• 천문학회보
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• 제46권1호
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• pp.54.3-54.3
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• 2021

The Coma cluster serves as an ideal laboratory to study the cluster assembly history. It is known as a typical example of relaxed galaxy clusters. However, recent X-ray, radio and optical observations revealed a number of substructures in Coma. The NGC 4839 group is an interesting substucture in the sense that it is overlappled with the X-ray bright component in the south-west region. Recent hydrodynamical simulations in the literature suggest that the NGC 4839 group came from the north-east direction of Coma, passed the apocenter about 1 Gyr ago, and started a second infall to the Coma core recently. Interestingly a number of E+A galaxies are located along the filament connecting the NGC 4839 group and the Coma core. We are surveying a wide area covering the NGC 4839 group to search for globular clusters and use them to investigate any connection between the globular clusters and the merger scenario of the NGC 4839 group. We utilized Subaru Hyper Suprime-Cam archival images of two circular fields with diameter ~1.8 deg, covering the Coma core and the NGC 4839 group. We discuss the results with regard to the formation history of the NGC 4839 group.

• 천문학회보
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• 제37권1호
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• pp.31.2-31.2
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• 2012

We present the Yonsei Evolutionary Population Synthesis (YEPS) models for spectroscopic and photometric evolutions of simple and composite stellar populations. The models are based on the most up-to-date Yonsei-Yale stellar evolutionary tracks and BaSel 3.1 flux libraries, and provide integrated spectroscopic quantities of Lick/IDS system including high-order Balmer absorption-lines. Special care has been taken to incorporate the systematic variation of horizontal branch (HB) morphology as functions of metallicity, age, alpha-element mixture, and helium abundance of simple stellar populations. Our models for normal-helium stellar populations indicate that the realistic modeling of HB and alpha-element brings about 5 Gyr and 0.1 dex differences in age and metallicity estimations, respectively, compared to those without these effects. The HB effect does not depend on the specific choice of stellar libraries and alpha-element enhancements, and this effect is non-negligible even in the metal sensitive absorption indices, such as Mg2 and Mg b. Comparison of the models to observations reveals that the HB and alpha-element effects are critical in understanding otherwise inexplicable phenomena found in globular cluster systems in the Milky Way and nearby galaxies, including the observed bimodality of the line strengths of globular clusters in massive galaxies. In addition, we found that helium-enhanced stellar populations, which are the major sources of extreme HB stars, bring about increased FUV, NUV fluxes, and thus the model colors of those filters become extremely blue. Age dating based on the YEPS model with normal-helium stellar populations reveals that the evidence for 'downsizing' of elliptical galaxies is found not only in the local field but also in Coma cluster, and that the mean age of elliptical galaxies in Coma cluster is about 1.4 Gyr younger than the mean age of those in the local field. We also find that our models with helium-enhanced subpopulations can naturally reproduce the strong UV-upturns observed in giant elliptical galaxies assuming an age similar to that of old GCs in the Milky Way.

• 천문학회보
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• 제38권1호
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• pp.30.2-30.2
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• 2013

The mean color of globular cluster (GCs) systems in early-type galaxies (ETGs) is, in general, bluer than the integrated color of field stars in their host galaxies. Recently, Goudfrooij & Kruijssen (2013) reported that even red GCs in the ETGs show bluer colors than their host field stars and suggested the different initial mass function (IMF) for red GCs and field stars to explain the observed offset in color. Here we suggest an alternative scenario that explains the observed color offsets between red GCs in ETGs and the field stars in the parent galaxies without invoking to the variation of the IMF. We find that the inclusion of second-generation (SG) helium-enhanced populations in the model fully explains the observed color offset between red GCs and field stars in the host galaxies. We have also tested the effect of the IMF slope on our models, but the effect is relatively small compared to the effect of the SG population. Our new model suggests that, in order to explain far-UV strong metal-rich GCs in M87 and the observed color offset between metal-rich GCs and the field stars in ETGs simultaneously, the inclusion of the SG populations with enhanced helium abundance is a more natural solution than the model that only adopted variations in the IMF.

• 천문학회보
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• 제41권1호
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• pp.33.1-33.1
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• 2016

Dwarf elliptical galaxies (dEs), the most abundant galaxy type in clusters, were recently shown to exhibit a wide variety in their properties. Particularly, the presence of blue cores in some dEs, what we call dE(bc), supports the scenario of late-type galaxy infall and subsequent transformation into red, quiescent dEs. While several transformation mechanisms for these dE(bc)s within cluster environment have been proposed, all these processes are able to explain only some of the observational properties of dEs. In this context, internal kinematic properties of dE(bc)s provide the most crucial evidence to discriminate different processes for the formation of these galaxies. We present Gemini Multi Object Spectrograph (GMOS) long-slit spectroscopy of two dE(bc)s in the Virgo cluster. We obtained radial profiles of velocity and velocity dispersion out to ~1.3 effective radius. We found that two dE(bc)s exhibit kinematically decoupled components as well as distinct peculiar features in velocity profiles, supporting the scenario of mergers. We also found that these galaxies are structurally compatible with low surface brightness component of blue compact dwarf galaxies. We suggest that a part of dE(bc)s in the Virgo Cluster were formed through galaxy merger in low density environment such as galaxy group or outskirt of the cluster, and then were quenched by subsequent effects within cluster environment.

• 천문학논총
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• 제30권2호
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• pp.389-392
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• 2015

We identify a strong lensing galaxy in the cluster IRC 0218 that is spectroscopically confirmed to be at z = 1.62, making it the highest-redshift strong lens galaxy known. The lens is one of the two brightest cluster galaxies and lenses a background source galaxy into an arc and a counterimage. With Hubble Space Telescope (HST) grism and Keck/LRIS spectroscopy, we measure the source redshift to be $z_S=2.26$. Using HST imaging, we model the lens mass distribution with an elliptical power-law profile and account for the effects of the cluster halo and nearby galaxies. The Einstein radius is $^{\theta}E=0.38^{+0.02{\prime}{\prime}}_{-0.01}$ ($3.2^{+0.2}_{-0.1}kpc$) and the total enclosed mass is $M_{tot}(<^{\theta}_E)=1.8^{+0.2}_{-0.1}{\times}10^{11}M_{\odot}$. We estimate that the cluster environment contributes ~ 10% of this total mass. Assuming a Chabrier IMF, the dark matter fraction within $^{\theta}E$ is $f^{Chab}_{DM}=0.3^{+0.1}_{-0.3}$, while a Salpeter IMF is marginally inconsistent with the enclosed mass ($f^{Salp}_{DM}=-0.3^{+0.2}_{-0.5}$).

• 천문학회지
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• 제46권1호
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• pp.41-47
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• 2013

I present a simple scheme for the treatment of gravitational interactions on galactic scales. In anal- ogy with known mechanisms of quantum field theory, I assume ad hoc that gravitation is mediated by virtual exchange particles-gravitons-with very small but non-zero masses. The resulting den- sity and mass profiles are proportional to the mass of the gravitating body. The mass profile scales with the centripetal acceleration experienced by a test particle orbiting the central mass, but this comes at the cost of postulating a universal characteristic acceleration $a_0{\approx}4.3{\times}10^{-12}msec^{-2}$ (or $8{\pi}a_0{\approx}1.1{\times}10^{-10}msec^{-2}$). The scheme predicts the asymptotic flattening of galactic rotation curves, the Tully-Fisher/Faber-Jackson relations, the mass discrepancy-acceleration relation of galaxies, the surface brightness-acceleration relation of galaxies, the kinematics of galaxy clusters, and "Renzo's rule" correctly; additional (dark) mass components are not required. Given that it is based on various ad-hoc assumptions and given further limitations, the scheme I present is not yet a consistent theory of gravitation; rather, it is a "toy model" providing a convenient scaling law that simplifies the description of gravity on galactic scales.

• 천문학회지
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• 제37권5호
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• pp.575-578
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• 2004

High resolution observations of cluster of galaxies by Chandra have revealed the existence of an X-ray emitting comet-like galaxy C153 in the core of cluster of galaxies A2125. The galaxy C153 moving fast in the cluster core has a distinct X-ray tail on one side, obviously due to ram pressure stripping, since the galaxy C153 crossed the central region of A2125. The X-ray emitting plasma in the tail is substantially cooler than the ambient plasma. We present results of two-dimensional magnetohydrodynamic simulations of the time evolution of a sub clump like C153 moving in magnetized intergalactic matter. Anisotropic heat conduction is included. We found that the magnetic fields are essential for the existence of the cool X-ray tail, because in non-magnetized plasma the cooler sub clump tail is heated up by isotropic heat conduction from the hot ambient plasma and does not form such a comet-like tail.

• 천문학회보
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• 제40권2호
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• pp.57.3-57.3
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• 2015

The formation of the Milky Way stellar halo is thought to be the result of merging and accretion of building blocks such as dwarf galaxies and massive globular clusters. Recently, Deason et al. (2015) suggested that the Milky Way outer halo formed mostly from big building blocks, such as dwarf spheroidal galaxies, based on the similar number ratio of blue straggler (BS) stars to blue horizontal-branch (BHB) stars. Here we demonstrate, however, that this result is seriously biased by not taking into detailed consideration on the formation mechanism of BHB stars from helium enhanced second-generation population. In particular, the high BS-to-BHB ratio observed in the outer halo fields is most likely due to a small number of BHB stars provided by GCs rather than to a large number of BS stars. This is supported by our dynamical evolution model of GCs which shows preferential removal of first generation stars in GCs. Moreover, there are sufficient number of outer halo GCs which show very high BS-to-BHB ratio. Therefore, the BS-to-BHB number ratio is not a good indicator to use in arguing that more massive dwarf galaxies are the main building blocks of the Milky Way outer halo. Several lines of evidence still suggest that GCs can contribute a signicant fraction of the outer halo stars.

• 천문학회보
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• 제40권2호
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• pp.33.3-34
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• 2015

We present the spectroscopic study of the globular clusters (GCs) in the massive elliptical galaxy M86 in the Virgo galaxy cluster. Using the spectra obtained from the Multi-Object Spectroscopy (MOS) mode of Faint Object Camera and Spectrograph (FOCAS) on the Subaru Telescope, we measure the radial velocities for 56 GCs in M86. The mean velocity of the GCs is derived to be $<v_p>=-335{\pm}41km/s$, which is different from the velocity of the M86 nucleus ($<v_{gal}>=-224{\pm}5km/s$) within ${\sim}2.5{\sigma}$. The mean velocity ($<v_p>=-342{\pm}60km/s$) of 33 blue GCs in M86 is similar to that ($<v_p>=-314{\pm}71km/s$) of 23 red GCs. We also derive the mean velocities of the GC systems in other 16 nearby early-type galaxies (ETGs) from the radial velocity data in the literature. The mean value of the differences between the mean velocity of the GC systems in each galaxy and the nucleus velocity of their host galaxies, is almost zero except the M86 GC system. But the scatter of the differences in the blue GC system is larger than that in the red GC system. We will discuss these results in the context of GC formation in ETGs.

• 천문학회보
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• 제43권1호
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• pp.36.1-36.1
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• 2018

Understanding the origin of strong galactic outflows and the suppression of star formation in dwarf galaxies is a key problem in galaxy formation. Using a set of radiation-hydrodynamic simulations of an isolated dwarf galaxy, we show that the momentum transferred from resonantly scattered Lyman-alpha(LyA) photons can suppress star formation by a factor of two in metal-poor galaxies by regulating the dynamics of star-forming clouds before the onset of supernova explosions (SNe). This is possible because each LyA photon resonantly scatters and imparts ~10-300 times greater momentum than in the single scattering limit. Consequently, the number of star clusters predicted in the simulations is reduced by a factor of ~5, compared to the model without the early feedback. More importantly, we find that galactic outflows become weaker in the presence of strong LyA radiation feedback, as star formation and associated SNe become less bursty. We also examine a model in which radiation field is arbitrarily enhanced by a factor of up to 10, and reach the same conclusion. The typical mass-loading factors in our metal-poor dwarf system are estimated to be ~5-10 near the mid-plane, while it is reduced to ~1 at larger radii.