arXiv subjects

Sungho Lee

Publications and source records attributed to Sungho Lee.

44 records · Page 3Linked to original sources

arXiv2010

Proto-Model of an Infrared Wide-Field Off-Axis Telescope

We develop a proto-model of an off-axis reflective telescope for infrared wide-field observations based on the design of Schwarzschild-Chang type telescope. With only two mirrors, this design achieves an entrance pupil diameter of 50 mm and an effective focal length of 100 mm. We can apply this design to a mid-infrared telescope with a field of view of 8 deg X 8 deg. In spite of the substantial advantages of off-axis telescopes in the infrared compared to refractive or on-axis reflective telescopes, it is known to be difficult to align the mirrors in off-axis systems because of their asymmetric structures. Off-axis mirrors of our telescope are manufactured at the Korea Basic Science Institute (KBSI). We analyze the fabricated mirror surfaces by fitting polynomial functions to the measured data. We accomplish alignment of this two-mirror off-axis system using a ray tracing method. A simple imaging test is performed to compare a pinhole image with a simulated prediction.

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arXiv2008

Multi-color Near Infra-red Intra-day and Short Term Variability of the Blazar S5 0716+714

In this paper, we report results of our near-infrared (NIR) photometric variability studies of the BL Lacertae object S5 0716+714. NIR photometric observations spread over 7 nights during our observing run April 2-9, 2007 at 1.8 meter telescope equipped with KASINICS (Korea Astronomy and Space Science Institute Near Infrared Camera System) and J, H, and Ks filters at Bohyunsan Optical Astronomy Observatory (BOAO), South Korea. We searched for intra-day variability, short term variability and color variability in the BL Lac object. We have not detected any genuine intra-day variability in any of J, H, and Ks passbands in our observing run. Significant short term variability ~ 32.6%, 20.5% and 18.2% have been detected in J, H, Ks passbands, respectively, and ~ 11.9% in (J-H) color.

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arXiv2007

Three-Dimensional Observations of H_2 Emission around Sgr A East - I. Structure in the Central 10 Parsecs of Our Galaxy

We have obtained velocity-resolved spectra of the H_2 v=1-0 S(1) (lambda = 2.1218 micron) emission line at 2 arcsec angular resolution (or 0.08 pc spatial resolution) in four regions within the central 10 pc of the Galaxy where the supernova-like remnant Sgr A East is colliding with molecular clouds. To investigate the kinematic, physical, and positional relationships between the important gaseous components in the center, we compared the H_2 data cube with previously published NH_3 data. The projected interaction-boundary of Sgr A East is determined to be an ellipse with its center offset 1.5 pc from Sgr A* and dimensions of 10.8 pc X 7.6 pc. This H_2 boundary is larger than the synchrotron emission shell but consistent with the dust ring which is believed to trace the shock front of Sgr A East. Since Sgr A East is driving shocks into its nearby molecular clouds, we can determine their positional relationships using the shock directions as indicators. As a result, we suggest a revised model for the three-dimensional structure of the central 10 pc. The actual contact between Sgr A East and all of the surrounding molecular material, including the circum-nuclear disk and the southern streamer, makes the hypothesis of infall into the nucleus and feeding of Sgr A* very likely.

astro-ph↗

arXiv2007

Detection of CFIRB with AKARI/FIS Deep Observations

The Cosmic Far-Infrared Background (CFIRB) contains information about the number and distribution of contributing sources and thus gives us an important key to understand the evolution of galaxies. Using a confusion study to set a fundamental limit to the observations, we investigate the potential to explore the CFIRB with AKARI/FIS deep observations. The Far-Infrared Surveyor (FIS) is one of the focal-plane instruments on the AKARI (formerly known as ASTRO-F) satellite, which was launched in early 2006. Based upon source distribution models assuming three different cosmological evolutionary scenarios (no evolution, weak evolution, and strong evolution), an extensive model for diffuse emission from infrared cirrus, and instrumental noise estimates, we present a comprehensive analysis for the determination of the confusion levels for deep far-infrared observations. We use our derived sensitivities to suggest the best observational strategy for the AKARI/FIS mission to detect the CFIRB fluctuations. If the source distribution follows the evolutionary models, observations will be mostly limited by source confusion. We find that we will be able to detect the CFIRB fluctuations and that these will in turn provide information to discriminate between the evolutionary scenarios of galaxies in most low-to-medium cirrus regions.

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arXiv2005

High Resolution Observations of the Near-Infrared Emission from NGC 6822 Hubble V

We observed Hubble V, the brightest HII region complex in the dwarf irregular galaxy NGC 6822, at near-infrared (1.8--2.4 um) wavelengths using the Cooled Grating Spectrometer 4 (CGS4) at the United Kingdom Infra-Red Telescope (UKIRT). The line emission maps of Hubble V show the typical structure of a photo-dissociation region (PDR) where an ionized core, traced by compact He I emission (2.0587 um) and Br_gamma emission (2.1661 um), is surrounded by an outer layer traced by molecular hydrogen (H_2) emission. The measured line ratios of H_2 2-1 S(1) (2.2477 um) / 1-0 S(1) (2.1218 um) from 0.2 to 0.6 and the un-shifted and un-resolved line profiles suggest that the H_2 emission originates purely from a PDR. By comparing the H_2 results with a PDR model, we conclude that Hubble V includes dense (10^{4.5} cm^{-3}) and warm PDRs. In this environment, most of the H_2 molecules are excited by far-UV photons (with a field strength of 10^{2-4} times that of the average interstellar field), although collisional processes de-excite H_2 and contribute significantly to the excitation of the first vibrational level. We expect that Hubble V is in the early stage of molecular cloud dissolution.

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arXiv2003

Interaction between the Northeastern Boundary of Sgr A East and Giant Molecular Clouds

We have detected the v=1-0 S(1) (2.1218 um) and v=2-1 S(1) (2.2477 um) lines of H_2 in the Galactic centre, in a 90 x 27 arcsec region between the northeastern boundary of the non-thermal source, Sgr A East, and the giant molecular cloud (GMC) M-0.02-0.07. The detected H_2 v=1-0 S(1) emission has an intensity of 1.6 - 21 x 10^-18 W/m^2/arcsec^2 and is present over most of the region. Along with the high intensity, the broad line widths (FWHM = 40 - 70 km/s) and the 2-1 S(1) / 1-0 S(1) line ratios (0.3 - 0.5) can be best explained by a combination of C-type shocks and fluorescence. The detection of shocked H_2 is clear evidence that Sgr A East is driving material into the surrounding adjacent cool molecular gas. The H_2 emission lines have two velocity components at about +50 km/s and 0 km/s, which are also present in the NH_3(3,3) emission mapped by McGary, Coil, and Ho (2001, ApJ, 559, 326). This two-velocity structure can be explained if Sgr A East is driving C-type shocks into both the GMC M-0.02-0.07 and the northern ridge of McGary, Coil, and Ho (2001).

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arXiv2002

Correlation Functions of the Abell, APM, and X-ray Clusters of Galaxies

We have measured the correlation functions of the optically selected clusters of galaxies in the Abell and the APM catalogs, and of the X-ray clusters in the X-ray-Brightest Abell-type Clusters of galaxies (XBACs) catalog and the Brightest Clusters Sample (BCS). The same analysis method and the same method of characterizing the resulting correlation functions are applied to all observational samples. We have found that the amplitude of the correlation function of the APM clusters is much higher than what has been previously claimed, in particular for richer subsamples. The correlation length of the APM clusters with the richness R >= 70 (as defined by the APM team) is found to be r_0 = 25.4(+3.1,-3.0) h^(-1)Mpc. The amplitude of correlation function is about 2.4 times higher than that of Croft et al. (1997). The correlation lengths of the Abell clusters with the richness class RC >= 0 and 1 are measured to be r_0 = 17.4(+1.2,-1.1) and 21.0(+2.8,-2.8) h^(-1)Mpc, respectively, which is consistent with our results for the APM sample at the similar level of richness. The richness dependence of cluster correlations is found to be r_0 = 0.40*d_c + 3.2 where d_c is the mean intercluster separation. This is identical in slope with the Bahcall & West (1992)'s estimate, but is inconsistent with the weak dependence of Croft et al. (1997). The X-ray bright Abell clusters in the XBACs catalog and the X-ray selected clusters in the BCS catalog show strong clustering. The correlation length of the XBACs clusters with L_x >= 0.65*10^44 h^(-2)erg/s is 30.3(+8.2,-6.5) h^(-1)Mpc, and that of the BCS clusters with L_x >= 0.70*10^44 h^(-2)erg/s is 30.2(+9.8,-8.9) h^(-1)Mpc. The clustering strength of the X-ray clusters is much weaker than what is expected from the optical clusters.

astro-ph↗

arXiv1998

Correlation Functions of the APM Clusters of Galaxies

We have found that the two-point correlation function of the APM clusters of galaxies has an amplitude much higher than that claimed by the APM group. As the richness limit increases from R = 53 to 80, the correlation length increases from 17.5 to 28.9 h^{-1}Mpc. This indicates that the richness dependence of the APM cluster correlation function is also much stronger than what the APM group has reported. The richness dependence can be represented by a fitting formula r_0 = 0.53 d_c + 0.01, which is consistent with the Bahcall's formula r_0 = 0.4 d_c. We have tried to find the possible reason for discrepancies. However, our estimates for the APM cluster correlation function are found to be robust against variation of the method of calculation and of sample definition.

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