Sang-Yoon Lee

Achievements “BVR Photometry of Supergiant Stars in Holmberg II”,Journal of Astronomy and Space Sciences, Vol. 23, no. 1, p. 1-10, 03/2006 Korea Space Science Society meeting (2006. 4.27-4.28 Suanbo, Korea)   Oral presentation on “Radial Variation of Color Distributions of Globular Clusters” Korea Space Science Society meeting (2006. 10. 26-10.27, Daejon, Korea) Poster presentation on “Radial Variation of Color Distributions of Globular Clusters in Giant Elliptical galaxies” Korean Astronomical Society meeting (2007. 4. 12-4.14, Jeju-do, Korea) Poster presentation on “Radial Variation of Color Distributions of Globular Clusters within Giant Elliptical Galaxies”

Research in Progress Age vs. [Fe/H] contours for globular clusters in NGC 4649. It shows K-S statistics between the observed color distribution and the model color distribution. The contours correspond to the 3δ , 2δ and 1δ confidence levels from center to outside. The left is for the innermost region of the galaxy and the diagrams to the right represent the regions radially further out. The age and [Fe/H] variations at each effective radius. Left panel: the age of GCs does not appear to correlate with radius. The error bars correspond to 2δ confidence level of the K-S test. The age difference in different parts of a galaxy seems within ±1 Gyr. Right panel: [Fe/H], on the other hand, appears to correlate well with radius; as is known from observations, [Fe/H] decrease radially outwards from the galaxy center, and at the outermost region [Fe/H] falls below -1.

Masters Thesis “ Radial Variation of Color Distributions of Globular Clusters in Giant Elliptical Galaxies” Supervisor : Suk-Jin Yoon Abstract The colors of globular clusters (GCs) in most large early-type galaxies are bimodal. It was thought that the radial variation of GC color distribution originates from the different spatial distribution of two GC sub-populations with distinct mean metallicity. Recently, Yoon, Yi, & Lee (2006) (YYL 2006 hereafter) present a theoretical metallicity-color relationship that has a significant inflection and show that such a relation can produce bimodal color distributions even when the underlying metallicity distribution is unimodal. We here apply this hypothesis to the radial variation of cluster color distributions within individual galaxies. YYL (2006) showed color distributions of various galaxies at fixed age. In this thesis, we analyzed the color distributions for a wider age range in order to look for GC age variation in the radial direction. The results show that there is almost no age variation of GCs along the radius, implying that the radial color distribution change is dependent on mean metallicity variation rather than age variation. The idea that the ratio change between blue and red GCs due to mean metallicity variation with the unimodal metallicity distribution function (MDF) offers a new perspective on galaxy formation. This can be interpreted that most GCs in giant elliptical galaxies have a short timescale for formation, and the formation epoch of metal enriched GCs by major merger must have been shortly after that of early, metal poor GCs

Papers in Preparation “Radial Variation of Color Distributions of Globular Clusters in Giant Elliptical Galaxies”