• 천문학논총
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• 제30권2호
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• pp.267-268
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• 2015

One of the long-standing problems in modern astronomy is the curious division of globular clusters (GCs) into two groups, according to the mean period (<$P_{ab}$>) of type ab RR Lyrae variables. In light of the recent discovery of multiple populations in GCs, we suggest a new model explaining the origin of the Sandage period-shift and the difference in mean period of type ab RR Lyrae variables between the two Oosterhoff groups. In our models, the instability strip in the metal-poor group II clusters, such as M15, is populated by second generation stars (G2) with enhanced helium and CNO abundances, while the RR Lyraes in the relatively metal-rich group I clusters like M3 are mostly produced by first generation stars (G1) without these enhancements. This population shift within the instability strip with metallicity can create the observed period-shift between the two groups, since both helium and CNO abundances play a role in increasing the period of RR Lyrae variables. The presence of more metal-rich clusters having Oosterhoff-intermediate characteristics, such as NGC 1851, as well as of most metal-rich clusters having RR Lyraes with the longest periods (group III) can also be reproduced, as more helium-rich third and later generations of stars (G3) penetrate into the instability strip with further increase in metallicity. Therefore, although there are systems where the suggested population shift cannot be a viable explanation, for the most general cases, our models predict that RR Lyraes are produced mostly by G1, G2, and G3, respectively, for the Oosterhoff groups I, II, and III.

• 천문학논총
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• 제25권3호
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• pp.101-105
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• 2010

Taking the great advantage of Subaru's wide field coverage both in the optical and in the near infrared, we have been providing panoramic views of distant clusters and their surrounding environments over the wide redshift range of 0:4 < z < 3. From our unique data sets, a consistent picture has been emerging that the star forming activity is once enhanced and then truncated in galaxy groups in the outskirts of clusters during the course of cluster assembly at z < 1. Such activity is shifted into cluster cores as we go further back in time to z ~ 1.5. At z = 2 - 2.5, we begin to enter the epoch when massive galaxies are actually forming in the cluster core. And by z ~ 3, we eventually go beyond the major epoch of massive galaxy formation. It is likely that the environmental dependence of star forming activity is at least partly due to the external environmental effects such as galaxy-galaxy interaction in medium density regions at z < 1, while the intrinsic effect of galaxy formation bias overtakes the external effect at higher redshifts, resulting in a large star formation activity in the cluster center.

• 천문학회보
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• 제40권1호
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• pp.74.1-74.1
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• 2015

'The one to one correspondence model' defines the relation between a dark matter halo (DM halo) and a galaxy. A basic assumption of this model is that a more massive DM subhalo hosts a brighter galaxy. In a more improved version of the model we may be able to assign a mock galaxy with a morphological type. In this study, we are building a mock galaxy catalog using massive halo merging trees from the Horizon Run 4. We test various merging models to calculate the merging time scale of a subhalo along its merging tree. And we obtain the halo mass functions for major subhalos and satellite subhalos, separately, and compare them with the observed luminosity functions of major galaxies and satellite galaxies from the SDSS group catalog. Furthermore, we are going to make a range of mock galaxy catalogs and investigate their properties, such as spatial distributions, environmental effects, and morphologies.

• 천문학회보
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• 제45권1호
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• pp.34.1-34.1
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• 2020

The recent discovery of diffuse dwarf galaxies that are deficient in dark matter appears to challenge the current paradigm of structure formation in our Universe. We describe the numerical experiments to determine if the so-called dark matter deficient galaxies (DMDGs) could be produced when two gas-rich, dwarf-sized galaxies collide with a high relative velocity of ~ 300km/s. Using idealized high-resolution simulations with both mesh-based and particle-based gravito-hydrodynamics codes, we find that DMDGs can form as high-velocity galaxy collisions separate dark matter from the warm disk gas which subsequently is compressed by shock and tidal interaction to form stars. Then using a large simulated universe ILLUSTRISTNG, we discover a number of high-velocity galaxy collision events in which DMDGs are expected to form. However, we did not find evidence that these types of collisions actually produced DMDGs in the ILLUSTRISTNG100-1 run. We argue that the resolution of the numerical experiment is critical to realize the "collision-induced" DMDG formation scenario. Our results demonstrate one of many routes in which galaxies could form with unconventional dark matter fractions.

• 천문학회보
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• 제46권1호
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• pp.36.1-36.1
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• 2021

Galaxy clusters host Mpc-scale diffuse radio emission giving us evidence of large-scale magnetic fields in the Universe. It is relevant to understand magnetic field amplification processes occurring at the center and outskirts of galaxy clusters. Each of these processes are believed to give rise to observed radio haloes and radio relics, respectively. In this work, we focus on studying the continuum and polarised emission in radio relics. We use threedimensional magnetohydrodynamical simulations of merger shock waves propagating through a magnetized, turbulent intracluster medium. Our model includes the diffusive shock acceleration (DSA) of cosmic ray electrons, their spatial advection and energy losses at run-time. We discuss the relation between the mock observation features and the underlying morphology of the magnetic field.

• 천문학회보
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• 제42권2호
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• pp.46.2-46.2
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• 2017

We will present a discovery of an elliptical jellyfish galaxy in Abell 2670 (Sheen et al. 2017, ApJL, 840, L7). Our MUSE IFU spectra revealed a rotating gas disk in the center of the galaxy and long ionised gas tails emanating from the disk. Its one-sided tails and a tadpole-like morphology of star-forming blobs around the galaxy suggested that the galaxy is experiencing strong ram-pressure stripping in the cluster environment. Stellar kinematics with stellar absorption lines in the MUSE spectra demonstrated that the galaxy is an elliptical galaxy without any hint of a stellar disk. Then, the primary question would be the origin of the rich gas component in the elliptical galaxy. A plausible scenario is a wet merger with a gas-rich companion. In order to investigate star formation history of the system (the galaxy and star-forming blobs), we derived star-formation rate and metallicity from the MUSE spectra. Photometric UV-Optica-IR SED fitting was also performed using GALEX, SDSS, 2MASS and WISE data, to estimate dust and gas masses in the system. For a better understanding of star formation history and environmental effect of this galaxy, FIR/sub-mm follow-up observations are proposed.

• 천문학회보
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• 제37권1호
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• pp.33.2-33.2
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• 2012

Low-ionization nuclear emission-line region (LINER) galaxies constitute a major fraction of low-luminosity AGN population in the local Universe. In contrast to Seyfert galaxies, it is theoretically expected that LINERs would not have an outflow due to their low Eddington ratio. Using Keck/LRIS spectroscopy on a nearby LINER galaxy SDSS J091628.05+420818.7, we find a significant radial velocity offset for [OIII]${\lambda}$5007 emission line as - 50 km $s^{-1}$ blueshifted compared to systemic velocity of the galaxy, while other emission lines exhibit no or little offset. The observed [OIII] velocity offset possibly indicates an outflow of gas in the LINER galaxy, and it is probable that we only detected the [OIII] velocity offset because [OIII] ionization region is closer to the accretion disk, hence, more affected by an outflow. We further investigate the [OIII] velocity offset of -4000 SDSS AGN-host galaxies to compare the strength of AGN outflow. We find that a number of both LINER and Seyfert galaxies show [OIII] velocity offset, but the fraction of LINER galaxies with velocity offset is smaller than that of Seyfert galaxies. The preliminary results imply the presence of gas outflow in LINER galaxies, although outflow strength is probably weaker compared to Seyfert galaxies.

• 천문학논총
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• 제30권2호
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• pp.133-137
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• 2015

The Galactic center uniquely provides opportunities to resolve how star clusters form in neutral gas overdensities engulfed in a large-scale accretion flow. We have performed sensitive Green Bank 100m Telescope (GBT), Karl G. Jansky Very Large Array (JVLA), and Submillimeter Array (SMA) mapping observations of molecular gas and thermal dust emission surrounding the Galaxy's supermassive black hole (SMBH) Sgr $A^{\ast}$. We resolved several molecular gas streams orbiting the center on ${\gtrsim}10$ pc scales. Some of these gas streams appear connected to the well-known 2-4 pc scale molecular circumnuclear disk (CND). The CND may be the tidally trapped inner part of the large-scale accretion flow, which incorporates inflow via exterior gas filaments/arms, and ultimately feeds gas toward Sgr $A^{\ast}$. Our high resolution GBT+JVLA $NH_3$ images and SMA+JCMT 0.86 mm dust continuum image consistently reveal abundant dense molecular clumps in this region. These gas clumps are characterized by ${\gtrsim}100$ times higher virial masses than the derived molecular gas masses based on 0.86 mm dust continuum emission. In addition, Class I $CH_3OH$ masers and some $H_2O$ masers are observed to be well associated with the dense clumps. We propose that the resolved gas clumps may be pressurized gas reservoirs for feeding the formation of 1-10 solar-mass stars. These sources may be the most promising candidates for ALMA to probe the process of high-mass star-formation in the Galactic center.

• 천문학회보
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• 제44권2호
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• pp.55.3-55.3
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• 2019

We present a pipeline to estimate baryonic properties of a galaxy inside a dark matter (DM) halo in DM-only simulations using a machine trained on high-resolution hydrodynamic simulations. As an example, we use the IllustrisTNG hydrodynamic simulation of a (75 h^-1 Mpc)³ volume to train our machine to predict e.g., stellar mass and star formation rate in a galaxy-sized halo based purely on its DM content. An extremely randomized tree (ERT) algorithm is used together with multiple novel improvements we introduce here such as a refined error function in machine training and two-stage learning. Aided by these improvements, our model demonstrates a significantly increased accuracy in predicting baryonic properties compared to prior attempts --- in other words, the machine better mimics IllustrisTNG's galaxy-halo correlation. By applying our machine to the MultiDark-Planck DM-only simulation of a large (1 h^-1 Gpc)³ volume, we then validate the pipeline that rapidly generates a galaxy catalogue from a DM halo catalogue using the correlations the machine found in IllustrisTNG. We also compare our galaxy catalogue with the ones produced by popular semi-analytic models (SAMs). Our so-called machine-assisted semi-simulation model (MSSM) is shown to be largely compatible with SAMs, and may become a promising method to transplant the baryon physics of galaxy-scale hydrodynamic calculations onto a larger-volume DM-only run. We discuss the benefits that machine-based approaches like this entail, as well as suggestions to raise the scientific potential of such approaches.

• 천문학회보
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• 제37권1호
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• pp.31.1-31.1
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• 2012

I will report recent progress in understanding properties of stellar and interstellar components of the Milky Way Galaxy on the two extremes - ongoing star formation activities in the Galactic center and stellar relics in the halo. Properties of the interstellar medium in the Galactic center and their relationship with star formation activities will be discussed based on by far the largest mid-IR spectroscopic data set in this region. Correlations between stellar kinematics and metallicities in the halo will be presented, along with a discussion on the estimation of fundamental stellar parameters from a set of empirically calibrated isochrones.