that when the magnitude of acceleration is smaller than the acceleration parameter $a_0$, the response of the system to gravity is stronger (larger acceleration) than the one given by Newtonian dynamics. The current value of $a_0$ is obtained mostly by observations of spiral galaxies (rotation curves and the Tully-Fisher relation). We attempt to estimate $a_0$ from the dynamics of elliptical galaxies. We seek elliptical galaxies that act as the lens of gravitational lensing systems and have velocity dispersion data available. We analysed 65 Einstein rings from the Sloan Len ACS survey (SLACS). The mass estimates from gravitation lensing and velocity dispersion agree well with each other, and are consistent with the estimates from population synthesis with a Salpeter IMF. The value of $a_0$ obtained from this analysis agrees with the current value.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.55.5-55.5
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• 2019

We present our first attempt at understanding the dual impact of the large-scale density and velocity environment on the formation of very first astrophysical objects in the Universe. Following the recently developed quasi-linear perturbation theory on this effect, we introduce the publicly available initial condition generator of ours, BCCOMICS (Baryon Cold dark matter COsMological Inital Condition generator for Small scales), which provides so far the most self-consistent treatment of this physics beyond the usual linear perturbation theory. From a suite of uniform-grid simulations of N-body+hydro+BCCOMICS, we find that the formation of first astrophysical objects is strongly affected by both the density and velocity environment. Overdensity and streming-velocity (of baryon against cold dark matter) are found to give positive and negative impact on the formation of astrophysical objects, which we quantify in terms of various physical variables.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.2
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• pp.52.3-52.3
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• 2019

We present our first attempt at understanding the dual impact of the large-scale density and velocity environment on the formation of very first astrophysical objects in the Universe. Following the recently developed quasi-linear perturbation theory on this effect, we introduce the publicly available initial condition generator of ours, BCCOMICS (Baryon Cold dark matter COsMological Inital Condition generator for Small scales), which provides so far the most self-consistent treatment of this physics beyond the usual linear perturbation theory. From a suite of uniform-grid simulations of N-body+hydro+BCCOMICS, we find that the formation of first astrophysical objects is strongly affected by both the density and velocity environment. Overdensity and streming-velocity (of baryon against cold dark matter) are found to give positive and negative impact on the formation of astrophysical objects, which we quantify in terms of various physical variables.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.1
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• pp.58.1-58.1
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• 2021

At the University of Seoul, we are investigating the following topics in cosmology: comparing traditional clustering algorithms to our new Mulguishin algorithms, analysis of 2-body Fuzzy Dark Matter 2-body collision, 2- and 3-point clustering statistics and its dependency on the cosmological model, and dynamics of dark-matter halos around the large-scale filamentary structures. In the following sections we present a brief introduction to our studies.