• 천문학논총
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• 제32권1호
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• pp.151-153
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• 2017

Early-type galaxies (ETGs) are generally dominated by old low-mass stars, which are not very productive of dust, and hot interstellar plasmas, which are very destructive of dust. Thus ETGs provide harsh environments for survival of dust. It has been found that some ETGs contain a large amount of dust, and yet its supply mechanism is not understood well. We present the result of a systematic study of dust in ETGs with the AKARI mid- and far-infrared all-sky surveys. From the AKARI result and the Ks band data obtained by ground-based telescopes, we find that there is a global correlation between the dust mass and stellar luminosity. We also compare the AKARI all-sky survey result with the CO data to discuss origins of dust in ETGs.

• 천문학회보
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• 제41권2호
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• pp.32.1-32.1
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• 2016

Early-type galaxies (ETGs) are supposed to follow the virial relation M ~ sigma^2 * R_e, with M being the galaxy mass, sigma being the stellar velocity dispersion, and R_e being the (2D) effective radius. I apply this relation to (a) the ATLAS3D sample and (b) the sample of Saglia et al. (2016). The two datasets reveal a statistically significant tilt of the empirical relation relative to the theoretical virial relation such that M ~ (sigma^2 * R_e)^0.92 with zero intrinsic scatter. This tilt disappears when replacing R_e with the semi-major axis of the projected half-light ellipse, a. Accordingly, a, not R_e, is the correct proxy for the scale radius of ETGs. By geometry, this implies that early-type galaxies are axisymmetric and oblate in general, in agreement with recent results from modeling based on kinematics and light distributions.

• Journal of Astronomy and Space Sciences
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• 제20권1호
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• pp.63-82
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• 2003

Schwarzschild의 선형 계획법을 사용하여 평형상태에서의 구형은하를 기술하는 비 충돌 볼쯔만 방정식의 일반적인 해를 얻었다. 이 계산으로부터 de Vaucouleurs의 광도 법칙을 만족하고 무거운 헤일로를 포함하는 등방성 구형은하의 존재를 확인할 수 있었다. 타원은하에서 속도 분산 곡선이 편평하거나 서서히 증가하는 현상은 암흑물질에 의한 질량대 광도비의 증가로 해석할 수 있다. 이런 경우 암흑물질의 밀도 분포는 core반경이 은하의 유효반경보다 훨씬 작은 값을 갖는다는 사실을 확인할 수 있었다.

• 천문학회보
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• 제37권1호
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• pp.61.1-61.1
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• 2012

We examine both optical and optical/near-infrared (NIR) color distributions of the globular cluster (GC) system in the core of the Coma cluster of galaxies (Abell 1656), centered on the giant elliptical galaxy NGC 4874, to study how non-linearities in the color-metallicity relations of GC systems in large elliptical galaxies are linked to bimodal optical color distributions. Since optical-NIR color distributions of extragalactic GC systems reflect the underlying features of the metallicity distributions, we also present the color-color relation for this GC system. In order to do this, we combine F160W ($H_{160}$) NIR imaging data acquired with the Wide Field Camera 3 IR Channel (WFC3/IR), newly installed on Hubble Space Telescope (HST), with F475W ($g_{475}$) and FF814W ($I_{814}$) optical imaging data from the HST Advanced Camera for Surveys (ACS). To quantitatively explain the feature of color distributions, we use the Gaussian Mixture Modeling (GMM) code. Finally, we show the radial distribution of the GCs in the field of NGC 4874.

• 천문학회보
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• 제36권2호
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• pp.70.2-70.2
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• 2011

We present a study of the metallicities, ages, and alpha-elements of globular clusters (GCs) in nearby giant elliptical galaxies (gEs) (M87, M49, M60, NGC 5128, NGC 1399, NGC 1407, and NGC 4636) using data in the literature. We used only the data for the GCs derived from the comparison of absorption line indices with the single stellar population model. The metallicity distributions of GCs in these gEs are bimodal, showing the existence of metal-poor and metal-rich populations. All these gEs harbor young GCs with ages less than 5 Gyr as well as old GCs. The mean age of the metal-rich GCs ([Fe/H]>-0.9) is about 3 Gyr younger than that of the metal-poor GCs. The mean values for the alpha-elements of the GCs are smaller than or similar to that of the Milky Way GCs. All the GCs in these gEs show an age-metallicity relation; the larger the metallicities of the GCs are, the younger the GCs are. Old GCs with ages > 10 Gyr have two distinct sub-populations (metal poor and metal rich), while young GCs show a broad metallicity distribution with a single peak. We discuss these results in relation with the formation of GCs in gEs.

• 천문학회보
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• 제35권2호
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• pp.42.1-42.1
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• 2010

We present ultraviolet (UV) photometric properties of dwarf galaxies in Fornax cluster and Ursa Major group in comparison with Virgo cluster using GALEX data. We construct UV color-magnitude relations (CMRs) of dwarf galaxies matching with available optical photometry and SDSS data. Majority of dwarf galaxies in Fornax cluster show sequence in UV CMRs consistent with that of dwarf elliptical (dEs) in Virgo cluster indicating similar age and metallicity properties of dEs in two clusters. The dS0 sequence in Fornax cluster is not distinct as much as that in Virgo cluster. Dwarf galaxies in outer region of the Fornax cluster show more bluer UV colors with a wide scatter in CMRs, which indicates recent star formation activity. We show that the UV colors of dwarf galaxies are related with the distribution and strength of the X-ray emission in the cluster. In contrast to the Fornax cluster, most dwarf galaxies in Ursa Major group are located in the blue cloud showing recent or on-going star formation, and few galaxies show characteristics of dEs. We discuss relationship between UV properties of dwarf galaxies and different environment of cluster.

• 천문학회보
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• 제36권2호
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• pp.76.1-76.1
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• 2011

We present ultraviolet (UV) photometric properties of galaxies in two clusters, the Fornax and Virgo, with different dynamical conditions. We construct UV color-magnitude relations (CMRs) of galaxies in the Fornax and Virgo clusters using GALEX UV data matching with optical B band data. Elliptical and lenticular galaxies locate on red sequence in UV CMRs and show UV upturn phenomenon in both clusters. While dwarf lenticular galaxies (dS0s) in the Fornax also follow the extension of red sequence of giant early type galaxies, they are redder than dS0s in the Virgo at a given magnitude. We also investigated the effect of neighbor galaxies and cluster environment to the UV properties. In the space of projected clustercentric radius and projected nearest neighbor galaxy distance, we found that red (NUV-B>3) galaxy fraction of the Fornax depends entirely on clustercentric radius. However, in the case of Virgo, galaxy colors are also affected by interactions between galaxies outside the cluster virial radius. We suggest that UV properties of early-type galaxies in the Fornax cluster is likely consistent with its dynamically evolved system compared to the Virgo cluster.

• 천문학회보
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• 제36권2호
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• pp.66.2-66.2
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• 2011

The unexpected rising flux of early-type galaxies at decreasing ultraviolet (UV) wavelengths is a long-standing mystery. One important observational constraint is the correlation between UV-optical colours and Mg2 line strengths found by Burstein et al. (1988). The simplest interpretation of this phenomenon is that the UV strength is related to the Mg line-strength. Under this assumption, we expect galaxies with larger Mg gradients to have larger UV colour gradients. By combining UV imaging from GALEX, optical imaging from MDM and SAURON integral-field spectroscopy, we investigate the spatially-resolved relationships between UV colours and stellar population properties of 34 early-type galaxies from the SAURON survey sample. We find that galaxies with old stellar populations show tight correlations between the FUV colours (FUV-V and FUV-NUV) and the Mg b index, $H{\beta}$ index and metallicity [Z/H]. We have also derived logarithmic internal radial colour, measured line strength and derived stellar population gradients for each galaxy and again found a strong dependence of the FUV-V and FUV-NUV colour gradients on both the Mg b line-strength and the metallicity gradients. In particular, global gradients of Mg b and [Z/H] with respect to the UV colour across galaxies are consistent with their local gradients within galaxies, suggesting that the global correlations also hold locally. From a simple model based on multi-band colour fits of UV upturn and UV-weak galaxies, we have identified a plausible range of parameters that reproduces the observed radial colour profiles. In these models, the centers of elliptical galaxies, where the UV flux is strong, are enhanced in metals by roughly 60% compared to UV-weak regions.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2009년도 한국우주과학회보 제18권2호
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• pp.35.3-36
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• 2009

To investigate properties of the stellar contents of the resolved asymptotic giant branch stars in the nearby dwarf galaxies, we obtained wide-field JHKs images of the dwarf irregular galaxies NGC 6822, IC 1613 and the dwarf elliptical galaxy NGC 205, using the WIRCam near-infrared imager of the CFHT. The obtained (J-Ks, Ks) and (H-Ks, Ks) color-magnitude diagrams for the resolved stars in the galaxies contain populations of foreground stars, super giant stars, red giant stars and the asymptotic giant branch (AGB) stars. Using corollary photometric data in the visible bands, AGB stars were selected in the color-magnitude diagrams with a wide wavelength baseline in color indices. In color-color diagrams of the resolved AGB stars, we identified C stars from M giant stars for each galaxies, i.e., 726 C stars in NGC 6822, 126 C stars in IC 1613 and 593 C stars in NGC 205. The number ratios of C stars to M-giants were estimated to be $0.59\pm0.03$ in NGC 6822, $0.30\pm0.03$ in IC 1613 and $0.14\pm0.01$ in NGC 205. From analyses of the correlations of the spatial distribution of the C/M ratios with the HI properties and dynamical structures of the target galaxies, we discuss environmental effects of the star formation in the galaxies. We also discuss the epochs of the AGB star formation in the galaxies by comparing theoretical isochrones with the color distributions and luminosity functions of the AGB stars.

• 천문학회보
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• 제40권1호
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• pp.48.2-48.2
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• 2015

van Dokkum and Conroy revisited the strong Na I lines at $8200{\AA}$ found in some giant elliptical galaxies and interpreted it as evidence for bottom-heavy initial mass function. Jeong et al. later found a lot of galaxies showing strong Na D doublet absorption line at $5900{\AA}$ (Na D excess objects; a.k.a. NEOs) and showed that their origins can be different for different types of galaxies. While the excess in Na D seems related with interstellar medium in late-type galaxies, smooth-looking early-type NEOs suggest no compelling sign of ISM contributions. To test this finding, we measured doppler shift in the Na D line. We hypothesized that ISM is more likely to show blueshift due to outflow caused by either star formation or AGN activities. In order to measure the doppler shift, we tried both Gaussian and Voigt functions to fit each galaxy spectrum near the Na D line. We found that Voigt profiles reproduce the shapes of the Na D lines markedly better. Many of late-type NEOs clearly show blueshift in their Na D lines, which is consistent with the former interpretation that the Na D excess found in them is related with star formation-caused gas outflow. On the contrary, early-type NEOs do not show any notable doppler component, which is also consistent with the interpretation of Jeong et al. that the Na D excess in early-type NEOs is likely not related with ISM activities but purely stellar in origin.