arXiv subjects
Publications and source records attributed to Myung Gyoon Lee.
Supernovae play a critical role in observational cosmology as well as in astrophysics of stars and galaxies. Recent era has seen dramatic progress in the research of supernovae. Several programs to search systematically supernovae in nearby to distant galaxies have been very successful. Recent progresses in the modern supernova search are reviewed.
We present a study of globular clusters in the inner region of the giant elliptical galaxy NGC 4472, based on the HST WFPC2 archive data. We have found about 1560 globular cluster candidates at the galactocentric radius r < 4 arcmin. V-(V-I) diagram of these objects shows a dominant vertical structure which consists obviously of two components: blue globular clusters (BGCs) and red globular clusters (RGCs). The luminosity function of the globular clusters is derived to have a peak at V(max)=23.50+/-0.16 from Gaussian fitting. The distance to NGC 4472 is estimated to be d=14.7+/-1.3 Mpc.The peak luminosity for the RGCs is similar to that for the BGCs, which indicates that the RGCs may be several Gyrs younger than the BGCs. The mean luminosity of the bright BGCs decreases by 0.2 mag with increasing galactocentric radius over the range of 9 arcmin, while that of the RGCs does not. The observed color distribution of these globular clusters is distinctively bimodal with peaks at (V-I) = 0.98 and 1.23. The mean observed color of all the globular clusters with V < 23.9 mag is derived to be (V-I)=1.11. These colors are exactly the same as those for the globular clusters in M87. It is found that the relative number of the BGCs to the RGCs is increasing with the increasing galactocentric radius. Surface number density profiles of both the BGCs and RGCs get flat in the central region, and the core radii of the globular cluster systems are measured to be r_c = 1.9 arcmin for the BGCs, r_c = 1.2 arcmin for the RGCs, and r_c = 1.3 arcmin for the total sample, which are much larger than the stellar core of the galaxy. In general the properties of the globular clusters in the inner region of NGC 4472 are consistent with those of the globular clusters in the outer region of NGC 4472.
We present deep wide field (16'.4 x 16'.4) Washington CT1 CCD surface photometry of the giant elliptical galaxy NGC 4472, the brightest member in the Virgo cluster. Our data cover a wider field than any previous CCD photometry as well as going deeper. Surface brightness profiles of NGC 4472 are not well fit by a single King model, but they can be fit approximately by two King models: with separate models for the inner and outer regions. Surface brightness profiles for the outer region can also be fit approximately by a deVaucouleurs law. There is clearly a negative color gradient within 3' of NGC 4472, in the sense that the color gets bluer with increasing radius. The slope of the color gradient for this region is derived to be $Δμ(C-T_1)$ = -0.08 mag arcsec$^{-2}$ for $Δ\log r =1$, which corresponds to a metallicity gradient of $Δ$ [Fe/H] $= -0.2$ dex. However, the surface color gets redder slowly with increasing radius beyond 3'. A comparison of the structural parameters of NGC 4472 in C and T1 images has shown that there is little difference in the ellipse shapes between isochromes and isophotes. In addition, photometric and structural parameters of NGC 4472 have been determined.
We present deep BVRI CCD photometry of the stars in the dwarf irregular galaxy SagDIG. The color-magnitude diagrams of the measured stars in SagDIG show a blue plume which consists mostly of young stellar populations, and a well-defined red giant branch (RGB). The foreground reddening of SagDIG is estimated to be E(B-V)=0.06. The tip of the RGB is found to be at I_(TRGB)=21.55 +/- 0.10 mag. From this the distance to this galaxy is estimated to be d = 1.18 +/- 0.10 Mpc. This result, combined with its velocity information, shows that it is a member of the Local Group. The mean metallicity of the red giant branch is estimated to be [Fe/H] < -2.2 dex. This shows that SagDIG is one of the most metal-poor galaxies. Total magnitudes of SagDIG (< r_H (= 107 arcsec)) are derived to be B^T=13.99 mag, V^T=13.58 mag, R^T=13.19 mag, and I^T=12.88 mag, and the corresponding absolute magnitudes are M_B=-11.62 mag, M_V=-11.97 mag, M_R=-12.33 mag, and M_I=-12.60 mag. Surface brightness profiles of the central part of SagDIG are approximately fit by a King model with a core concentration parameter c = log (r_t / r_c) ~ 0.6, and those of the outer part follow an exponential law with a scale length of 37 arcsec. The central surface brightness is measured to be mu_B (0) = 24.21 mag arcsec^(-2) and mu_V (0) =23.91 mag arcsec^(-2). The magnitudes and colors of the brightest blue and red stars in SagDIG (BSG and RSG) are measured to be, respectively, _BSG = 19.89 +/- 0.13 mag, <(B-V)(3)>_BSG = 0.08 +/- 0.07 mag, _RSG = 20.39 +/- 0.10 mag, and <(B-V)(3)>_RSG = 1.29 +/- 0.12 mag. The corresponding absolute magnitudes are derived to be _BSG = -5.66 mag and _RSG = -5.16 mag, which are about one magnitude fainter than those expected from conventional correlations with galaxy luminosity.
We present deep BVI CCD photometry of the stars in the dwarf galaxy DDO 210. The color-magnitude diagrams of DDO 210 show a well-defined red giant branch (RGB) and a blue plume. The tip of the RGB is found to be at I_TRGB = 20.95 +/- 0.10 mag. From this the distance to DDO 210 is estimated to be d = 950 +/- 50 kpc. The corresponding distance of DDO 210 to the center of the Local Group is 870 kpc, showing that it is a member of the Local Group. The mean metallicity of the red giant branch stars is estimated to be [Fe/H] = -1.9 +/- 0.1 dex. Integrated magnitudes of DDO 210 within the Holmberg radius (r_H=110 arcsec = 505 pc) are derived to be M_B=-10.6 +/- 0.1 mag and M_V=-10.9 +/- 0.1 mag. B and V surface brightness profiles of DDO 210 are approximately consistent with an exponential law with scale lengths r_s(B) = 161 pc and r_s(V) = 175 pc. The brightest blue and red stars in DDO 210 (BSG and RSG) are found to be among the faintest in the nearby galaxies with young stellar populations: _{BSG} = -3.41 +/- 0.11 mag and _{RSG} = -4.69 +/- 0.13 mag. An enhancement of the star formation rate in the recent past (several hundred Myrs) is observed in the central region of DDO 210. The opposite trend is observed in the outer region of the galaxy, suggesting a possible two-component structure of the kind disk/halo found in spiral galaxies. The real nature of this two-component structure must, however, be confirmed with more detailed observations.
We present deep BVRI CCD photometry of the stars in the dwarf irregular galaxy UKS 2323-326 in the Sculptor Group. The color-magnitude diagrams of the measured stars in UKS 2323-326 show a blue plume which consists mostly of young stellar populations, and a well-defined red giant branch (RGB). The tip of the RGB is found to be at I_TRGB = 22.65 +/- 0.10 mag. From this the distance to this galaxy is estimated to be d = 2.08 +/- 0.12 Mpc. The corresponding distance of this galaxy from the center of the Local Group is 1.92 Mpc, showing that it is located outside the Local Group. The large distance, combined with its velocity information, indicates that it is very likely to be a member of the Sculptor Group. The mean metallicity of the red giant branch is estimated to be [Fe/H] = -1.98 +/- 0.17 dex. Total magnitudes of UKS 2323-326 (< r_H = 70 arcsec) are derived to be B^T=14.07 mag, V^T=13.50 mag, R^T=13.18 mag, and I^T=12.83 mag, and the corresponding absolute magnitudes are M_B=-12.58 mag, M_V=-13.14 mag, M_R=-13.45 mag, and M_I=-13.79 mag. Surface brightness profiles of the central part of UKS 2323-326 are approximately consistent with a King model with a core concentration parameter c = log (r_t / r_c) ~= 1.0, and those of the outer part follow an exponential law with a scale length of 21 arcsec. The magnitudes and colors of the brightest blue and red stars in UKS 2323-326 (BSG and RSG) are measured to be, respectively, _{BSG} = 20.33 +/- 0.25 mag, <(B-V)(3)>_{BSG} = 0.14 +/- 0.07 mag, _{RSG} = 20.74 +/- 0.18 mag, and <(B-V)(3)>_{RSG} = 1.35 +/- 0.08 mag. The corresponding absolute magnitudes are derived to be _{BSG} = -6.31 mag and _{RSG} = -5.91 mag, which are about half magnitude fainter than those expected from conventional correlations with galaxy luminosity.
We present UBVI CCD photometry of two open clusters NGC 1798 and NGC 2192 which were little studied before. Color-magnitude diagrams of these clusters show several features typical for old open clusters: a well-defined main-sequence, a red giant clump, and a small number of red giants. The main sequence of NGC 1798 shows a distinct gap at V ~ 16.2 mag. From the surface number density distribution we have measured the size of the clusters, obtaining 8'.3 (= 10.2 pc) for NGC 1798 and 7'.3 (= 7.5 pc) for NGC 2192. Then we have determined the reddening, metallicity, and distance of these clusters using the color-color diagrams and color-magnitude diagrams: E(B-V)=0. 51\pm0.04, [Fe/H] = -0.47\pm0.15 dex and (m-M)_0=13.1\pm0.2 (d=4.2\pm 0.3 kpc) for NGC 1798, and E(B-V)=0.20\pm0.03, [Fe/H] = -0.31\pm0.15 dex and (m-M)_0=12.7\pm0.2 (d=3.5\pm 0.3 kpc) for NGC 2192. The ages of these clusters have been estimated using the morphological age indicators and the isochrone fitting with the Padova isochrones: 1.4 \pm 0.2 Gyrs for NGC 1798 and 1.1 \pm 0.1 Gyrs for NGC 2192. The luminosity functions of the main sequence stars in these clusters are found to be similar to other old open clusters. The metallicity and distance of these clusters are consistent with the relation between the metallicity and galactocentric distance of other old open clusters.
We present BVRI CCD surface photometry for the central (6'.35 X 6'.35) regions of the dwarf elliptical galaxies NGC 185 and NGC 205 in the Local Group. Surface brightness profiles of NGC 185 (R < 225") and NGC 205 (R < 186") show excess components in the central regions. The colors of NGC 185 get bluer inward at R < 25", while they remain constant at R >= 25". The colors of NGC 205 get bluer inward at 1" < R < 50", and remain flat outside. Our photometry, supplemented by the photometry based on the far-ultraviolet and visual images of the HST archive data, shows that there is an inversion of color at the very nucleus region (at about 1"). The implications of the redder color of the core part of the nucleus compared with neighboring regions are discussed. The amount of the excess components in the central regions of these galaxies is estimated to be ~10^5 solar luminosity. Distributions of dust clouds in the central regions of the two galaxies are also investigated.
We present a comprehensive study of the luminosity function and spatial structure of the globular cluster system of NGC 4472, based on deep wide field Washington CT1 CCD images. The globular cluster luminosity function shows a peak at T1=23.3 +/- 0.1 mag, about 1.5 mags brighter than our 50% completeness limit. We estimate the true distance modulus to NGC 4472 to be (m-M)o = 31.2 +/- 0.2. With our large sample (~ 2000) of bright globular clusters over a wide field, we make a definitive investigation of the spatial structures of the metal-poor and metal-rich cluster populations and find that they are systematically different: (1) The metal-rich clusters are more centrally concentrated than the metal-poor clusters; and (2) The metal-rich clusters are elongated roughly along the major axis of the parent galaxy, while the metal-poor clusters are essentially spherically distributed. In general, the metal-rich clusters closely follow the underlying halo starlight of NGC 4472 in terms of spatial structure and metallicity, while the metal-poor clusters do not. The global value of the specific frequency of the globular clusters in NGC 4472 is estimated to be S_N = 4.7 +/- 0.6. The local specific frequency increases linearly outward from the center of NGC 4472 until ~ 5'.5, beyond which it levels off at S_N ~ 8.5 until the limit of our data at 7'. The specific frequency of both the metal-rich and metal-poor populations shows similar behavior. However, S_N of the metal-poor clusters is about a factor 2 larger than that of the metal-rich clusters in the outer regions. Implications of these results for the origin of the globular clusters in NGC 4472 are discussed.
We present integrated Washington CT1 photometry of 18 bright blue objects discovered in the dwarf galaxy UGC 7636 which is located 5'.5 southeast of the giant elliptical galaxy NGC 4472, the brightest galaxy in the Virgo cluster. Several lines of evidence indicate that UGC 7636 is interacting violently with NGC 4472. These objects are very blue with colors of -0.4 < (C-T1) < 0.6, and their magnitudes are in the range of 20.6 < T1 < 22.9 mag which corresponds to absolute magnitudes of -10.6 < M(T1) < -8.3 mag for a distance modulus of (m-M)o = 31.2. These objects are grouped spatially in three regions: the central region of UGC 7636, the tidal tail region, and the HI cloud region. No such objects were found in the counter tail region. It is concluded that these objects are probably young star clusters which formed < 0.1Gyr ago during the interaction between UGC 7636 and NGC 4472. Surface photometry of UGC 7636 (r < 83") shows that there is a significant excess of blue light along the tidal tail region compared with other regions. The star clusters are bluer than the stellar light in the tidal tail region, indicating that these clusters might have formed later than most stars in the tidal tail region which were formed later than most stars in the main body of the galaxy.