• Publications of The Korean Astronomical Society
• /
• v.30 no.2
• /
• pp.75-78
• /
• 2015

A growing body of evidence has been supporting the existence of so-called "dark molecular gas" (DMG), which is invisible in the most common tracer of molecular gas, i.e., CO rotational emission. DMG is believed to be the main gas component of the intermediate extinction region from Av~0.05-2, roughly corresponding to the self-shielding threshold of $H_2$ and $^{13}CO$. To quantify DMG relative to $H{\small{I}}$ and CO, we are pursuing three observational techniques; $H{\small{I}}$ self-absorption, OH absorption, and THz $C^+$ emission. In this paper, we focus on preliminary results from a CO and OH absorption survey of DMG candidates. Our analysis shows that the OH excitation temperature is close to that of the Galactic continuum background and that OH is a good DMG tracer co-existing with molecular hydrogen in regions without CO. Through systematic "absorption mapping" by the Square Kilometer Array (SKA) and ALMA, we will have unprecedented, comprehensive knowledge of the ISM components including DMG in terms of their temperature and density, which will impact our understanding of galaxy evolution and star formation profoundly.

• Publications of The Korean Astronomical Society
• /
• v.30 no.2
• /
• pp.629-631
• /
• 2015

In this proceedings, preliminary results of the KVN Source-Frequency Phase-Referencing (SFPR) observation of 3C 66A and 3C 66B are presented. The motivation of this work is to measure the core-shift of these 2 sources and study the temporal evolution of the jet opacity. Two more sources were observed as secondary reference calibrators and each source was observed at 22, 43, and 86 GHz simultaneously. Our preliminary results show that after using the observations at the lower frequency to calibrate those at the higher frequency of the same source, the residual visibility phases for each source at the higher frequencies became more aligned, and the coherence time became much longer; also, the residual phases for different sources, within 10 degrees angular separations, follow similar trends. After reference to the nearby calibrator, the SFPRed maps were obtained as well as the astrometric measurements, i.e. the combined coreshift. The measurements were found to be affected by structural blending effects because of the large beamsize of KVN, but this can be corrected with higher resolution maps (e.g. KAVA maps).

• Publications of The Korean Astronomical Society
• /
• v.30 no.2
• /
• pp.341-343
• /
• 2015

We present an analysis of the relation between star-formation (SF) and accretion luminosities of local type-2 active galactic nuclei (AGNs) at $0.01{\leq}z<0.22$. We match type-2 AGNs found in the Sloan Digital Sky Survey to current far-infrared (FIR) survey catalogues based on AKARI and Herschel. Estimating AGN luminosities from [$O{\small{III}}$]${\lambda}5007$ and [$O{\small{I}}$]${\lambda}6300$ emission lines, we find a positive linear trend between FIR and AGN luminosities over a wide dynamical range. This result appears to be inconsistent with recent reports that low-luminosity AGNs show no correlation between FIR and X-ray luminosities; this contradiction is likely due to Malmquist and sample selection biases. Moreover, we also find that pure-AGN candidates, for which the FIR radiation is thought to be AGN-dominated, show significant low-SF activities. These AGNs hosted by low-SF galaxies are rare in our sample. However, it is possible that the low fraction of low-SF AGN is caused by observational limitations, as recent FIR surveys are not sufficient to examine the population of high-luminosity AGNs hosted by low-SF galaxies.

• The Bulletin of The Korean Astronomical Society
• /
• v.37 no.2
• /
• pp.74.2-74.2
• /
• 2012

Metallicity is an important tracer of star formation in galaxy evolution. Based on the flux ratios of broad emission lines, AGN metallicity has shown a correlation with AGN luminosity. However, it is not clear what physical parameter drives the observed correlation. Using a sample 69 Palomar-Green QSOs at low-z (z<0.5), we determine BLR gas metallicity from emission line flux ratios, i.e., N V1240/C IV1549, (Si IV1398+O IV1402)/C IV1549 and N V1240/He II1640 based on the UV spectra from the HST and IUE archives. We compare BLR gas metallicity with various AGN properties, i.e., black hole mass, AGN luminosity and Eddington ratio, in order to investigate physical connection between metal enrichment and AGN activity. In contrast to high-z QSOs, which show the correlation between metallicity and black hole mass, we find that the metallicity of low-z QSOs correlates with Eddington ratio, but not with black hole mass, suggesting that metallicity enrichment mechanism is different between low-z and high-z.

• The Bulletin of The Korean Astronomical Society
• /
• v.37 no.2
• /
• pp.70.1-70.1
• /
• 2012

We investigate 53 late-type galaxies in Virgo to get better understanding galaxy evolution driven by environmental effects in the cluster. The goal is to study how galaxies are strongly affected gravitationally by their surroundings and/or how interstellar medium (ISM) of galaxies changes through the interaction with intracluster medium (ICM). To quantify these, a variety of diagnostic methods have been introduced. Our diagnostics have two different perspectives. First, we have carefully examined the morphological and kinematical properties of individual galaxies using high resolution HI images and compared with multi-wavelength data. Based on the visual inspection, we have identified signatures of the interactions with other galaxies or the ICM. Second, we have quantified influence of local environments of individual galaxies using X-ray data and optical catalog of the cluster. By combining all the diagnostics, we have identified the environmental effect(s) at work on individual galaxies. We also probe the environmental processes as a function of the cluster centric distance. Various gravitational interactions are found throughout the cluster, while the ICM-ISM interaction is mainly dominant near the cluster center. However, we find some evidence that galaxies start losing their gas already in the low density outskirts of the cluster.

• The Bulletin of The Korean Astronomical Society
• /
• v.36 no.1
• /
• pp.86.2-86.2
• /
• 2011

The faint wing-like features at velocities beyond the velocity boundaries of the Galactic rotation (Forbidden-Velocity Wings, FVWs) in the large-scale position-velocity diagrams of the HI surveys are thought to be associated with dynamical Galactic events. The primary candidates of these FVWs are rapidly expanding HI shells of the old Galactic supernova remnants (SNRs), which are too faint to be visible in other frequencies. The unprecedented sensitivity and resolution of the I-GALFA survey enable detection of "all" HI shells of Galactic SNRs at forbidden velocities predicted by Koo and Kang (2004). Therefore, comparing the distribution of the FVWs visible in the I-GALFA survey and that of the model will improve our understanding on the interstellar medium and the evolution of SNRs. We have been developing an automatic searching algorithm for FVWs in the I-GALFA survey to minimize the selection effects of visual inspection used in the previous FVW study. We present the searching mechanism for FVWs and the statistical properties of the automatically searched FVWs. Also, we discuss the similarities and the differences between the distribution of the identified FVWs and that of the SNRs predicted by Koo and Kang (2004).

• Journal of The Korean Astronomical Society
• /
• v.33 no.1
• /
• pp.1-17
• /
• 2000

We have performed extensive simulations of response of gaseous disk in barred galaxies using SPH method. The gravitational potential is assumed to be generated by disk, bulge, halo, and bar. The mass of gaseous disk in SPH simulation is assumed to be negligible compared to the stellar and dark mass component, and the gravitational potential generated by other components is fixed in time. The self-gravity of the gas is not considered in most simulations, but we have made a small set of simulations including the self-gravity of the gas. Non-circular component of velocity generated by the rotating, non-axisymmetric potential causes many interesting features. In most cases, there is a strong tendency of concentration of gas toward the central parts of the galaxy. The morphology of the gas becomes quite complex, but the general behavior can be understood in terms of simple linear approximations: the locations and number of Lindblad resonances play critical role in determining the general distribution of the gas. We present our results in the form of 'atlas' of artificial galaxies. We also make a brief comment on the observational implications of our calculations. Since the gaseous component show interesting features while the stellar component behaves more smoothly, high resolution mapping using molecular emission line for barred galaxies would be desirable.

• The Bulletin of The Korean Astronomical Society
• /
• v.44 no.1
• /
• pp.74.3-75
• /
• 2019

Recent Integral Field Spectroscopy (IFS) studies revealed that not only late type galaxies (LTGs) but also early type galaxies (ETGs) have various kinds of kinematic rotation. (e.g. not clearly detectable rotation, disk-like rotation, kinematically distinct core (Cappellari 06)) Among the various studies about galactic kinematics, one of the most notable anomalies is the star-gas misalignment. The gas forms stars and stars release gas through mass-loss. In this process, their angular momentum is conserved. Therefore, kinematic decoupling between stars and gas can occur due to external gas inflow or perturbation of components. There are some possible origins of misalignment: cold gas from filaments, hot gas from outer halo, interaction or merging events with galaxies and environmental effects. Misalignment, the black box from mixture of internal and external gas, can be an important keyword for understanding further about galaxies' kinematics and external processes. Using both SAMI IFS data(Sydney-AAO Multi-object Integral field spectrograph Galaxy Survey, Croom+12) and Horizon-AGN simulation(Dubois+14), we examined misaligned galaxies properties and distribution. Because the simulation has lots of galaxies at various z, we were able to study history of formation, evolution and extinction of misalignment, which was hard to be done with observation only.

• The Bulletin of The Korean Astronomical Society
• /
• v.43 no.2
• /
• pp.32.3-32.3
• /
• 2018

We use the synthetic light-cone from the cosmological hydrodynamical simulation Horizon-AGN to produce a mock photometric galaxy catalogue on the redshift range 0

• The Bulletin of The Korean Astronomical Society
• /
• v.46 no.1
• /
• pp.56.2-56.2
• /
• 2021

Constraining the structure and thus the fate of giant molecular clouds (GMCs), the primary sites of star formation in galaxies, is crucial to understand the evolution of galaxies themselves. Exploiting the unprecedented sensitivity and angular resolution of the Atacama Large Millimeter/sub-millimeter Array (ALMA), we have measured the spatially-resolved (~ 20 pc resolution) properties of the GMCs in two nearby late-type galaxies, NGC 5806 (SAB(s)b) and NGC 6753 ((R)SA(r)b), as part of the WISDOM project. Although these results are preliminary, we identified ~ 200 resolved GMCs in NGC 5806 within a radius of 500 pc, most within a nuclear ring structure, and ~ 400 resolved GMCs in NGC 6753 within a radius of 2 kpc, most within a flocculent spiral structure. The GMCs of NGC 5806 have similar sizes but slightly higher linewidths than clouds in the Milky Way disc. Because the GMCs also have higher surface densities, the calculated cloud Virial parameters are nevertheless about unity, suggesting that the GMCs of NGC 5806 are in gravitational equilibrium and thus long lived. This is contrary to other WISDOM results on earlier-type galaxies, where large cloud linewidths are likely due to shear associated with the local (circular) orbital motions (rather than the clouds' self-gravity), and the clouds are either marginally or not gravitationally bound. These results support the notion that spheroids alter the dynamical states of clouds (morphological quenching), that are otherwise (i.e. in galaxy discs) fairly homogenous and similar to those of the Milky Way.