• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.59.2-59.2
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• 2018

Recent studies of galaxy clusters have shown that the galaxy clusters in dense environment tend to have lower star formation rate in local universe with z < 1. However, this correlation is not significant in galaxy clusters with z > 1. The study of galaxy clusters around z=1 can yield insight into cosmological galaxy evolution. Nevertheless, the identification of galaxy clusters beyond the scope of immediate local universe requires wide field data in optical and near-infrared bands. By incorporating data from Canada-France-Hawaii Telescope Legacy Survey(CFHTLS) and Infrared Medium-Deep Survey(IMS), the photometric redshifts of galaxies in CFHTLS W2 field were calculated. Using spatial distribution and photometric redshifts, the galaxies in the field were divided into redshift bins. The image of each redshift bin was analyzed by measuring the number density within proper distance of 1Mpc. By comparing high density regions in consecutive redshift bins, we identified the cluster candidates and mapped the large-scale structure within the CFHTLS W2 field.

• Journal of The Korean Astronomical Society
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• v.49 no.5
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• pp.225-232
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• 2016

We present an optical imaging survey of AKARI Deep Field South (ADF-S) using the Korea Microlensing Telescope Network (KMTNet), to find optical counterparts of dusty star-forming galaxies. The ADF-S is a deep far-infrared imaging survey region with AKARI covering around 12 deg², where the deep optical imaging data are not yet available. By utilizing the wide-field capability of the KMTNet telescopes (~4 deg²), we obtain optical images in B, R and I bands for three regions. The target depth of images in B, R and I bands is ~24 mag (AB) at 5σ, which enables us to detect most dusty star-forming galaxies discovered by AKARI in the ADF-S. Those optical datasets will be helpful to constrain optical spectral energy distributions as well as to identify rare types of dusty star-forming galaxies such as dust-obscured galaxy, sub-millimeter galaxy at high redshift.

• Journal of The Korean Astronomical Society
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• v.29 no.2
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• pp.245-253
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• 1996

We have analyzed infrared (IR) images of Jupiter which was observed at the McDonald Observatory, Texas, U.S.A., during the P/SHoemaker-LEvy 9 (SL9) impact period and about one week after the last impact. The IR images were obtained on the 2.7m telescope using a NICMOS array with filters to isolate the $1.5{\mu}m\;NH_3\; band,\;the\;2.3{\mu}m\;CH_4\;band,\;the\;2.12{\mu}m\;H_2\;S(0)$ pressure-induced absorption, and the continua at $1.58{\mu}m\;and\;2.0{\mu}m$ (short K-band). All images except those with the $1.58{\mu}m$ continuum filter show bright impact sites against the relatively dark Jovian disk near the impact latitude of about $45^{\circ}$ S. This implies that dusts originated from the impacts reflect the solar radiation at high altitudes before absorbed by stratospheric $CH_4,\;NH_3 \;or\;H_2$. The impact sites observed with the $2.3{\mu}m$ filter are conspicuously bright against a very dark background. The morphology of impact sites, G, L, and H at 2.3 and $2.12{\mu}m$ filters shows clearly an asymmetric structure toward the incident direction of the comet fragments, in agreement with the studies of visible impact images obtained with the Hubble Space Telescope. Comparisons of reflectances of G, L, and H sites with simple radiative transfer models suggest that optically thick dust layers were formed at high altitudes at which methane absorption attenuates incoming sunlight only by about $1\%$. The dust layers in these sites seem to form at about the same altitude regardless of the magnitude of the impacts, but they appear to descend gradually after the impacts. The dust layers have optical depths of 2-5, according to the models.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.65.3-66
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• 2018

We present the results of high-resolution optical (R ~ 30,000) and near-infrared (R ~ 45,000) spectroscopic monitoring observations of a new FU Orionis-like young stellar object, 2MASS J06593158-0405277. FU Orionis objects (FUors) are well-studied examples of episodic accretion because of their outburst phenomenon. Recently, 2MASS J06593158-0405277 exhibited an outburst and was identified as a FUor. It provides an important opportunity to investigate the whole FUors phenomenon from its pre-outburst to its post-outburst phase. We monitored 2MASS J06593158-0405277 with the BOES and the IGRINS since Dec 25, 2014 (UT). We detected several wind and disk features and present here our analysis for time variations of those spectral lines.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.1
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• pp.45.2-46
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• 2021

The Hayabusa 2 mission target asteroid (162173) Ryugu is a near-Earth, carbonaceous (C-type) asteroid. Before the arrival, this asteroid is expected to be covered with mm- to cm- sized grains through the thermal infrared observations [1]. These grains are widely understood to be formed by past impacts with other celestial bodies and fractures induced by thermal fatigue [2]. However, the close-up images by the MASCOT lander showed lumpy boulders but no abundant fine grains [3]. Morota et al. suggested that there would be submillimeter particles on the top of these boulders but not resolved by Hayabusa 2's onboard instruments [4]. Hence, we conducted polarimetry of Ryugu to investigate microscopic grain sizes on its surface. Polarimetry is a powerful tool to estimate physical properties such as albedo and grain size. Especially, it is known that the maximum polarization degree (Pmax) and the geometric albedo (pV) show an empirical relationship depending on surface grain sizes [5]. We observed Ryugu from UT 2020 November 30 to December 10 at large phase angles (ranging from 78.5 to 89.7 degrees) to derive Pmax. We modified TRIPOL (Triple Range Imager and POLarimeter, [6]) to attach to the 1.8-m telescope at the Bohyunsan Optical Astronomy Observatory (BOAO). With this instrument, we observed the asteroid and determined linear polarization degrees at the Rc-band filter. We obtained sufficient data sets from 7 nights at this observatory to determine the Pmax value, and collaborated with other observatories in Japan (i.e., Hokkaido University, Higashi-Hiroshima, and Nishi-Harima) to acquire linear polarization degrees of the asteroid from total 24 nights observations with large phase angle coverage (From 28 to 104 degrees). The observational results have been published in Kuroda et al. (2021) [7]. We thus found the dominance of submillimeter particles on the surface of Ryugu from the comparison with other meteorite samples from the campaign observation. In this presentation, we report our activity to modify the TRIPOL for the 1.8-m telescope and the polarimetric performance. We also examine the rotational variability of the polarization degree using the TRIPOL data.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.1
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• pp.49.1-49.1
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• 2014

The NISS (Near-infrared Imaging Spectrometer for Star formation history) onboard NEXTSat-1 is the near-infrared instrument onboard NEXTSat-1 which is being developed by KASI. The imaging low-resolution spectroscopic observation in the near-infrared range for nearby galaxies, low background regions, star-forming regions and so on will be performed on orbit. After the System Requirement Review, the optical design is changed from on-axis to the off-axis telescope which has a wide field of view (2 deg. ${\times}$ 2 deg.) as well as the wide wavelength range from 0.95 to $3.8{\mu}m$. The mechanical structure is considered to endure the launching condition as well as the space environment. The design of relay optics is optimized to maintain the uniform optical performance in the required wavelength range. The stray light analysis is being made to evade a light outside a field of view. The dewar is designed to operate the infrared detector at 80K stage. From the thermal analysis, we confirmed that the telescope can be cooled down to around 200K in order to reduce the large amount of thermal noise. Here, we report the current status of the NISS development.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.50.2-50.2
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• 2013

Galaxy clusters, the largest gravitationally bound systems, are an important means to place constraints on cosmological models. Moreover, they are excellent places to test galaxy evolution models in connection to the environments. To this day, massive clusters have been found unexpectedly at high redshfit (Kang & Im 2009, Durret et al. 2011, Tashikawa et al. 2012), and evolution of galaxies in cluster has not been fully understood. Finding galaxy cluster candidates at z > 1 in wide, deep imaging survey data will enable us to solve such issues of modern extragalactic astronomy. We report new candidates of galaxy clusters in the wide and deep survey fields, European Large Area ISO Survey North1(ELAIS-N1) and North2(ELAIS-N2) fields, covering sky area of $8.75deg^2$ and $4.85deg^2$ each. We also suggest a new useful colour-colour selection technique to separate 1 < z < 2 galaxies from low-z galaxies by combining multi-wavelength data from the UKIRT Infrared Deep Sky Survey Deep Extragalactic Survey (UKIDSS DXS, JK bands), Spitzer Wise-area InfraRed Extragalactic survey (SWIRE, Optical-Infrared bands), Canada France Hawaii Telescope (CFHT, z band) and Infrared Medium-deep Survey(IMS, J band).

• Journal of Astronomy and Space Sciences
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• v.33 no.1
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• pp.1-11
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• 2016

Over the last few years, the data obtained using the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope has provided new insights on high-energy processes in globular clusters, particularly those involving compact objects such as MilliSecond Pulsars (MSPs). Gamma-ray emission in the 100 MeV to 10 GeV range has been detected from more than a dozen globular clusters in our galaxy, including 47 Tucanae and Terzan 5. Based on a sample of known gammaray globular clusters, the empirical relations between gamma-ray luminosity and properties of globular clusters such as their stellar encounter rate, metallicity, and possible optical and infrared photon energy densities, have been derived. The measured gamma-ray spectra are generally described by a power law with a cut-off at a few gigaelectronvolts. Together with the detection of pulsed γ-rays from two MSPs in two different globular clusters, such spectral signature lends support to the hypothesis that γ-rays from globular clusters represent collective curvature emission from magnetospheres of MSPs in the clusters. Alternative models, involving Inverse-Compton (IC) emission of relativistic electrons that are accelerated close to MSPs or pulsar wind nebula shocks, have also been suggested. Observations at >100 GeV by using Fermi/LAT and atmospheric Cherenkov telescopes such as H.E.S.S.-II, MAGIC-II, VERITAS, and CTA will help to settle some questions unanswered by current data.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.2
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• pp.35.2-35.2
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• 2010

It is crucial to understand the dust properties in the early universe since they provide important clues about how the early cosmic star formation should be interpreted in the presence of dust extinction. GRB 071025 is an unusually red GRB that occured at high redshift, offering an unique opportunity to study the dust properties in the early universe. We investigate the extinction properties of GRB 071025 through the analysis of RIJHK data obtained with the 1-m telescope at Mt. Lemmon Optical Astronomy Observatory (LOAO) and Simultaneous Quad Infrared Imaging Device (SQIID) on the Kitt-Peak Mayall 4-m telescope. Our dataset is independent from that in a previous work (Perley et al. 2010) where a small systematic photometric errors could complicate the interpretation. After determining the temporal power law exponent with five I-band frames from LOAO, we construct a multi-band monochromatic SED of the GRB afterglow. By using various extinction laws, we find that the SED is best fitted with models that incorporate SNe II dust and derive a photometric redshift of 4.99(+0.12/-0.03). Our results strongly support the prior claim that dusts in GRB 071025 originate mainly from supernovae, implying SNe II predominantly contributed to the dust enrichment in the early universe (z ~ 5).

• Journal of Astronomy and Space Sciences
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• v.31 no.1
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• pp.83-90
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• 2014

The NISS onboard NEXTSat-1 is being developed by Korea astronomy and space science institute (KASI). For the study of the cosmic star formation history, the NISS performs the imaging spectroscopic observation in the near-infrared range for nearby galaxies, low background regions, star-forming regions and so on. It is designed to cover a wide field of view ($2{\times}2$ deg) and a wide wavelength range from 0.95 to $3.8{\mu}m$ by using linear variable filters. In order to reduce the thermal noise, the telescope and the infrared sensor are cooled down to 200 K and 80 K, respectively. Evading a stray light outside the field of view and making the most use of limited space, the NISS adopts the off-axis reflective optical system. The primary and the secondary mirrors, the opto-mechanical part and the mechanical structure are designed to be made of aluminum material. It reduces the degradation of optical performance due to a thermal variation. This paper presents the study on the conceptual design of the NISS.