Mun-Suk Chun

The Intrinsic Shapes of Low Surface Brightness Dwarf Irregular Galaxies and Comparison to Other Types of Dwarf Galaxies

In this paper, we measure the ellipticities of 30 low surface brightness (LSB) dwarf irregular (dI) galaxies and compare the ellipticity distribution with that of 80 dwarf elliptical (dEs) and 62 blue-compact dwarfs (BCDs). We find that the ellipticity distribution of LSB dIs is very similar to that of BCDs, and marginally different from that of dEs. We then determine the distribution of intrinsic shapes of dI galaxies and compare this to the distributions of other types of dwarf galaxies under various assumptions. First, we assume that LSB dIs are either all oblate or all prolate, and use a nonparametric analysis to find the best-fitting distribution of intrinsic shapes. With this assumption, we find that the scarcity of nearly circular LSB dIs implies, at the 99% confidence level, that they cannot be a population of randomly oriented oblate or prolate objects, implying that LSB dIs are highly unlikely to be disk-shaped systems. Next, we assume that dIs are triaxial, and use a parametric analysis to find permissible distributions of intrinsic shapes. We find that if the intrinsic axis ratios β and γ are distributed according to a Gaussian with means β0 and γ0 and a common standard deviation of σ, the best-fitting set of parameters for LSB dIs is (β0, γ0, σ) = (0.66, 0.50, 0.15), and the best fit for BCDs is (β0, γ0, σ) = (0.66, 0.55, 0.16), while the best fit for dEs is (β0, γ0, σ) = (0.78, 0.69, 0.24). The dIs and BCDs thus have very similar shape distributions, given this triaxial hypothesis, while the dEs peak at a somewhat more spherical shape. Therefore, our results provide strong observational evidence to support the evolutionary scenario in which the three types of dwarf galaxy have a close relation with each other.

Astrometric Properties of Gravitational Binary-Microlens Events and Their Applications

In this paper, we study the astrometric properties of gravitational microlensing events caused by binary lenses. By investigating the centroid shifts for various types of binary-lens events, we find that the deviations of the centroid shift trajectories from the elliptical ones of single-lens events are characterized by distortions, twistings, and big jumps. We study the conditions of binary-lens system configurations and source star trajectories for individual types of deviations. We find dramatic differences in the astrometric centroid shifts for binary-lens microlensing events that would be degenerate had their parameters been determined photometrically. Therefore, when additional astrometric observations of a binary-lens event are available, one can resolve the ambiguity of the binary-lens fit and uniquely determine the binary-lens parameters.

A COMPARISON OF THE INTRINSIC SHAPES OF TWO DIFFERENT TYPES OF DWARF GALAXIES BLUE COMPACT DWARFS AND DWARF ELLIPTICALS

We measure the apparent shapes for a sample of 62 blue compact dwarf galaxies (BCDs) and compare them with the apparent shapes for a sample of 80 dwarf elliptical galaxies (dEs). The BCDs are flatter, on average, than the dEs, but the difference is only marginally significant. We then use both nonparametric and parametric techniques to determine possible distributions of intrinsic shapes for the BCDs. The hypothesis that BCDs are oblate spheroids can be ruled out with a high confidence level (>99%), but the hypothesis that they are prolate spheroids cannot be excluded. The apparent shapes of BCDs are totally consistent with the hypothesis that they are triaxial ellipsoids. If the intrinsic axis ratios β and γ are distributed according to a Gaussian with means β0 and γ0 and standard deviation σ, we find that the best-fitting distribution for BCDs has (β0, γ0, σ) = (0.66, 0.55, 0.16), while that for dEs has (β0, γ0, σ) = (0.85, 0.64, 0.24). Our results are consistent with the hypothesis that BCDs have a close evolutionary relation with dEs.

Color gradients in four king type globular clusters: NGC 2298, NGC 6402, NGC 6934, and NGC 7089

We present CCD surface photometry for four globular clusters: NGC 2298, NGC 6402, NGC 6934 and NGC 7089. Our photometry was aimed at the central ∼3 arcmin region in each cluster. We show that the surface brightness distributions of these clusters are well described by the isothermal King (1966) models. Our color analysis shows that NGC 7089 exhibits a remarkable color gradient in which the central region is bluer than the outer parts by the amplitude inB-V ∼0.1 mag/□″. Color gradients have also been detected in the central regions of NGC 6402 and NGC 6934, although the variation does not extend to outer part. No gradient has been detected in NGC 2298. Similar color gradients have been previsouly observed exclusively in highly concentrated systems classified as post core collapse clusters.

UBVIJHK PHOTOMETRIC STUDY OF THE OPEN CLUSTER NGC 2849

We present UBVIJHK photometry of the open cluster NGC 2849. Color-magnitude diagrams (CMDs) of this cluster show a well-defined main sequence and a sparse red giant branch. In spite of a relatively large color excess of E(B - V) = 0.50 ± 0.04, our color-color analysis shows that the interstellar reddening toward this cluster is compatible with the standard extinction model. The cluster also has a broad main sequence indicative of binary sequence rather than differential reddening or field contamination, as suggested elsewhere. From the ultraviolet excess, we estimate the metallicity to be [Fe/H] = -0.24 ± 0.12. A distance modulus of (m - M)0 = 13.93 ± 0.17 is obtained from zero-age main-sequence fitting. A CMD comparison with the Padua theoretical models published by Bertelli and coworkers gives an age of log t = 8.8 ± 0.1.

Discovery of remote star clusters in the halo of the Irregular galaxy NGC 6822

We report the discovery of three new star clusters in the halo of the Local Group dwarf irregular galaxy NGC 6822. These clusters were found in the deep images taken with the MegaPrime at the CFHT, covering a total field of 2 deg x 2 deg. The most remote cluster is found to be located as far as 79 arcmin away from the center of NGC 6822. This distance is several times larger than the size of the region in NGC 6822 where star clusters were previously found. Morphological structures of the clusters and color-magnitude diagrams of the resolved stars in the clusters show that at least two of these clusters are proabably old globular clusters.

THE CORE COLLAPSE OF GLOBULAR CLUSTERS WITH STELLAR EVOLUTION

The orbit averaged Fokker-Planck equation is used to study the dynamical evolution of globular clusters. Stellar evolutions according to their masses are incorporated in the model. The initial density distribution is chosen by Plummers model with the initial mass function index α=0.65, 1.35, 2.35, and 3.35. The mass-loss rate is given by the model of Fusi-Pecci and Renzini. It is found that the stellar mass loss acts as an energy source, and thereby affects the dynamical evolution of globular clusters by slowing down the evolution rate and extending the core collapse time. Also, the dynamical length scale is extended.

Radial colour gradient in 47 tuc. Preliminary results

SummaryAfter a brief review of the literature concerning radial colour gradients in globular clusters, we give preliminary results on the radial colour gradient in 47 Tuc.

Correlation between the physical parameters and morphological type of spiral galaxies

From PDS scans of the late-type spiral galaxies NGC 1313, 1365, 6946, and 7793, we determine the surface brightness distributions. These distributions enable us to calculate physical parameters such as R e * μ e , α−1, μ0, D/B, C21, and M/L. The values of some of these parameters were also compiled from other studies, and all were compared with morphological type T. One of the meaningful results shows that there is a certain correlation between D/B and T, which is constant until T ≤ 1 or 2, and then increases sharply for T ≥ 1 or 2. This may support the validity of the intrinsic formation theory.