• The Bulletin of The Korean Astronomical Society
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• v.36 no.1
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• pp.60.2-60.2
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• 2011

Galactic bars are supposed to be a channel of gas inflow to the galactic center and thus possibly help nuclear star-formation and AGN activities. However, previous studies based on small local samples did not agree with this expectation. We find it necessary to examine the expectation using a large sample and so investigate the effects of bar structures on galactic nuclear activities, based on the Sloan Digital Sky Survey (SDSS) DR7. We used 6,348 late-type galaxies brighter than Mr = -19.0 in the redshift range $0.01{\leq}z{\leq}0.05$. Late-type galaxies are visually classified into barred or unbarred galaxies using SDSS color composite images. We compare the fractions of galaxies showing star-formation and AGN activities among barred and unbarred galaxies as a function of optical color, stellar mass, and black-hole mass. We have found that bar enhances nuclear star-formation activity on galaxies having low stellar mass, and low black-hole mass. This effect is stronger in redder galaxies. In the case of AGN, bar effects are higher in intermediate-mass galaxies. Bars also have an effect on the strength(!) of the star-formation and AGN activity in our sample as well. Thus, it seems that nuclear activities are powered by gas inflow from galactic bar structures perhaps not always but under certain conditions.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.32.3-33
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• 2020

Jellyfish galaxies show spectacular features such as star-forming knots and tails due to strong ram-pressure stripping in galaxy clusters. Thus, jellyfish galaxies are very useful targets to investigate the effects of ram-pressure stripping on the star formation activity in galaxies. Integral field spectroscopy (IFS) studies are the best way to study star formation in jellyfish galaxies, but they have been limited to those in low-mass galaxy clusters until now. In this study, we present a Gemini GMOS/IFU study of three jellyfish galaxies in very massive clusters (M_200 > 10^15 Mo). The host clusters (Abell 2744, MACSJ0916.1-0023, and MACSJ1752.0+4440) are X-ray luminous and dynamically unstable, suggesting that ram-pressure stripping in these clusters is much stronger than in low-mass clusters. We present preliminary results of star formation rates, kinematics, dynamical states, and ionization mechanisms of our sample galaxies and discuss how ram-pressure stripping relates with the star formation activity of jellyfish galaxies in massive clusters.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.491-494
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• 2015

We present 12CO (2-1) data for four spiral galaxies (NGC 4330, NGC 4402, NGC 4522, NGC 4569) in the Virgo cluster that are undergoing different ram pressure stages. The goal is to probe the detailed molecular gas properties under strong intra-cluster medium (ICM) pressure using high-resolution millimeter data taken with the Submillimeter Array (SMA). Combining this with Institut de RadioAstronomie $Millim{\acute{e}}trique$ (IRAM) data, we also study spatially resolved temperature and density distributions of the molecular gas. Comparing with multi-wavelength data (optical, $H\small{I}$, UV, $H{\alpha}$), we discuss how molecular gas properties and star formation activity change when a galaxy experiences $H\small{I}$ stripping. This study suggests that ICM pressure can modify the physical and chemical properties of the molecular gas significantly even if stripping does not take place. We discuss how this affects the star formation rate and galaxy evolution in the cluster environment.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.65-65
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• 2011

Early-type galaxies represent the end point of galaxy evolution and, despite pervasive residual star formation, are generally considered "red and dead", that is composed exclusively of old stars with no star formation. Here, their molecular gas content is constrained and discussed in relation to their evolution, supporting the continuing importance of minor mergers and/or cold gas accretion. First, as part of the Atlas3D survey, the first complete, large, volume-limited survey of CO in normal early-type galaxies is presented. At least of 23% of local early-types possess a substantial amount of molecular gas, the necessary ingredient for star formation, independent of mass and environment but dependent on the specific stellar angular momentum. Second, using CO synthesis imaging, the extent of the molecular gas is constrained and a variety of morphologies is revealed. The kinematics of the molecular gas and stars are often misaligned, implying an external gas origin in over a third of all systems, more than half in the field, while external gas accretion must be shot down in clusters. Third, many objects appear to be in the process of forming regular kpc-size decoupled disks, and a star formation sequence can be sketched by piecing together multi-wavelength information on the molecular gas, current star formation, and young stars. Fourth, early-type galaxies do not seem to systematically obey all our usual prejudices regarding star formation (e.g. Schmidt-Kennicutt law, far infrared-radio continuum correlation), suggesting a greater diversity in star formation processes than observed in disk galaxies and the possibility of "morphological quenching". Lastly, a first step toward constraining the physical properties of the molecular gas is taken, by modeling the line ratios of density- and opacity-sensitive molecules in a few objects. Taken together, these observations argue for the continuing importance of (minor) mergers and cold gas accretion in local early-types, and they provide a much greater understanding of the gas cycle in the galaxies harbouring most of the stellar mass. In the future, better dust masses and dust-to-gas mass ratios from Herschel should allow to place entirely independent constraints on the gas supply, while spatially-resolved high-density molecular gas tracers observed with ALMA will probe the interstellar medium and star formation laws locally in a regime entirely different from that normally probed in spiral galaxies.

• Publications of The Korean Astronomical Society
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• v.25 no.3
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• pp.53-58
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• 2010

The origin of the galaxies represents an important focus of current cosmological research, both observational and theoretical. Its resolution involves a comprehensive understanding of star formation and evolution, galaxy dynamics, supermassive black holes, and the cosmology of the very early universe. In this paper, I will review our current understanding of galaxy formation and review some of the challenges that lie ahead. Specific issues that I address include the galaxy luminosity function, feedback by supernovae and by AGN, and downsizing. I argue that current evidence favours two distinct modes of star formation in the early universe, in order to account for the origin of disk and massive spheroidal galaxies. However perhaps the most urgent need is for a robust theory of star formation.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.2
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• pp.43.2-43.2
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• 2021

Ram-pressure stripping (RPS) is known as the main driver of quenching the star formation (SF) activity in cluster galaxies. However, galaxies undergoing RPS in galaxy clusters often show blue star-forming knots in their disturbed disks and tails. The existence of these "jellyfish galaxies" implies that RPS can temporarily boost the SF activity of cluster galaxies. Thus, jellyfish galaxies are very unique and interesting targets to study the influence of RPS on their SF activity, in particular with integral field spectroscopy (IFS). While there have been many IFS studies of jellyfish galaxies in low-mass clusters (e.g., the GASP survey), IFS studies of those in massive clusters have been lacking. We present an IFS study of five jellyfish galaxies in massive clusters at intermediate redshifts using the Gemini GMOS/IFU. Their star formation rates (SFRs) are estimated to be up to 15 Mo/yr in the tails and 50 Mo/yr in the disks. These SFRs are by a factor of 10 higher than those of star-forming galaxies on the main sequence in the M*-SFR relation at similar redshifts. Our results suggest that the SF activity of jellyfish galaxies tends to be more enhanced in massive clusters than in low-mass clusters. This implies that strong RPS in massive clusters can trigger strong starbursts.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.1
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• pp.55.1-55.1
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• 2021

Ram-pressure stripping (RPS) is known as a typical mechanism of quenching star formation (SF) of galaxies orbiting in clusters, but it can also boost the SF activity within a short period of time. Jellyfish galaxies, with eye-catching blue tails and knots, are such starburst galaxies undergoing strong RPS in galaxy clusters. Thus, they are very useful targets to understand their SF activity in relation to RPS. We study the SF activity of three jellyfish galaxies in massive clusters at z=0.3-0.4 (MACSJ1752-JFG2, MACSJ0916-JFG1, and A2744-F0083) with Gemini GMOS/IFU and compare our results to those of jellyfish galaxies in low-mass clusters. We obtain total star formation rates (SFRs) of up to 60 Mo/yr and SFRs in the tails of up to 15 Mo/yr, which are much higher than those of jellyfish galaxies in low-mass clusters with the median SFRs of 1.1 Mo/yr in total and 0.03 Mo/yr in tails. In addition, these SFRs are also significantly higher than the SF main sequence of galaxies at the redshifts of the three jellyfish galaxies. This implies that their SF activity is much more enhanced compared to jellyfish galaxies in low-mass clusters due to extreme RPS in massive clusters.

• Journal of The Korean Astronomical Society
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• v.29 no.spc1
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• pp.77-78
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• 1996

We perfom a large project for complex study of Blue Compact Galaxies (BCGs) with strong star formation, which includes optical spectroscopy, BVR CCD photometry and HI 21 cm radio survey. The most interesting galaxies are studied also with HST and VLA. In the frame of this project we began the study of H$\alpha$ morphology of BCGs with 6-m telescope. We present and discuss here the results for the first 6 galaxies. We found the noticeable variety of forms for H$\alpha$ morphology comparing to broad band images: from very compact HII region in very center of stellar body (Mark 996, possible dwarf post-merger, old galaxy experiencing strong star formation burst), to very extended gas emission encompassing the whole area traced by stars (SBS 0335-052, the most probable young galaxy in formation).

• Journal of Astronomy and Space Sciences
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• v.30 no.1
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• pp.59-67
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• 2013

We present a spectroscopic study of 343 blue compact galaxies (BCGs) at 0.20 < z < 0.35 from the Sloan Digital Sky Survey (SDSS) DR7 data. We derive gas phase oxygen abundance using the empirical and direct method. Stellar masses of galaxies are derived from the STARLIGHT code. We also derive star formation rates of galaxies based on $H{\alpha}$ emission line from the SDSS as well as far-ultraviolet (FUV) flux from the Galaxy Evolution Explorer GR6 data. Evolution of the luminosity-metallicity and mass-metallicity (M-Z) relations with redshift is observed. At a given luminosity and mass, galaxies at higher redshifts appear to be biased to low metallicities relative to the lower redshift counterparts. Furthermore, low mass galaxies show higher specific star formation rates (SSFRs) than more massive ones and galaxies at higher redshifts are biased to higher SSFRs compared to the lower redshift sample. By visual inspection of the SDSS images, we classify galaxy morphology into disturbed or undisturbed. In the M-Z relation, we find a hint that morphologically disturbed BCGs appear to exhibit low metallicities and high SSFRs compared to undisturbed counterparts. We suggest that our results support downsizing galaxy formation scenario and star formation histories of BCGs are closely related with their morphologies.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.31.1-31.1
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• 2020

A merger or interaction between galaxy clusters is one of the most violent events in the universe. Thus, an interacting cluster is an optimum laboratory to understand how galaxy properties are influenced by a drastic change of the large-scale environment. Here, we present the observational evidence that bars in disk galaxies can form by cluster-cluster interaction and the bar formation is associated with star-formation enhancement. We investigated 105 galaxy clusters at 0.015