• The Bulletin of The Korean Astronomical Society
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• v.31 no.2
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• pp.64.1-64.1
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• 2006

• The Bulletin of The Korean Astronomical Society
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• v.30 no.1
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• pp.66.1-66.1
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• 2005

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.37.1-37.1
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• 2013

We present our study of analyzing the photometric properties of the globular cluster (GC) system which resides in the extended halo of the central bright Coma cluster galaxy NGC 4874. The core of the Coma cluster of galaxies (Abell 1656) was observed with both the HST Advanced Camera for Surveys (ACS) in the F475W (g475) and F814W (I814) and Wide Field Camera 3 IR Channel (WFC3/IR) in the F160W (H160) filters. The data analysis procedure and GC candidate selection criteria are briefly described. We investigate the interesting "tilt" features in color-magnitude diagrams for this GC system and their link to the nonlinear color-metallicity relation for GCs. The NGC 4874's GC system exhibits a bimodal distribution in the optical g475-I814 color and much more than half the GCs fall in the red side at g475-I814 ~ 1.1. This bimodality is weakened in the optical-IR I814-H160 color; the quantitative analysis on the features of both color distributions using the Gaussian Mixture Modeling code proves the bimodalities are different. Both colors, thus, cannot linearly reflect the bimodality of an underlying metallicity, supporting the suggestion that observed bimodalities in extragalactic GC colors are the metallicity-to-color projection effect.

• 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.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.535-537
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• 2015

Interpreting ultraviolet-to-infrared (UV-to-IR) observations of galaxies in terms of constraints on physical parameters-such as stellar mass ($M_{\ast}$) and star formation rate (SFR)-requires spectral synthesis modelling. We investigate how increasing the level of sophistication of the standard simplifying assumptions of such models can improve estimates of galaxy physical parameters. To achieve this, we compile a sample of 1048 galaxies at redshifts 0.7 < z < 2.8 with accurate photometry at rest-frame UV to near-IR wavelengths from the 3D-HST Survey. We compare the spectral energy distributions of these galaxies with those from different model spectral libraries to derive estimates of the physical parameters. We find that spectral libraries including sophisticated descriptions of galaxy star formation histories (SFHs) and prescriptions for attenuation by dust and nebular emission provide a much better representation of the observations than 'classical' spectral libraries, in which galaxy SFHs are assumed to be exponentially declining functions of time, associated with a simple prescription for dust attenuation free of nebular emission. As a result, for the galaxies in our sample, $M_{\ast}$ derived using classical spectral libraries tends to be systematically overestimated and SFRs systematically underestimated relative to the values derived adopting a more realistic spectral library. We conclude that the sophisticated approach considered here is required to reliably interpret fundamental diagnostics of galaxy evolution.

• Journal of The Korean Astronomical Society
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• v.44 no.1
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• pp.1-11
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• 2011

A sample of 145 JHK-2MASS observations of NGC open star clusters is studied, of which 132 have never been studied before. Twelve are classified as non-open clusters and 13 are re-estimated self-consistently, after applying the same methods in order to compare and calibrate our reduction procedures. The fundamental and structural parameters of the 120 new open clusters studied here are derived using color-magnitude diagrams of JHK Near-IR photometry with the fitting of solar metallicity isochrones. We provide here, for the first time, a catalog of the main parameters for these 120 open clusters, namely, diameter, distance, reddening and age.

• Bulletin of the Korean Space Science Society
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• 2006.10a
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• pp.33-33
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• 2006

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.355-358
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• 2015

We have conducted a near-infrared monitoring campaign at the UK InfraRed Telescope (UKIRT), of the Local Group galaxy M33. The main aim was to identify stars in the very final stage of their evolution, and for which the luminosity is more directly related to the birth mass than the more numerous less-evolved giant stars that continue to increase in luminosity. The pulsating giant stars (AGB and red supergiants) are identified and their distributions are used to derive the star formation rate as a function of age. These stars are also important dust factories; we measure their dust production rates from a combination of our data with Spitzer Space Telescope mid-IR photometry. The mass-loss rates are seen to increase with increasing strength of pulsation and with increasing bolometric luminosity. Low-mass stars lose most of their mass through stellar winds, but even super-AGB stars and red superginats lose ~40% of their mass via a dusty stellar wind. We construct a 2-D map of the mass-return rate, showing a radial decline but also local enhancements due to agglomerations of massive stars. By comparing the current star formation rate with total mass input to the ISM, we conclude that the star formation in the central regions of M33 can only be sustained if gas is accreted from further out in the disc or from circum-galactic regions.

• Solar Energy
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• v.12 no.2
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• pp.18-27
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• 1992

Multilayer coatings of $WO_3$ were deposited by the sol-gel technique on microscope slide glass and ITO coated glass. These films were characterized optically, chemically, and structurally by XRD, spectro-photometry, DTA/TGA, SEM/EDAX and RBS. Uniform $WO_3$ sol-gel films were dip coated on slide glass at dipping speed of 5mm/s. This sample indicated a low near IR transmittance in optical properties as a result of coloration using a dilute HCI electrolyte as the $H^+$ion sources. Differential thermal analysis results have allowed the accurate determination of the formation temperature of the $WO_3$ crystalline phase from the gel data in the range of $380^{\circ}C{\sim}500^{\circ}C$, consistent with crystallization temperature of sol-gel film. RBS spectrometry was performed on the uncolored $WO_3$ sol-gel film, yielding a chemical composition of $WO_3$.