• 천문학회지
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• 제50권5호
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• pp.151-155
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• 2017

We investigate the radio properties of the dwarf galaxy SDSS J133245.62+263449.3 which shows optical signatures of black hole activity. Dwarf galaxies are known to host intermediate mass black holes (IMBHs) with masses $M_{BH}{\sim}10^{4-6}M_{\odot}$, some of them being radio loud. Recently, Reines et al. (2013) found dwarf galaxy candidates which show signatures of being black hole hosts based on optical spectral lines. SDSS J133245.62+263449.3 is one of them; it shows a flux density of ~ 20 mJy at 1.4 GHz, which corresponds to $L_{1.4GHz}{\sim}10^{23}W\;Hz^{-1}$. This is much brighter than other black hole host dwarf galaxies. However, star formation activity can contribute to radio continuum emission as well. To understand the nature of the radio emission from SDSS J133245.62+263449.3, we imaged this radio loud dwarf galaxy at low frequencies (325 MHz and 610 MHz) using the Giant Metrewave Radio Telescope (GMRT). We present here the high resolution images from our GMRT observations. While we detect no obvious extended emission from radio jets from the central AGN, we do find the emission to be moderately extended and unlikely to be dominated by disk star formation. VLBI observations using the Korean VLBI Network (KVN) are now being planned to understand the emission morphology and radiation mechanism.

• 천문학회보
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• 제43권1호
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• pp.38.3-38.3
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• 2018

The properties of unseen high-redshift sources (and sinks) are encoded in the 3D structure of the cosmic 21-cm signal. Here I introduce a flexible parametrization for high-z galaxies' properties, including their star formation rates, ionizing escape fraction and their evolution with the mass of the host dark matter halos. With this parametrization, I self-consistently calculate the corresponding 21-cm signal during reionization and the cosmic dawn. Using a Monte Carlo Markov Chain sampler of 3D simulations, 21CMMC, I demonstrate how combining high-z luminosity functions with a mock 21-cm signal can break degeneracies, resulting in ~ percent level constraints on early universe astrophysics.

• 천문학회보
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• 제43권1호
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• pp.59.2-59.2
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• 2018

Recent studies of galaxy clusters have shown that the galaxy clusters in dense environment tend to have lower star formation rate in local universe with z < 1. However, this correlation is not significant in galaxy clusters with z > 1. The study of galaxy clusters around z=1 can yield insight into cosmological galaxy evolution. Nevertheless, the identification of galaxy clusters beyond the scope of immediate local universe requires wide field data in optical and near-infrared bands. By incorporating data from Canada-France-Hawaii Telescope Legacy Survey(CFHTLS) and Infrared Medium-Deep Survey(IMS), the photometric redshifts of galaxies in CFHTLS W2 field were calculated. Using spatial distribution and photometric redshifts, the galaxies in the field were divided into redshift bins. The image of each redshift bin was analyzed by measuring the number density within proper distance of 1Mpc. By comparing high density regions in consecutive redshift bins, we identified the cluster candidates and mapped the large-scale structure within the CFHTLS W2 field.

• 천문학회지
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• 제52권5호
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• pp.181-205
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• 2019

There is evidence that the luminosities of Type Ia supernova (SN Ia) depend on their environments. While the impact of this trend on estimating cosmological parameters is widely acknowledged, the origin of this correlation is still under debate. In order to explore this problem, we first construct the YONSEI (YOnsei Nearby Supernova Evolution Investigation) SN catalog. The catalog consists of 1231 spectroscopically confirmed SNe Ia over a wide redshift range (0.01 < z < 1.37) from various SN surveys and includes light-curve fit data from two independent light-curve fitters, SALT2 and MLCS2k2. For a sample of 674 host galaxies, we use the stellar mass and the star formation rate data in Kim et al. (2018). We find that SNe Ia in low-mass and star-forming host galaxies are $0.062{\pm}0.009mag$ and $0.057{\pm}0.010mag$ fainter than those in high-mass and passive hosts, after light-curve corrections with SALT2 and MLCS2k2, respectively. When only local environments of SNe Ia (e.g., locally star-forming and locally passive) are considered, this luminosity difference increases to $0.081{\pm}0.018mag$ for SALT2 and $0.072{\pm}0.018mag$ for MLCS2k2. Considering the significant difference in the mean stellar population age between the two environments, this result suggests that the luminosity evolution of SNe Ia with redshift is most likely the origin of the environmental dependence.

• 천문학회보
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• 제42권1호
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• pp.49.2-49.2
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• 2017

Star forming dwarf galaxies in various environments are attractive objects for investigating the environmental effects on chemical evolution of dwarf galaxies. Using SDSS DR7 spectroscopic data and GALEX ultraviolet (UV) imaging data, we study the chemical properties of star forming dwarf galaxies in various environments of the Virgo cluster, Ursa Major group, and field. We derived gas-phase abundance, galaxy mass, and UV specific star formation rate (sSFR) of subsample, early-type (ETD) and late-type star forming dwarf (LTD) galaxies, which are divided by visually classified galaxy morphology. We found no O/H enhancement of LTDs in cluster and group environments compared to the field, implying no environmental dependence of the mass-metallicity relation for LTDs. LTDs in the Virgo cluster and Ursa Major group have similar sSFR at a given galaxy mass, but they exhibit systematically lower sSFR than those in isolated field environment. We suggest that LTDs in the Virgo cluster are an infalling population that was recently accreted from the outside of the cluster. We found that ETDs in the Virgo cluster and Ursa Major group exhibit enhanced O/H compared to those in the field. However, no distinct difference of N/O of galaxies between different environments. The chemically evolved ETDs in the Virgo cluster and Ursa Major group also show similar mass-sSFR relation, but systematically lower sSFR at a fixed galaxy mass compared to the field counterparts. We suggest that ETDs in the Virgo cluster and Ursa Major group have evolved under the similar local environments. We also discuss the evolutionary path of ETDs and LTDs with respect to the environmental effects of ram pressure stripping and galaxy interaction/merging.

• 천문학회보
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• 제36권2호
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• pp.69.1-69.1
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• 2011

While the Virgo Cluster Catalog (VCC) is well established catalog from deep photographic plate survey, with available survey data recently released (e.g., SDSS), it can be further updated concerning the membership and morphology of galaxies. While membership and morphology of galaxies included in the VCC are based on the single band imaging data, thanks to the multi-color imaging and spectroscopic observations of SDSS, we are able to revise the membership and morphology of sample galaxies in the fields of the Virgo cluster. We present a new catalog of galaxies in the Virgo cluster using SDSS DR7 data, the extended Virgo cluster catalog. Using SDSS imaging and spectroscopic data, we introduce two kinds of galaxy classifications which are complementary each other. In addition to traditional morphological classification by visual inspection of the images ("Primary Classification"), we also attempt to classify galaxies with the spectroscopic features ("Secondary Classification"). The primary classification is basically based on the scheme of galaxy morphological classification of VCC. The secondary classification relies on the SED shape and presence of emission/absorption lines returned from SDSS. Our morphological classifications allow to study the evolution and associated star formation histories of galaxies in the Virgo cluster.

• 천문학회보
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• 제37권2호
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• pp.77.1-77.1
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• 2012

We have performed cosmological hydrodynamic simulations that include the effects of radiative heating/cooling, star formation, feedback by supernova explosions, and metallicity evolution. Our simulations cover a cubic box of a side length 4 Mpc/h with 130 million particles. The mass of each particle is $3.4{\times}10^4M_{\odot}$, thus sub-galactic mini-halos can be resolved with more than hundred particles. Our simulation follows the whole formation process of the mini-halos (M< $10^7M_{\odot}$) around dwarf galaxies. We discuss various properties of the mini halos such as mass function, specific frequency, baryon-to-dark matter ratio, metallicity, spatial distribution, and orbit eccentricity distribution as functions of redshift and host galaxy mass. We also discuss how the formation and evolution of the mini halos are affected by the epoch of the reionization.

• 천문학회보
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• 제39권2호
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• pp.47.2-47.2
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• 2014

Bulges of galaxies are thought to have formed and grown at least in part through galaxy mergers, and thus an accurate derivation of their properties can be an effective course to test/confirm our understanding on their formation and evolution in the standard hierarchical merger paradigm. We have generated a sample of galaxy bulges (n = 15,423) in the nearby (0.005 < z < 0.05) universe from the SDSS DR7 and GALEX GR6plus7 databases and derived their structural and photometric properties by means of SExtractor and GALFIT application. Most notable properties include bulge-to-total luminosity ratio, effective radius, disk scale length, ellipticity, and position angle. The UV properties of the bulges have also been analyzed to infer their recent star formation history. A spectroscopic analysis has been performed using their absorption and emission line strengths measured and released by the OSSY team. We present our preliminary results from our investigation mainly focused on stellar population properties and discuss their implications on the formation of bulges.

• 천문학회보
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• 제44권1호
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• pp.37.2-37.2
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• 2019

Recent investigations of the double red clump in the color-magnitude diagram of the Milky Way bulge cast serious doubts on the structure and formation origin of the outer bulge. Unlike previous interpretation based on an X-shaped bulge, stellar evolution models and CN-band observations have suggested that this feature is another manifestation of the multiple stellar population phenomenon observed in globular clusters (GCs). This new scenario requires a significant fraction of the outer bulge stars with chemical patterns uniquely observed in GCs. Here we show from homogeneous high-quality spectroscopic data that the red giant branch stars in the outer bulge ($>5.5^{\circ}$ from the Galactic center) are clearly divided into two groups according to Na abundance in the [Na/Fe] - [Fe/H] plane. The Na-rich stars are also enhanced in Al, while the differences in O and Mg are not observed between the two Na groups. The population ratio and the Na and Al differences between the two groups are also comparable with those observed in metal-rich GCs. Since these chemical patterns and characteristics are only explained by stars originated in GCs, this is compelling evidence that the outer bulge was mostly assembled from disrupted proto-GCs in the early history of the Milky Way. We will also discuss the implications of this result on the formation of the early-type galaxies in general.

• 천문학회보
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• 제44권2호
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• pp.39.1-39.1
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• 2019

We trace the cosmological origin of satellites around isolated dwarf galaxies using a very high resolution (12 pc/h) cosmological hydrodynamic zoom simulation. To realistically describe the formation and evolution of small-mass stellar satellites, our model includes a full baryonic physics treatment. We find that the mini-halos form objects resembling dwarf galaxies. The majority of their star forming gas is accreted after reionization, thus the survival of a mini-halo's gas to reionization is not an important factor. Instead, the key factor seems to be the ability for a mini-halo to cool its recently accreted gas, which is more efficient in more massive halos. Although the host galaxy is only a dwarf galaxy itself, we find that ram pressure is an efficient means by which accreted mini-halos lose their gas content, both by interacting with hot halo gas but also in direct collisions with the gas disk of the host. The satellites are also disrupted by the tidal forces near the center of the host galaxy. Compared to the disrupted satellites, surviving satellites are relatively more massive, but tend to infall later into the host galaxy, thus reducing the time they are subjected to destructive environmental mechanisms and dynamical friction.