• The Bulletin of The Korean Astronomical Society
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• v.30 no.1
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• pp.27.1-27.1
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• 2005

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.51.1-51.1
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• 2017

We present the dependence of halo properties on two different Galactic potentials: the $St{\ddot{a}}ckel$ potential and the Milky Way-like potential known as "Galpy". Making use of the Sloan Digital Sky Survey Data Release 12 (SDSS DR12), we find that the shape of the metallicity distribution and rotation velocity distribution abruptly changes at 15 kpc of $Z_{max}$ (the maximum distance of stellar orbit above or below the Galactic plane) and 32 kpc of $r_{max}$ (the maximum distance of an orbit from the Galactic center) in the $St{\ddot{a}}ckel$, which indicates that the transition from the inner to outer halo occurs at those distances. When adopting the $St{\ddot{a}}ckel$ potential, stars with $Z_{max}$ > 15 kpc show a retrograde motion of $V_{\phi}=-60km\;s^{-1}$, while stars with $r_{max}$ > 32 kpc show $V_{\phi}=-150km\;s^{-1}$. If we impose $V_{\phi}$ < $-150km\;s^{-1}$ to the stars with $Z_{max}$> 15 kpc or $r_{max}$> 32, we obtain the peak of the metallicity distribution at [Fe/H] = -1.9 and -1.7 respectively. However, there is the transition of the metallicity distribution at $Z_{max}=25kpc$, whereas there is no noticeable retrograde motion in the Galpy. The reason for this is that stars with high retrograde motion in the $St{\ddot{a}}ckel$ potential are unbound and stars with low rotation velocity reach to larger region of $Z_{max}$ and $r_{max}$ due to shallower potential in the Galpy. These results prove that as the adopted Galactic potential can affect the interpretation of the halo properties, it is required to have a more realistic Galactic potential for the thorough understanding of the dichotomy of the Galactic halo.

• Journal of the Korean Magnetics Society
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• v.25 no.2
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• pp.31-38
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• 2015

Recently, a first-principles study led to a prediction that quaternary Heusler compounds, CoFeCrZ (Z = Ga, Ge) are excellent half-metallic ferromagnets. In this study, we investigate the electronic and the magnetic properties at the (001) surfaces of CoFeCrGa and CoFeCrGe by means of the full-potential linearized augmented plane wave (FLAPW) method within generalized gradient approximation. We considered two types of surface termination: CoFe-terminated and CrZ-terminated surfaces, Z being either Ga or Ge. From the calculated total magnetic moments and the local density of states, we found that half-metallicity is not preserved for all the surfaces. But the calculated atomic density of states showed that CrGa-terminated surface of the CoFeCrGa is almost half-metallic. The magnetic moment of the Co, Fe, or Cr atoms at the surface or subsurface layers in each system had very different values.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.29.1-29.1
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• 2017

I present new near-infrared (NIR) photometry of globular cluster (GC) systems associated to a cD galaxy NGC 4874 in the core of the Coma cluster and 16 early-type galaxies in the Fornax and Virgo clusters of galaxies using the Infrared Channel of the Wide Field Camera 3 (WFC3/IR) on board the Hubble Space Telescope (HST). Combining these high-resolution NIR data with new HST Advanced Camera for Surveys (ACS) optical photometry for NGC 4874 and existing ACS GC catalogs from the ACS Fornax and Virgo Cluster Surveys, I have examined for the first time the GC systems in a statistically significant optical/NIR sample of galaxies spanning a wide range of luminosities and colors. A primary goal of this study is to explore empirically whether the distributions of purely optical and hybrid optical - NIR color indices for extragalactic GCs have different forms and whether the relations between these color indices are nonlinear, indicating that they behave differently with underlying metallicity. I find that some GC systems of large galaxies in our sample show color bimodalities that differ between the optical and optical - NIR colors, in the sense that they have disparate ratios of "blue" and "red" peak GCs, as well as differing ratios in their color dispersions. Consistent with these results, I find empirically that the dependence of hybrid optical-NIR color on purely optical color is nonlinear, with an inflection at intermediate metallicities. These findings underscore the importance of understanding the nature of galaxy-to-galaxy variations in the GC color distributions and color-color relations, as well as the exact forms of the color-metallicity transformations, in interpreting the observational data on GC color bimodality. Our ACS data for NGC 4874 shows that its GC system exhibits a very strong blue tilt, implying a very steep mass-metallicity scaling, and the centroid of this GC system is offset by $4{\pm}1kpc$ from the luminosity center of NGC 4874, in the direction of NGC 4872. Finally, I discuss the asymmetrical GC distribution around a dwarf elliptical galaxy in Coma that has a very high relative velocity with respect to the cluster mean at small clustercentric radius.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.2
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• pp.77.3-77.3
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• 2019

We present an analysis of the stellar population of compact elliptical galaxies (cEs) in various environments. Following conventional selection criteria of cEs, we created a list of cE candidates in the redshift range of z < 0.05 using SDSS DR12 catalog. We finally selected cEs with low-luminosity (Mg > 18.7 mag), small effective radius (Re < 600 pc), and high velocity dispersion (> 60 kms-1). We divide our cE sample into those inside and outside of the one virial radius of the bright (Mr < -21 mag) nearby host galaxy which is then defined as cEs with (cEw) and without (cEw/o) host galaxy, respectively. We investigated the stellar population properties of cEs based on the Hb, Mgb, Fe 5270, and Fe 5335 line strengths from the OSSY catalog. We found that cEw has a systematically higher metallicity than cEw/o. In the velocity dispersion-Mgb distribution, while cEw/o follows the relation of early-type galaxies, cEw are found to have a systematically higher metallicity than cEw/o at a given velocity dispersion. The different feature in the metallicity between cEw and cEw/o can suggest that two different scenarios can be provided in the formation of cEs. cEw would be the remnant cores of the massive progenitor galaxies that their outer parts have been tidally stripped by massive neighbor galaxies (i.e., nurture origin). On the other hand, cEw/o are likely to be faint-end of early-type galaxies maintaining in-situ evolution (i.e., nurture origin).

• Journal of The Korean Astronomical Society
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• v.56 no.1
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• pp.59-73
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• 2023

The second generation of stars in the globular clusters (GCs) of the Milky Way (MW) exhibit unusually high N, Na, or Al, compared to typical Galactic halo stars at similar metallicities. The halo field stars enhanced with such elements are believed to have originated in disrupted GCs or escaped from existing GCs. We identify such stars in the metallicity range -3.0 < [Fe/H] < 0.0 from a sample of ~36,800 giant stars observed in the Sloan Digital Sky Survey and Large Sky Area Multi-Object Fiber Spectroscopic Telescope survey, and present their dynamical properties. The N-rich population (NRP) and N-normal population (NNP) among our giant sample do not exhibit similarities in either in their metallicity distribution function (MDF) or dynamical properties. We find that, even though the MDF of the NRP looks similar to that of the MW's GCs in the range of [Fe/H] < -1.0, our analysis of the dynamical properties does not indicate similarities between them in the same metallicity range, implying that the escaped members from existing GCs may account for a small fraction of our N-rich stars, or the orbits of the present GCs have been altered by the dynamical friction of the MW. We also find a significant increase in the fraction of N-rich stars in the halo field in the very metal-poor (VMP; [Fe/H] < -2.0) regime, comprising up to ~20% of the fraction of the N-rich stars below [Fe/H] = -2.5, hinting that partially or fully destroyed VMP GCs may have in some degree contributed to the Galactic halo. A more detailed dynamical analysis of the NRP reveals that our sample of N-rich stars do not share a single common origin. Although a substantial fraction of the N-rich stars seem to originate from the GCs formed in situ, more than 60% of them are not associated with those of typical Galactic populations, but probably have extragalactic origins associated with Gaia Sausage/Enceladus, Sequoia, and Sagittarius dwarf galaxies, as well as with presently unrecognized progenitors.

• Journal of the Korean earth science society
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• v.38 no.3
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• pp.209-221
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• 2017

We investigate the star formation rate, stellar mass, and gas-phase metallicity of local starburst galaxies with different star formation time scales based on their optical spectra. The observation is made using the longslit spectrograph attached to the 4K CCD on the Bohyunsan Optical Astronomy Observatory 1.8m telescope, targeting 21 Wolf-Rayet galaxies as young starbursts and 13 UV excess galaxies as slightly older starbursts. A Baldwin-Phillips-Terlevich diagram analysis shows that 50% of the observed targets are pure star-forming galaxies while only 15% are classified as Active Galactic Nuclei. Fraction of galaxies that reside in composite region is higher in UV excess galaxies than in Wolf-Rayet galaxies, suggesting that the AGN development requires extra time after the onset of the star formation. Most of the observed starburst galaxies have stellar masses of $10^{9-11}M_{\odot}$ and stellar formation rates of $0.01-100M_{\odot}yr^{-1}$, and their star formation rates are consistent with that of the SDSS star forming main sequence galaxies of similar stellar mass. There is no significant difference between Wolf-Rayet galaxies and UV excess galaxies in terms of the stellar mass and star formation rate. We also see a mass-metallicity relation for local starbursts with slightly lower metallicity for a given stellar mass, which implies the existence of a strong feedback activity due to the star formation in these galaxies.

• Journal of the Korean Magnetics Society
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• v.25 no.6
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• pp.175-179
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• 2015

The half-metallicity and magnetism of the (001) surfaces of $(AlP)_1/(CrP)_1$ superlattice were investigated by means of FLAPW (Full-potential Liniarized Augmented Plane Wave) method. We considered four types of (001) surface termination, i.e., Al(S)-, Cr(S)-, P(S)Al(S-1)- and P(S)Cr(S-1)-term systems. We found that only Cr(S)-term system maintains the half-metallicity at the surface as only this system has the calculated magnetic moment of integer number of bohr magnetons. The magnetic moment of Cr(S) atom in the system was $3.02{\mu}_B$ which was increased from the bulk value by the effects of band narrowing and increased spin-splitting at the surface. The electronic density of states of the P(S) atom in the P(S)Al(S-1)-term showed very sharp surface states due to the broken $p_z$ bonds at the surface. We found there is still a strong p-d hybridization between the P(S) and Cr(S-1) layers in the P(S)Cr(S-1)-term which causes a considerable increase of magnetic moment of P(S) atom.

• The Bulletin of The Korean Astronomical Society
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• v.33 no.1
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• pp.27.2-27.2
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• 2008

• The Bulletin of The Korean Astronomical Society
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• v.33 no.1
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• pp.30.1-30.1
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• 2008