• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.511-512
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• 2015

Star formation activities dominate the evolution of galaxies. Elliptical galaxies are believed to be old galaxies in the Hubble sequence, and elliptical galaxies at different evolution epochs might have different star formation activities and/or morphologies. We investigate the connection between star formation rates and the morphology of elliptical galaxies. With the Sloan Digital Sky Survey (SDSS) and the Galaxy Zoo, we select a sample of elliptical galaxies by morphology and consider their infrared emission as an index of star formation rate to study the relation between the star formation rates and their morphological properties, such as ellipticities. In addition, we select some nearby spiral galaxies with very low MIR emission to probe the mechanisms of these red spiral galaxies. We display our preliminary results and discuss their implication on the evolution of galaxies in this poster.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.33.1-33.1
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• 2018

The differential rotational properties of early-type galaxies (ETGs) revealed by integral field spectroscopy surveys is arguably one of the most exciting findings in the galaxy evolution study during the past decade. Numerical studies have shown that galaxy mergers under various configurations can reproduce the observed distribution of ETG spin. However, we suggest an alternative scenario for the spin evolution of a large fraction of ETGs. Using the Horizon-AGN simulation, we follow the spin evolution of 10037 color-selected ETGs more massive than 1010 Msun that are divided into four groups: cluster centrals (3%), cluster satellites (33%), group centrals(5%), and field ETGs (59%). We find a strong mass dependence of the slow rotator fraction, fSR, and the mean spin of massive ETGs. Although the environmental dependence is not clear in the fSR, it is visible in the mean value of the spin parameter. The environmental dependence is driven by the satellite ETGs whose spin gradually decreases as their environment becomes denser. Galaxy mergers appear to be the main cause of total spin changes in 94% of central ETGs of halos with Mvir > 1012.5 Msun, but only 22% of satellite and field ETGs. We find that non-merger induced tidal perturbations better correlate with the galaxy spin-down in satellite ETGs than mergers. Given that the majority of ETGs are not central in dense environments, we conclude that non-merger tidal perturbation effects played a key role in the spin evolution of ETGs observed in the local (z < 1) universe.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.52.1-52.1
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• 2017

Galaxy clusters are the most massive gravitationally bound systems and thus probably the most recent objects to form. One of promising routes to understand the assembly history of galaxy clusters is to measure observable quantities of components in clusters that are sensitive to the evolutionary state of the cluster. Recent deep observations on the nearby clusters show distinct diffuse intracluster light (ICL), that the light from stars are not bound any individual cluster galaxy, however until now this component has not been well studied due to its faint nature, with typical brightness of ~100 times fainter than the sky background. As shown in galaxy cluster simulation studies, the ICL abundance increases during various dynamical exchanges of galaxies such as the disruption of dwarf galaxies, major mergers between galaxies and the tidal stripping of galaxies. Thus, the ICL is an effective tool to measure the evolutionary stage of galaxy clusters. Moreover, the investigation of the ICL evolution mechanism will allow us understand the galaxy evolution process therein. In this pilot study, we target the Coma cluster, where the existing ICL studies are limited only in the central region. With large and uniform deep optical images from the Subaru telescope, available only recently (Okabe et al. 2014), we are developing a robust ICL measurement technique, extracting the ICL surface brightness and color profiles, which will allow us to study the origin of the ICL and its connection to the evolutionary history of the Coma cluster. For the next phase, we plan to utilize the plenty of spectroscopy data from the MMT telescope to compare ICL properties with the star formation history of the brightest cluster galaxies (BCG), and discuss the ICL formation mechanism of the Coma cluster by comparing the distribution of cluster galaxies with the distribution of diffuse light inside the Coma cluster.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.58.2-58.2
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• 2018

Fossil groups of galaxies have characteristic features of a dominant central elliptical galaxy (${\Delta}M_{12}$ > 2 in $0.5R_{vir}$) embedded in highly relaxed X-ray halo, which indicates dynamically stable and evolved systems. These are thought as a final stage of the evolution of galaxy groups in the hierarchical structure formation scenario. However, the formation and evolution of fossil clusters are still unclear due to lack of detailed studies. Therefore, we perform a kinematic research of a known fossil cluster Abell 2261 (A2261 hereafter) using spectroscopic data of 589 galaxies in the A2261 field. Even though A2261 is known as a fossil cluster, previous studies found several unusual features such as quite high X-ray entropy for a stable cluster, and an elongated shape, which are not expected in standard fossil clusters. Using the caustic method, we identify cluster member galaxies and discover a second bright galaxy (${\Delta}M_{12}=1.68$) at ${\sim}1.5R_{vir}$. The presence of such a bright galaxy can break the current fossil state of cluster in the near future. In addition, with two independent substructure finding methods, we confirm that the previously detected elongated galaxy distribution of the cluster is a real feature. These findings indicate that A2261 is not in a fully stable state, unlike the existing fossil definition diagnostic. We require a more stringent criterion for the fossil definition to represent a genuinely final stage of cluster evolution.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.2
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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.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.49.1-49.1
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• 2017

In order to investigate the origin of multiple stellar populations in the halo and bulge of the Milky Way, we have constructed chemical evolution models for the low-mass proto-Galactic subsystems such as globular clusters (GCs). Unlike previous studies, we assume that supernova blast waves undergo blowout without expelling the pre-enriched gas, while relatively slow winds of massive stars, together with the winds and ejecta from low and intermediate mass asymptotic giant branch stars, are all locally retained in these less massive systems. We first applied these models to investigate the origin of super-helium-rich red clump stars in the metal-rich bulge as recently suggested by Lee et al. (2015). We find that chemical enrichments by the winds of massive stars can naturally reproduce the required helium enhancement (dY/dZ = 6) for the second generation stars. Disruption of these "building blocks" in a hierarchical merging paradigm would have provided helium enhanced stars to the bulge field. Interestingly, we also find that the observed Na-O anticorrelation in metal-poor GCs can be reproduced, when multiple episodes of starbursts are allowed to continue in these subsystems. Specific star formation history with decreasing time intervals between the stellar generations, however, is required to obtain this result, as would be expected from the orbital evolution of these subsystems in a proto-Galaxy. The "mass budget problem" is also much alleviated by our models without ad-hoc assumptions on star formation efficiency and initial mass function.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.2
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• pp.32.2-32.2
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• 2018

As computational resolution of modern cosmological simulations reach ever so close to resolve individual star-forming clumps in a galaxy, a need for "resolution-appropriate" physics for a galaxy-scale simulation has never been greater. To this end, we introduce a self-consistent numerical framework that includes explicit treatments of feedback from star-forming molecular clouds and massive black holes. We perform a state-of-the-art cosmological simulation of a quasar-host galaxy at z~7.5, and demonstrate that previously undiscussed types of interplay between galactic components may hold important clues about the growth and impact of quasar-host galaxies.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.37.2-37.2
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• 2017

Since the discovery of quasars about a half century ago, it has been an open question what triggers supermassive black holes at the center of galaxies. I will talk about AGNs and their host galaxies in the context of galaxy assembly and evolution. In particular, I will focus on the fundamental, but unanswered question whether black holes play a key role in galaxy evolution from my personal perspective.

• Journal of Astronomy and Space Sciences
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• v.29 no.2
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• pp.175-180
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• 2012

White dwarf stars have played important roles in rather diverse areas of astrophysics. This paper outlines how these stellar remnants, especially those in widely separated "fragile" binaries, have provided unique leverage on difficult astrophysical problems such as the ages of stars, the structure and evolution of the Galaxy, the nature of dark matter and even the discovery of dark energy.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.1
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• pp.61.2-61.2
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• 2011

HI gas disks are known to be the largest ISM reservoir in most late type galaxies. When the HI properties of galaxies such as total mass, density, and distribution change, the galaxies may evolve quite differently. In this talk, I will present two groups of galaxies, one undergoing HI stripping and one accreting more gas. I will discuss causes of gas stripping and accretion, and possible consequences in galaxy evolution.