• The Bulletin of The Korean Astronomical Society
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• v.38 no.1
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• pp.31.1-31.1
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• 2013

Barred-spiral galaxies possess double patterns: a bar and spiral arms. While their angular speeds play an important role in governing gas dynamical evolution of barred spiral galaxies, there is no direct way to observe them. The Tremaine-Weinberg (TW) method has been one of the most reliable indirect methods to estimate pattern speeds, although it requires a few strict assumptions, notably one that the gas tracer is in a quasi-steady state. In barred-spiral galaxies, however, non-steady gas flows are significant especially when the double patterns have different angular speeds. Using numerical models, we explore the effect of non-steady gas motions on the determination of double pattern speeds based on the TW method. We find that the TW method is accurate within 15% when there is only a single pattern or when double patterns have the same angular speed. When double patterns have different speeds, on the other hand, neglecting the non-steady flows leads to quite large errors (> 30%) in the derived pattern speeds, and severely underestimate the real values for the viewing angle parallel to the bar minor axis. This suggests that one should be cautious when applying the TW method to galaxies with double patterns with different speeds.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.71.2-71.2
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• 2014

processes at the origin of the star formation in the galaxies over the last 10 billions years. While it was proposed in the past that merging of galaxies has a dominant role to explain the triggering of the star formation in the distant galaxies having high star formation rates. In the opposite, more recent studies revealed scaling laws linking the star formation rate in the galaxies to their stellar mass or their gas mass. The small dispersion of these laws seems to be in contradiction with the idea of powerful stochastic events due to interactions, but rather in agreement with the new vision of galaxy history where the latter are continuously fed by intergalactic gas. I was especially interested in one of this scaling law, the relation between the star formation (SFR) and the stellar mass (M*) of galaxies, commonly called the main sequence of star forming galaxies. I have studied this main sequence, SFR-M*, in function of the morphology and other physical parameters as the radius, the colour, the clumpiness. The goal was to understand the origin of the sequence's dispersion related to the physical processes underlying this sequence in order to identify the main mode of star formation controlling this sequence. This work needed a multi-wavelength approach as well as the use of galaxies profile simulation to distinguish between the different galaxy morphological types implied in the main sequence.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.1
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• pp.40.2-40.2
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• 2012

Early-type dwarf galaxies are the most numerous galaxies in dense environments, making them ideal probes of the mechanisms that govern galaxy formation and evolution. Despite the common picture of an early-type dwarf galaxy as a quiescent one with no star formation and little gas, recent systematic investigations of early-type dwarf galaxies in the cluster revealed an unexpected variety among these apparently simple objects. In this talk, I review intriguing complexity of early-type dwarf galaxies in the cluster. I will also briefly introduce a new catalog of galaxies in the Virgo cluster using SDSS data, extended Virgo Cluster Catalog (EVCC).

• 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.

• Journal of The Korean Astronomical Society
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• v.50 no.4
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• pp.111-124
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• 2017

The environmental dependence of the morphology of dwarf galaxies in isolated satellite systems is analyzed to understand the origin of the dwarf galaxy morphology using the visually classified morphological types of 5836 local galaxies with $z{\leq}0.01$. We consider six sub-types of dwarf galaxies, dS0, dE, $dE_{bc}$, dSph, $dE_{blue}$, and dI, of which the first four sub-types are considered as early-type and the last two as late-type. The environmental parameters we consider are the projected distance from the host galaxy ($r_p$), local and global background densities, and the host morphology. The spatial distributions of dwarf satellites of early-type galaxies are much different from those of dwarf satellites of late-type galaxies, suggesting the host morphology combined with $r_p$ plays a decisive role on the morphology of the dwarf satellite galaxies. The local and global background densities play no significant role on the morphology of dwarfs in the satellite systems hosted by early-type galaxies. However, in the satellite system hosted by late-type galaxies, the global background densities of dE and dSph satellites are significantly different from those of $dE_{bc}$, $dE_{blue}$, and dI satellites. The blue-cored dwarf satellites ($dE_{bc}$) of early-type galaxies are likely to be located at $r_p$ > 0.3 Mpc to keep their cold gas from the ram pressure stripping by the hot corona of early-type galaxies. The spatial distribution of $dE_{bc}$ satellites of early-type galaxies and their global background densities suggest that their cold gas is intergalactic material accreted before they fall into the satellite systems.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.33.2-33.2
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• 2020

Our HI monster sample is a set of local HI-rich galaxies identified by the ALFALFA survey (Arecibo Legacy Fast Survey ALFA) at z<0.08. Intriguingly, they are also found with a relatively large molecular gas reservoir compared to the galaxies with similar stellar mass and color, yet their star formation rate is quite comparable to normal spirals. This makes our HI monsters good candidates of galaxies in the process of gas accretion which may lead to the stellar mass growth. One feasible explanation for their relatively low star formation activity for a given high cool gas fraction is the gas in monsters being too turbulent to form stars as normal spirals. In order to verify this hypothesis, we probe the molecular gas kinematics of 10 HI monsters which we observed using the Atacama Large Millimeter/sub-millimeter Array (ALMA). We utilize the tilted ring model to investigate what fraction of the molecular gas in the sample is regularly and smoothly rotating. In addition, we model the molecular gas disk using the GALMOD package of the Groningen Image Processing System (GIPSY) and compare with the observations to identify the gas which is offset from the 'co-planar differential rotation'. Based on the results, we discuss the possibility of gas accretion in the sample, and the potential origin of non-regularly rotating gas and the inefficient star formation.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.61.2-61.2
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• 2014

A non-axisymmetric mass distribution of galactic structures including bulge (or bar) causes gas inflow from the disk to the nuclear region, including intense star formation within few hundred parsecs of galactic central. In order to investigate the relation between the ellipticity of the bulge and the presence of a nuclear starburst, we use a volume-limited sample of galaxies with Mr < -19.5 mag at 0.02 < z < 0.05 from the Sloan Digital Sky Survey Data Release 7. Total sample is 3252 spiral galaxies, which include nuclear starburst galaxies. We find that the occurrence of nuclear starbursts has a moderate correlation with bulge ellipticity of intermediate-type spiral galaxies (morphology classes Sab-Sb) in low galaxy number density environments and isolated regions where the distance between the target galaxies and the closest galaxies is relatively far. In high galaxy number density environments and interacting regions, close encounters and mergers between galaxies can cause gas inflow to the nuclear region even without the presence of non-axisymmetric bulges.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.76.3-76.3
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• 2019

We present the results from our comparisons of HCN and HCO+ (J=4-3) with HI and $H_2$ gas in NGC 6946, a sample from a mapping study of the dense molecular gas in the strongest star-forming galaxies (MALATANG). The MALATANG is one of the JCMT legacy surveys on the nearest 23 IR-brightest galaxies beyond the Local Group, which aims to study the relations of dense molecular gas with more general cool gas such as atomic and molecular hydrogen gas, and star formation properties in active galaxies. In this work, we particularly focus on the comparisons between the JCMT HCN/HCO+ (J=4-3) data and the THINGS HI/the NRO CO (J=1-0) data. We probe the dense molecular gas mass as a function of HI and $H_2$ mass in different locations in the central ${\sim}1.5kpc^2$ region. We discuss how the excess/deficit of $HI/H_2$ or total cool gas ($HI+H_2$) mass controls the presence and/or the fraction of dense molecular gas.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.1
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• pp.33.2-33.2
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• 2013

We present 12CO ($J=1{\rightarrow}0$) observations of a set of local galaxies (0.04 < z < 0.08) with a large cool gas reservoir, dubbed "HI Monsters". The data were obtained using the Redshift Search Receiver (RSR) on the Five College Radio Astronomy Observatory (FCRAO) 14 m telescope. The sample consists of 20 galaxies with $M_{HI}$ > $3{\times}10^{10}M_{\odot}$ identified by the ALFALFA survey and 8 additional objects with comparable HI mass from a separate LSB galaxy study ($M_{HI}$ > $1.5{\times}10^{10}M_{\odot}$). Our sample selection is purely based on the amount of neutral hydrogen in galaxies, thereby providing a chance to study how atomic and molecular gas relate to each other in these extremely HI-rich systems. We have detected CO in 15 out of 20 ALFALFA selected HI Monsters and 4 out of 8 LSB HI Monsters. We present the global molecular gas properties of the sample and discuss how their molecular gas properties correlate with their star formation activities.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.1
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• pp.47.1-47.1
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• 2014

We examine the effect of environment on star formation activity of a sample of galaxy group catalogue given in Tempel et al.(2012) constructed from the Sloan Digital Sky Survey Data Release 8. In order to compare galaxies in different environment, we classify galaxies into two groups: galaxies in low density environment and galaxies in high density environment. After matching colors and apparent magnitudes of the galaxies, we are left with 5912 galaxies in each of the environment category. The fraction of star-forming galaxies in low-density environment is ~34%, higher than ~15% in high-density environment. Star-forming galaxies in low density environment have a higher average SFR value than those in high density environment. The bulge-to-disk ratio for galaxies in two different environment shows bimodal distribution. Regardless of the environment, we find galaxies with high star formation rate despite their red (g-r) color, for which the origin enhancing their star formation rate is investigated.