• 천문학회보
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• 제38권2호
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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.

• 천문학회보
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• 제35권2호
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• pp.27.1-27.1
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• 2010

Globular cluster (GC) systems in most galaxies, particularly in ellipticals, show bimodal color distributions. Because broadband colors trace metallicity at old ages, this phenomenon has been commonly interpreted as bimodal metallicity distributions, implying the presence of two sub-populations in the globular cluster system within a galaxy. However, a new explanation has recently been proposed, in which the non-linear nature of color-metallicity relations induced by horizontal-branch stars can produce bimodal color distributions even from unimodal metallicity distributions. In this study, we put these two explanations to the test on the origin of color bimodality, using multi-band (U,B,V and I) photometry of globular clusters in NGC 1399, the central giant elliptical galaxy in Fornax galaxy cluster. We find significant changes in the morphology of color distributions when using different colors. The observation is also well reproduced by the Monte Carlo realization of GC color when a unimodal metallicity distribution and the theoretical non-linear color-metallicity relations are assumed. We discuss the implications regarding theories on galaxy formation and evolution.

• 대한원격탐사학회지
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• 제32권5호
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• pp.539-550
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• 2016

육상의 수체를 탐지하는 것은 홍수, 태풍, 지진해일과 같은 재해 모니터링에 있어 핵심적인 사항이며, 습지, 빙하 등 지표 수자원의 시 공간적 변화를 파악하는데 중요한 역할을 한다. 본 연구에서는 Kompsat-5 SAR 영상으로부터 육상의 수체를 탐지하기 위하여 임계값에 기반한 접근방법의 적용성을 분석하고, 다양한 임계값 설정 기법의 탐지 성능을 평가하였다. 또한 SAR 영상의 스펙클 필터링이 임계값 설정에 미치는 영향을 분석하였으며, 영상에서 수체가 차지하는 비율에 따른 탐지 성능의 변화에 대한 정량적인 평가를 수행하였다. 추가적으로 탐지 성능을 향상시키기 위해 히스토그램의 bimodality 검정과 majority filtering 처리를 활용하는 새로운 알고리즘을 제안하였다. 세종시 지역의 사례의 경우 제안된 알고리즘을 통해 최종적으로 약 96%의 탐지율과 0.3%의 오탐지율로 수체를 탐지할 수 있음을 보였다.

• Journal of Astronomy and Space Sciences
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• 제29권2호
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• pp.191-194
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• 2012

The observed distribution of a blending-corrected sample of Einstein ring crossing times, $t_E$, for microlensing events toward the galactic bulge/bar are analyzed. An inspection of the distribution of crossing times suggests that it may be bimodal, indicating that two populations of lenses could be responsible for observed microlensing events. Given the possibility that microlensing in this direction can be due to the two most common classes of stars, main-sequence and white dwarf, we analyze and show via Monte Carlo simulations that the observed bimodality of $t_E$ can be derived from their accepted mass functions, and the density distributions of both stellar populations in the galactic disk and bulge/bar, with a transverse velocity distribution that is consistent with the density distribution. Kolmogorov-Smirnov (KS) one sample tests shows that a white dwarf population of about 25% of all stars in the galaxy agrees well with the observed bimodality with a KS significance level greater than 97%. This is an expanded and updated version of a previous investigation (Wickramasinghe, Neusima, & Struble, in Mao 2008). A power-point version of the talk, with introductory figures, is found at: https://sites.google.com/site/rhkochconference/agenda-1/program.

• 천문학회보
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• 제34권1호
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• pp.145.2-145.2
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• 2009

• 천문학논총
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• 제30권2호
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• pp.255-259
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• 2015

Recent narrow-band Ca photometry discovered two distinct red giant branch (RGB) populations in some massive globular clusters (GCs) including M22, NGC 1851, and NGC 288. In order to investigate the differences in light/heavy elements abundances between the two subpopulations, we have performed low-resolution spectroscopy for stars on the two RGBs in these GCs. We find a significant difference (more than $4{\sigma}$) in calcium abundance from the spectroscopic HK' index for both M22 and NGC 1851. We also find a more than $8{\sigma}$ difference in CN band strength between the Ca-strong and Ca-weak subpopulations. For NGC 288, however, we detect the presence of a large difference only in the CN strength. The calcium abundances of the two subpopulations in this GC are identical within errors. We also find interesting differences in CN-CH relations among these GCs. While CN and CH indices are correlated in M22, they show an anti-correlation in NGC 288. However, NGC 1851 shows no difference in CH between two groups of stars having different CN strengths. The CN bimodality in these GCs could be explained by pollution from intermediate-mass asymptotic giant branch stars and/or fast-rotating massive stars. For the presence or absence of calcium bimodality and the differences in CN-CH relations, we suggest these would be best explained by how strongly type II supernovae enrichment has contributed to the chemical evolutions of these GCs.

• 한국데이터정보과학회:학술대회논문집
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• 한국데이터정보과학회 2003년도 추계학술대회
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• pp.19-30
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• 2003

In this paper we present a new family of distributions that allows a continuous variation not only from normality to non-normality but also from unimodality to bimodality. Its properties are especially useful in studying and making inferences about models involving the univariate truncated normal distribution. The properties of the family and its applications are given.

• 응용통계연구
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• 제22권4호
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• pp.879-891
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• 2009

We developed the projected l-axial skew-normal(LASN) family of distributions for I-axial data. The LASN family of distributions contains the semicircular skew-normal(SCSN) and the circular skew-normal(CSN) families of distributions as special cases. The LASN densities are similar to the wrapped skew-normal densities for the small values of the scale parameter. However CSN densities have more heavy tails than those of the wrapped skew-normal densities on the circle. Furthermore the CSN densities have two modes as the scale parameter increases. The LASN distribution has very convenient mathematical features. We extend the LASN family of distributions to a bivariate case.

• Communications for Statistical Applications and Methods
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• 제16권2호
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• pp.389-396
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• 2009

Approximations for the null distribution of a test statistic arising in multivariate analysis to test homogeneity of variances and a mixture of two beta distributions by making use of a product of beta baseline density function and a polynomial adjustment, so called beta-polynomial density approximant, are discussed. Explicit representations of density and distribution approximants of interest in each case can easily be obtained. Beta-polynomial density approximants produce good approximation over the entire range of the test statistic and also accommodate even the bimodal distribution using an artificial example of a mixture of two beta distributions.

• 천문학회보
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• 제35권2호
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• pp.31.1-31.1
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• 2010

We present a spectroscopic study of the globular clusters (GCs) in the giant elliptical galaxy (gE) NGC 4636 in the Virgo cluster. Line indices of GCs in NGC 4636 are measured from the integrated spectra obtained with Faint Object Camera and Spectrograph (FOCAS) on the Subaru Telescope. We derive the [Fe/H] of the GCs in NGC 4636 using Brodie & Huchra (1990) method. The [Z/H], age, and alpha-element of the GCs are obtained from the comparison of the Lick line indices with the SSP model grid. We investigate the distribution and radial variation of the metallicity, age, and alpha-element of NGC 4636 GCs. The metallicity distribution of NGC 4636 GCs shows a bimodality. The chemical properties of these GCs show little radial variation. These results will be discussed with regard to the formation and evolution of NGC 4636.