• The Bulletin of The Korean Astronomical Society
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• v.46 no.2
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• pp.41.1-41.1
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• 2021

Neutral (HI) gas clouds associated with galaxies are responsible for fuelling the star-formation in the universe. In literature, the extremely strong damped Lyman-alpha absorbers (or ESDLAs) have been known to be sensitive to the effects of HI-H2 transition and star-formation in galaxies. Yet, ESDLAs are rare to probe due to the smaller cross section they subtend on the sky (similar to galaxies). In my talk, I will focus primarily on my study of the nature of ESDLAs that are observed as absorption signature along the line-of-sight (LOS) of a quasar (QSO). I will further look at the HI-H2 transition and interesting results relevant to diffuse molecular gas and the multi-phase medium (gas in different ionization states) that are associated with ESDLAs. Furthermore, I will also discuss how the ESDLA environments differ from the high star-forming and molecular environments detected in blind optical and radio surveys consecutively.

• Journal of The Korean Astronomical Society
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• v.34 no.4
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• pp.243-246
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• 2001

We summarize various aspects of the interaction of supernova remnants (SNRs) with the ambient medium. We discuss the evolution' of SNRs in environments sculpted by the progenitor star, and summarize the factors on which this evolution depends. As a specific example, we consider the evolution of the medium around a 35 M$\bigodot$ star, and the interaction of the shock wave with this medium when the star explodes as a SN. We also discuss the interaction of Type Ia SNe with the ambient medium, especially the formation and growth of hydrodynamic instabilities.

• The Bulletin of The Korean Astronomical Society
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• v.34 no.2
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• pp.43.2-43.2
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• 2009

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.44.1-44.1
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• 2013

We investigate the origin of radio emission in nearby early-type galaxies using the European VLBI Network (EVN) at 1.4 GHz. The sample included NGC 1277, which was found to have an over-massive black hole of $1.7{\times}10^{10}M_{\odot}$, and four other early-type galaxies in the Perseus cluster. All the sources were detected above $5{\sigma}$. They show compact radio cores and high brightness temperatures, $10^7{\sim}10^9K$, which implies that radio emission in these objects is non-thermal. While the observed radio luminosities could be consistent with star formation (${\sim}1M_{\odot}yr^{-1}$), the small source size would imply a specific star formation rate (sSFR) of ${\sim}10^6M_{\odot}yr^{-1}kpc^{-2}$. Such a high sSFR rules out ongoing star formation. Supernovae (SNe) are ruled out as well because it is unlikely that we see SNe in all galaxies at the same time, and there is no significant radio variability either. The most plausible scenario is that these galaxies show low luminosity AGN activity in the radio, although there is no sign of AGN activity in other bands. If our interpretation is correct, then regular early-type galaxies may harbor active AGN more often than suspected from observations at other wavelengths.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.48.2-48.2
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• 2013

To understand the connection between active galactic nuclei (AGNs) and star formation, we investigated the relation between AGN bolometric and far-infrared (FIR) luminosities, using type-2 AGNs. By matching type-2 AGNs at z < 0.3 selected from the SDSS based on the emission-line diagnostics, against the AKARI/FIS All-Sky Survey Catalogue and the COSMOS PEP (PACS Evolutionary Probe) Survey Catalogue, we obtained a sample of 729 type-2 AGNs detected in the AKARI survey ($90{\mu}m$) and 17 ones detected in the PEP survey ($100{\mu}m$). For AGN bolometric luminosities, we adopted an estimate based on the [OIII] and [OI] line luminosities. We confirmed that there is a correlation between the AGN bolometric and FIR luminosities with a large scatter, which is consistent with previous studies. However, we claim that this correlation suffers from various artificial effects, e.g., FIR detection limits, survey volumes, and so on. We will discuss the limitations of studying the connection between AGN and star formation using currently available facilities.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.1
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• pp.79.1-79.1
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• 2012

We present here $UBVI$ and H${\alpha}$ photometric results of stellar sources in the bubble nebula NGC 7635. The early type members are selected from the photometric membership criteria. H${\alpha}$ photometry allows us to detect 11 pre-main sequence candidates with H${\alpha}$emission. In addition, we performed PSF photometry for the Spitzer IRAC and MIPS 24${\mu}m$ images from archive (program ID 20726, PI: J. Hester) in order to search for the young stellar objects (YSOs). Total 19 sources are classified as YSOs (7 class I, 11 class II, and 1 transitional disk candidates) in the color-color diagrams according to the classification scheme of Gutermuth et al.. Among them, 7 YSOs have counterparts in optical photometric data. These stars can be divided into two groups at given color indices. It implies that there occurred the star formation events more than twice. We would like to discuss the star formation history in the bubble nebula using the results from SED fitter (Robitaille et al.), color composite image from IRAC bands, and spatial distribution of early type stars and YSOs.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.32.2-32.2
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• 2017

Stars form in dense core within the molecular clouds. The prestellar cores provide information of the physical characteristics at the very early stages of star formation. The low dust temperature (<14K) of Planck cold clumps/cores (PGCCs) make them likely to be prestellar objects or at the very initial stage of protostellar collapse. We have been conducting the legacy surveys of Planck cold clumps with the JCMT, the TRAO 14-m and many other telescopes. We aim to study of the initial conditions of star formation and chemical evolutions of the cores in the different environments. From JCMT SCUBA-2 $850{\mu}m$ survey (SCOPE), we have already identified hundreds of dense cores, which may be at the earliest phase of star formation. Therefore in order to explore the chemical evolution of these dense cores, we used KVN telescopes in order to observe 75 well selected SCUBA-2 cores in many molecules as the follow-up project of KVN Pilot Observation of SCUBA-2. These observations will help advance our understanding of the propoerties of these SCUBA-2 cores in PGCCs.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.62.1-62.1
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• 2018

Galaxies in the cluster environment interact with the intracluster medium (ICM), losing the interstellar medium (ISM) and alternating their evolution. Observational evidences of the extraplanar ISM stripped by the ICM's ram pressure are prevalent in HI imaging studies of cluster galaxies. However, current theoretical understanding of the ram pressure stripping (or ICM-ISM interaction in general) is still limited mainly due to the lack of numerical resolution at ISM scales in large-scale simulations. Especially, self-consistent modeling of the turbulent, multiphase ISM is critical to understand star formation in galaxies interacting with the ICM. To achieve this goal, we utilize the TIGRESS simulation suite, simulating a local patch of galactic disks with high resolution to resolve key physical processes in the ISM, including cooling/heating, self-gravity, MHD, star formation, and supernova feedback. We then expose the ISM disk to ICM flows and investigate the evolution of star formation rate and the properties of the ISM. By exploring ICM parameter space, we discuss an implication of the simple ram pressure stripping condition (so called the Gunn-Gott condition) to the realistic ISM.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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• pp.33.3-34
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• 2016

We explored the GALEX UV properties of optical red sequence galaxies in 4 rich Abell clusters at z ~ 0.1. In particular, we tried to find a hint of merger-induced recent star formation (RSF) in red sequence galaxies. Based on the NUV - r' colors of the galaxies, about 36% of the post-merger galaxies were classified as RSF galaxies with a conservative criterion (NUV - $r^{\prime}{\leq}5$), and that number was doubled (~ 72%) when using a generous criterion (NUV - $r^{\prime}{\leq}5.4$). Post-merger galaxies with strong UV emission showed more violent, asymmetric features on the deep optical images. Also it turned out that all massive RSF galaxies (Mr' < -22 and NUV - $r^{\prime}{\leq}5$) exhibited post-merger signatures. Our results suggested that only 30% of RSF red sequence galaxies show morphological hints of recent galaxy mergers. This implies that internal processes (e.g., stellar mass-loss or hot gas cooling) for the supply of cold gas to early-type galaxies may play a significant role in the residual star formation of early-type galaxies at a recent epoch.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.355-358
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• 2015

We have conducted a near-infrared monitoring campaign at the UK InfraRed Telescope (UKIRT), of the Local Group galaxy M33. The main aim was to identify stars in the very final stage of their evolution, and for which the luminosity is more directly related to the birth mass than the more numerous less-evolved giant stars that continue to increase in luminosity. The pulsating giant stars (AGB and red supergiants) are identified and their distributions are used to derive the star formation rate as a function of age. These stars are also important dust factories; we measure their dust production rates from a combination of our data with Spitzer Space Telescope mid-IR photometry. The mass-loss rates are seen to increase with increasing strength of pulsation and with increasing bolometric luminosity. Low-mass stars lose most of their mass through stellar winds, but even super-AGB stars and red superginats lose ~40% of their mass via a dusty stellar wind. We construct a 2-D map of the mass-return rate, showing a radial decline but also local enhancements due to agglomerations of massive stars. By comparing the current star formation rate with total mass input to the ISM, we conclude that the star formation in the central regions of M33 can only be sustained if gas is accreted from further out in the disc or from circum-galactic regions.