2006 ~ 2007
The astrophysics and cosmology group of Korea Institute for Advanced Study consists of Prof. Changbom Park, and five research fellows. Prof. Park, as the director of the Korean Scientist Group (KSG), has continued leading the international consortium of the Sloan Digital Sky Survey (SDSS). The SDSS is an ambitious international collaboration aiming to survey a hugh volume of the universe by taking a million galaxy spectra, and the data from the survey is expected to contribute to the development of cosmology profoundly. Now the KSG consists of 10 professors and 27 postdocs and graduate students in eight institutes in Korea. In September, 2006, KIAS and KSG have hosted the 2006 SDSS Collaboration Meeting at Seoul National University. The meeting, participated by 120 people, was open to all Korean scientists, and provided a momentum for stimulating great interests in galactic and extra-galactic astronomy in Korea. The astrophysics and cosmology group has organized the biennial international workshop "The 2nd KIAS International Workshop on Cosmology and Structure Formation". There were 12 invited talks on inflation, cosmic microwave background (CMB), large-scale structure and galaxy formation. Prof. Park and Dr. Juhan Kim have formed a group called 'Computational Astrophysics Group' to share knowledges on computational hydrodynamics, MHD, N-body simulation, and parallel computation among astrophysicists in Korea. The first meeting was held in June, 2006, and five meetings have been held at KIAS.
Individual members of the astrophysics and cosmology group have also been active in his/her research in 2006. Dr. Chan-Gyung Park together with Prof. Park have produced a foreground-cleaned three-year (CMB) temperature fluctuation map from the WMAP three-year data. They examined the statistical properties of the large scale modes, and found that most known peculiarities of large scale modes were only marginally significant. The genus statistic was measured from the CMB map to test for the Gaussianity of the primordial fluctuations, and it was found that the Galactic north-south asymmetry was also marginally significant. Using the SDSS galaxy sample, Dr. Chan-Gyung Park and Prof. Park have also been measuring the velocity dispersion functions of the early and late type galaxies.
Dr. Yun-Young Choi and Prof. Park have studied the relations among various physical properties of the SDSS galaxies. It was found that, at fixed morphology and luminosity, early types show very small dispersions in color, color gradient, concentration, size, and velocity dispersion. Many parameters manifest different behaviors across the absolute magnitude of about M*. They have also used a new local density estimator to explore the relation between the environment and galaxy properties. It was found that variations of galaxy properties with environment were almost entirely due to the dependence of morphology and luminosity on environment. When morphology and luminosity are fixed, other physical properties are nearly independent of local density without any break or feature. Weak residual dependences on environment include that of the color of late-types and of the luminosity-velocity dispersion relation of early types. Motivated by the study of effects of local density and the nearest companion on galaxy morphology, they have proposed a mechanism that the morphology of galaxies in galaxy systems is transformed by the tidal force.
Dr. Juhan Kim has improved the cosmological N-body simulation code for better parallel performance and memory-cost. Dr. Kim, Prof. Park, and Dr. Choi are using new simulation results to investigate the environmental effects on the luminosity, spin, and shape of mock galaxies (dark halos) in the one-to-one monotonic correspondence model and compared the results with those found from the SDSS sample.
Dr. Pravabati Chingangbam has studied the effect of a fast rolling period in between two slow roll inflationary periods on the evolution of curvature perturbations generated by a Dirac-Born-Infeld scalar field, and showed that the amplitude of perturbations get amplified at super-Hubble scales. The entropy perturbations are shown to act as the source for this amplification. Dr. Chingangbam has also worked on the dynamics of DBI scalar field, minimally coupled to gravity, with a potential which is negative for a range of field values. The fate of the universe at late times is found to depend on the negative depth of the potential.
Recently, Dr. Jaiseung Kim joined the group from Brown University. Dr. Kim is working on a new method to reduce the Galactic foreground emission in the CMB maps by requiring randomness of spherical harmonics modes of the CMB.