• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.331-333
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• 2015

Gravitational interactions - mergers and fly-by encounters - between galaxies play a key role as the drivers of their evolution. Here we perform a cosmological N-body simulation using the tree-particle-mesh code GOTPM, and attempt to separate out the effects of mergers and fly-bys between dark matter halos. Once close pair halos are identified by the halo finding algorithm PSB, they are classified into mergers ($E_{12}$ < 0) and fly-by encounters ($E_{12}$ > 0) based on the total energy ($E_{12}$) between two halos. The fly-by and merger fractions as functions of redshift, halo masses, and ambient environments are calculated and the result shows the following.(1) Among Milky-way sized halos ($0.33-2.0{\times}10^{12}h^{-1}M{\odot}$), $5.37{\pm}0.03%$ have experienced major fly-bys and $7.98{\pm}0.04%$ have undergone major mergers since z ~ 1; (2) Among dwarf halos ($0.1-0.33{\times}10^{12}h^{-1}M{\odot}$), $6.42{\pm}0.02%$ went through major fly-bys and $9.51{\pm}0.03%$ experienced major mergers since z ~ 1; (3) Milky-way sized halos in the cluster environment experienced fly-bys (mergers) 4-11(1.5-1.7) times more frequently than those in the field since z ~ 1; and (4) Approaching z = 0, the fly-by fraction decreases sharply with the merger fraction remaining constant, implying that the empirical pair/merger fractions (that decrease from z ~ 1) are in fact driven by the fly-bys, not by the mergers themselves.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.48.3-48.3
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• 2015

Using numerical simulations, we study the effects of ram pressure stripping on dwarf galaxies. It is commonly assumed that ram pressure only affects the gas component of a galaxy. We find that it actually can affect the dynamics of the stars too, and even the dark matter surrounding the disk - an effect dubbed 'ram pressure drag'. We study the effects of ram pressure drag on tidal dwarf galaxies, and find the response is very strong. Tidal dwarfs may be entirely destroyed by gas removal, and their stellar dynamics may appear heavily dark matter dominated where no dark matter exists. We discuss the consequences for tidal dwarf evolution, tidal streams, and disk galaxy evolution in general.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.2
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• pp.43.2-43.2
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• 2010

We reconstruct the large-scale initial density field from the distribution of galaxies observed by the Sloan Digital Sky Survey (SDSS). After adding the small-scale fluctuations to match the power spectrum to that of the standard LCDM model, we make a cosmological N-body simulation of formation of structures from the initial conditions. Properties of the objects formed in the simulation can be statistically compared with those of the observed SDSS galaxies. The simulation makes it possible to know the past history of evolution of objects located in different environments, and also gives us information on the environmental parameters that cannot be directly obtained observationally. It is hoped that this comparative study leads us to better understanding of formation and evolution of galaxies in conjunction with large-scale structures in the universe.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.2
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• pp.37.2-37.2
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• 2019

To study the effects of central mass concentration on the formation and evolution of galactic bars, we run fully self-consistent simulations of Milky Way-sized, isolated galaxies with initial classical bulges. We let the mass of a classical bulge mass less than 20% of the total disk mass, and vary the central concentration of a dark matter halo. We find that both classical bulge and halo concentration delay the bar formation and weaken the bar strength. The presence of a bulge increases the initial rotational velocity near the center and hence the bar pattern speed. Bars in galaxies with a more concentrated halo slowdown relatively rapidly as they lose their angular momentum through interaction with the halo. In some of our models, bars do not experience slowdown at the expense of the decrease in their moment of inertia as the bar evolves, with the resulting pattern speed similar to that of the bar in the Milky Way.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.35.2-35.2
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• 2020

Submillimeter galaxies (SMGs) have played an important role in the understanding of galaxy evolution and cosmic star formation history at high redshift because they are known as being located at z ~ 2 and harbor a vigorous star formation. Therefore studying properties of SMGs can lead us to understand evolution of massive and actively star forming galaxies and distribution of cosmic star formation density. Recently we detected 548 SMGs near North Ecliptic Pole with JCMT/SCUBA-2 from the JCMT large program covering about 2 deg2 so far. To derive their physical parameters, we compiled a multi-wavelength photometry ranging from optical (0.3 ㎛) to submillimeter (850 ㎛) by cross-identifying counterparts at different wavelengths. In order to find counterparts, we used either VLA-1.4 GHz image and/or Spitzer/IRAC 3.6 ㎛, 4.5 ㎛ image. The number of SMGs with relatively robust counterparts is 349. In this talk, we present photometric redshifts, stellar mass, star formation rates, total infrared luminosity, and AGN fraction of these 349 SMGs derived through SED fitting analysis.

• Journal of The Korean Astronomical Society
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• v.38 no.2
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• pp.149-151
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• 2005

We have studied the environmental effect on optical-NIR color gradients of 273 nearby elliptical galaxies. Color gradient is a good tool to study the evolutionary history of elliptical galaxies, since the steepness of the color gradient reflects merging history of early types. When an elliptical galaxy goes through many merging events, the color gradient can be get less steep or reversed due to mixing of stars. One simple way to measure color gradient is to compare half-light radii in different bands. We have compared the optical and near infrared half-light radii of 273 early-type galaxies from Pahre (1999). Not surprisingly, we find that $r_e(V)s$ (half-light radii measured in V-band) are in general larger than $r_e(K)s$ (half-light radii measured in K-band). However, when divided into different environments, we find that elliptical galaxies in the denser environment have gentler color gradients than those in the less dense environment. Our finding suggests that elliptical galaxies in the dense environment have undergone many merging events and the mixing of stars through the merging have created the gentle color gradients.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.54.1-54.1
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• 2017

Intrinsic alignments (IA), the coherent alignment of intrinsic galaxy orientations, can be a source of a systematic error of weak lensing surveys. The redshift evolution of IA also contains information about the physics of galaxy formation and evolution. We present the first measurement of IA at high redshift, z~1.4, using the spectroscopic catalog of blue star-forming galaxies of the FastSound redshift survey, with the galaxy shape information from the Canada-Hawaii-France telescope lensing survey. The IA signal is consistent with zero with power-law amplitudes fitted to the projected correlation functions for density-shape and shape-shape correlation components, $A_{\delta+}=-0.0040\pm 0.0754$ and $A_{++}=-0.0159\pm 0.0271$, respectively. These results are consistent with those obtained from blue galaxies at lower redshifts (e.g., $A_{\delta+}=0.0035_{-0.0389}^{+0.0387}$ and $A_{++}=0.0045_{-0.0168}^{+0.0166}$ at z=0.51 from the WiggleZ survey), suggesting no strong redshift evolution of IA. The upper limit of the constrained IA amplitude corresponds to a few percent contamination to the weak-lensing shear power spectrum, resulting in systematic uncertainties on the cosmological parameter estimations by $-0.035<\Delta \sigma_8<0.026$ and $-0.025<\Delta \Omega_{\mathrm m}<0.019$.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.1
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• pp.55.1-55.1
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• 2011

We investigate dynamical evolution of gas in barred galaxies using a high-resolution, grid-based hydrodynamic simulations on two-dimensional cylindrical geometry. Non-axisymmetric gravitational potential of the bar is represented by the Ferrers ellipsoids independent of time. Previous studies on this subject used either particle approaches or treated the bar potential in an incorrect way. The gaseous medium is assumed to be infinitesimally-thin, isothermal, unmagnetized, and initially uniform. To study the effects of various environments on the gas evolution, we vary the gas sound speed as well as the mass of a SMBH located at the center of a galaxy. An introduction of the bar potential produces bar substructure including a pair of dust lane shocks, a nuclear ring, and nuclear spirals. The sound speed affects the position and strength of the bar substructure significantly. As the sound speed increases, the dust lane shocks tend to move closer to the bar major axis, resulting in a smaller-size nuclear ring at the galactocentric radius of about 1 kpc. Nuclear spirals that develop inside a nuclear ring can persist only when either sound speed is low or in the presence of a SMBH; they would otherwise be destroyed by the ring material with eccentric orbits. The mass inflow rates of gas toward the galactic center is also found to be proportional to the sound speed. We find that the sound speed should be 15 km/s or larger if the mass inflow rate is to explain nuclear activities in Seyfert galaxies.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.1
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• pp.56.3-57
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• 2021

We use N-body/hydrodynamic simulations to study the evolution of the spin of a Milky Way-like galaxy through interactions. We perform a controlled experiment of co-planner galaxy-galaxy encounters and study the evolution of disk spins of interacting galaxies. Specifically, we consider the cases where the late-type target galaxy encounters an equally massive companion galaxy, which has either a late or an early-type morphology, with the closest approach distance of about 50 kpc, in prograde or retrograde sense. By examining the time change of the circular velocity of the disk material of the target galaxy from each case, we find that the target galaxy tends to lose the spin through prograde collisions but hardly through retrograde collisions, regardless of the companion galaxy type. The decrease of the spin results mainly from the deflection of the orbit of the disk material by tidal disruption. It is found that the spin angular momentum of the disk of the target galaxy decreases by 15 - 20% after a prograde collision. We conclude that the accumulated effects of galaxy-galaxy interactions will play an important role in determining the angular momentum of late-type galaxies at current stage.

• Publications of The Korean Astronomical Society
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• v.25 no.3
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• pp.71-76
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• 2010

The current status of numerical simulations of galaxy formation is reviewed. After a description of the main numerical simulation techniques, I will present several applications in order to illustrate how numerical simulations have improved our understanding of the galaxy formation process.