• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.381-383
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• 2015

MOdified Newtonian Dynamics (MOND) is an alternative to the dark matter paradigm. MOND asserts that when the magnitude of acceleration is smaller than the acceleration parameter $a_0$, the response of the system to gravity is stronger (larger acceleration) than the one given by Newtonian dynamics. The current value of $a_0$ is obtained mostly by observations of spiral galaxies (rotation curves and the Tully-Fisher relation). We attempt to estimate $a_0$ from the dynamics of elliptical galaxies. We seek elliptical galaxies that act as the lens of gravitational lensing systems and have velocity dispersion data available. We analysed 65 Einstein rings from the Sloan Len ACS survey (SLACS). The mass estimates from gravitation lensing and velocity dispersion agree well with each other, and are consistent with the estimates from population synthesis with a Salpeter IMF. The value of $a_0$ obtained from this analysis agrees with the current value.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.52.1-52.1
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• 2013

We present new analyses of FLS 1718+59, a galaxy-galaxy gravitational lens system in the Spitzer First Look Survey (FLS) Field. A background galaxy (z = 0.245) is severely distorted by a nearby elliptical galaxy (z = 0.08), which can be interpreted as a result of gravitational lensing. We analyze this system by multiple methods, including ELLIPSE fitting, gravitational lens modeling, and surface brightness fitting. From this analysis, we obtain parameters of the lens galaxy using varying approaches and compare them. In the future, we will conduct SED fitting for the lens galaxy and estimate the stellar mass, and compare this with the total mass of the lens to check the M-L relation.

• Publications of The Korean Astronomical Society
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• v.15 no.1
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• pp.15-19
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• 2000

The lens mass determined from the photometrically obtained Einstein time scale suffers from large uncertainty due to the lens parameter degeneracy. The uncertainty can be substantially reduced if the mass is determined from the lens proper motion obtained from astrometric measurements of the source image centroid shifts, ${\delta}{\theta}_c$, by using high precision interferometers from space-based platform such as the Space Interferometry Mission (SIM), and ground-based interferometers soon available on several 8-10m class telescopes. However, for the complete resolution of the lens parameter degeneracy it is required to determine the lens parallax by measuring the parallax-induced deviations in the centroid shifts trajectory, ${\Delta}{\delta}{\theta}_c$ aloe. In this paper, we investigate the detectabilities of ${\delta}{\theta}_c$ and ${\Delta}{\delta}{\theta}_c$ by determining the distributions of the maximum centroid shifts, $f({\delta}{\theta}_{c,max})$, and the average maximum deviations, $(<{\Delta}{\delta}_{c,max}>)$, for different types of Galactic microlensing events caused by various masses. From this investigation, we find that as long as source stars are bright enough for astrometric observations it is expected that $f({\delta}{\theta}_c)$ for most events caused by lenses with masses greater than 0.1 $M_\bigodot$ regardless of the event types can be easily detected from observations by using not only the SIM (with a detection threshold but also the ${\delta}{\theta}_{th}\;\~3{\mu}as)$ but also the ground-based interferometers $(with\;{\delta}{\theta}_{th}\;\~3{\mu}as)$. However, from ground-based observations, it will be difficult to detect ${\Delta}{\delta}{\theta}_c$ for most Galactic bulge self-lensing events, and the detection will be restricted only for small fractions of disk-bulge and halo-LMC events for which the deviations are relatively large. From observations by using the SIM, on the other hand, detecting ${\Delta}{\delta}{\theta}_c$ will be possible for majority of disk and halo events and for a substantial fraction of bulge self-lensing events. For the complete resolution of the lens parameter degeneracy, therefore, SIM observations will be essential.

• Journal of The Korean Astronomical Society
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• v.34 no.2
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• pp.81-97
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• 2001

Since the first proposal by Paczynski, great efforts to detect Galactic dark matter by detecting light variations of stars located in the Magellanic Clouds and Galactic bulge caused by gravitational microlensing have been and are being carried out and more than 1,000 events have been successfully detected. In this paper, we review the progress in the theoretical and experimental progresses in microlensing. We begin with basics of microlensing and summarize the results obtained from the last 8 year observations along with the implications of the results. We also discuss the usefulness of microlensing in other fields of astronomy such as the stellar atmosphere, Galactic binary systems, and extra-solar planets. We finally discuss the problems of the current experiments and the new types of observations that can overcome these problems.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.397-399
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• 2015

Recently, cosmic voids have been recognized as a powerful cosmological probe. A number of studies have focused on the effects of the gravitational lensing by voids on the temperature (and in some cases polarization) anisotropy of the Cosmic Microwave Background (CMB) background at relatively large to medium scales, l ~ 1000. Many of these studies attempt to explain the unusually large cold spot in CMB temperature maps and dynamical evidence of dark energy via detections of late-time integrated Sachs Wolfe (ISW) effect. Here, the effects of lensing by voids on the CMB temperature anisotropy at small scales, up to l = 3000, will be investigated. This work is carried out in the light of the benefits of adding large catalogues of cosmic voids, to be identified by future large galaxy surveys such as EUCLID and LSST, to the analysis of CMB data such as those from Planck mission. Our numerical simulation utilizes two methods, namely, the small-de ectionangle approximation and full ray-tracing analysis. Using the fitted void density profiles and radius (RV ) distribution available in the literature from N-body simulations, we simulated the secondary temperature anisotropy (lensing) of CMB photons induced by voids along a line of sight from redshift 0 to 2. Each line of sight contains approximately 1000 voids of effective radius $RV_{,eff}=35h^{-1}Mpc$ with randomly distributed radial and projected positions. Both methods are used to generate temperature maps. The two methods will be compared for their accuracy and effciency in the implementation of theoretical modeling.

• Journal of The Korean Astronomical Society
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• v.38 no.2
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• pp.191-195
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• 2005

We used Subaru observations of A1689 (z = 0.183) to derive an accurate, model-independent mass profile for the entire cluster, r$\le$2Mpc/h, by combining magnification bias and distortion measurements. The projected mass profile steepens quickly with increasing radius, falling away to zero at r${\~}$1.0Mpc/h, well short of the anticipated virial radius. Our profile accurately matches onto the inner profile, r $\le$200kpc/ h, derived from deep HST / ACS images. The combined ACS and Subaru information is well fitted by an NFW profile with virial mass, $(1.93 \pm 0.20) {\times}10^{15} M_{\bigodot}$, and surprisingly high concentration, $C_{vir} = 13.7^{+1.4}_{-1.1}$, significantly larger than theoretically expected ($C_{vir} {\le}4$), corresponding to a relatively steep overall profile. These results are based on a reliable sample of background galaxies selected to be redder than the cluster E/SO sequence. By including the faint blue galaxy population a much smaller distortion signal is found, demonstrating that blue cluster members significantly dilute the true signal for r $\le$ 400kpc/ h. This contamination is likely to affect most weak lensing results to date.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.70.1-70.1
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• 2014

We analyze a newly discovered galaxy-galaxy scale gravitational lens system, FLS 1718+59 in the Spitzer First Look Survey (FLS) field. A background galaxy (z = 0.245) is severely distorted by a foreground galaxy (z = 0.08), via gravitational lensing. We analyze this system by several methods, including surface brightness fitting (Galfit and Ellipse), gravitational lens modeling (gravlens), and spectral energy distribution fitting (Magphys). From Galfit and Ellipse we measure properties of the lens galaxy, such as the effective radius and the average surface brightness inside it, the ellipticity, and the position angle. gravlens gives us the total mass inside the Einstein radius ($R_{Ein}$), and Magphys provides us an estimate of the stellar mass inside $R_{Ein}$. By comparing these obtained parameters, we confirm that the lens galaxy is an elliptical galaxy on the Fundamental Plane, and calculate the stellar mass function inside $R_{Ein}$, and discuss the implications of the results regarding the initial mass function.

• Publications of The Korean Astronomical Society
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• v.32 no.1
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• pp.293-297
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• 2017

We discuss a new IRAS Faint Source Catalog galaxy redshift catalogue (RIFSCz) which incorporates data from Galex, SDSS, 2MASS, WISE, AKARI and Planck. AKARI fluxes are consistent with photometry from other far infrared and submillimetre missions provided an aperture correction is applied. Results from the Hermes-SWIRE survey in Lockman are also discussed briefly, and the strong contrast between the galaxy populations selected at 60 and $500{\mu}m$ is summarized.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.79.2-79.2
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• 2015

The $M_{BH}-{\sigma}$ relation for galaxies is a stand-out illustration of the co-evolution of galaxies and their central supermassive black holes (SMBHs); however, how this co-evolution occurs and whether this relation holds for SMBHs of the early universe is still a matter of debate. In order to study this at higher redshifts, quasi-stellar objects (QSOs) are the best targets, due to their large sample size and effective $M_{BH}$ estimation. Nevertheless, it is difficult to examine properties of their host galaxies, simply due to the sheer brightness of the QSO itself. Here, we discuss a distinctive method in studying these QSO host galaxies, via gravitational lensing (GL). GL offers a unique approach in determining the mass of the lens object, in this case the host galaxy. QSOs from the SDSS quasar catalog were searched in the Hubble Space Telescope archives, and GL features around them were visually inspected. One such candidate is SDSS J1114-00; to increase its robustness as a GL system candidate, it was observed with the Inamori-Magellan Areal Camera & Spectrograph (IMACS) on the Magellan Baade Telescope at Las Campanas Observatory, to check whether the GL features have identical colors, meaning they are likely to originate from the same source. After confirmation of such GL systems, a sufficiently large sample will enable us to examine the $M_{BH}-{\sigma}$ relation at various redshifts, and in turn, investigate the co-evolution of SMBHs and their host galaxies.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.2
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• pp.27.2-27.2
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• 2015

Strategic plan of Subaru science and operation will be introduced. Currently, Subaru has wide variety of instruments, conducts only classical observations, with less than 5 nights allocation for each proposal. Near future, Subaru will emphasize on surveys, introduce queue mode observations, reduce the number of instruments, and concentrate on large size programs. Large surveys are called Subaru Strategic Programs (SSPs). HSC-SSP is on-going (300 nights for 5 years), PFS-SSP will start at around 2020 (360 nights for 5 years), and IRD-SSP from 2016 (TBD). HSC science includes 1) cosmology with gravitational lensing, 2) lensing studies of galaxies and clusters, 3) photometric redshifts, 4) the Solar system, 5) the Milky Way and the Local Group, 6) AGN/quasars, 7) transients, 8) galaxies at low/high redshifts, and 9) clusters of galaxies. PFS science includes 1) cosmology, 2) galaxy & AGN, and 3) galactic archaeology. Subaru is planning the third pillar instrument, so called ULTIMATE-Subaru, which is the GLAO optical-NIR wide field camera & multi-IFU spectrograph for finding galaxies at ultra high redshift (z>10). Finally the strategy from Subaru to TMT will be presented. Subaru will conduct four major SSPs (HSC, PFS, IRD, ULTIMATE-Subaru) in coming decade to provide targets to TMT. HSC performs wide field surveys to reveal the distribution of dark matter in the Universe. IRD surveys Earth-like young planets to discover ~20 Earth-like habitable planets. PFS studies the expanding Universe to provide a few million emission line galaxies to TMT.