• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.56.2-56.2
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• 2019

Measurement of the location of the baryon acoustic oscillation (BAO) feature in the clustering of galaxies has proven to be a robust and precise method to measure the expansion of the Universe. The best constraints so far have been provided from spectroscopic surveys because the errors on the redshift obtained from spectroscopy are minimal. This in turn means that the errors along the line-of-sight are reduced and so one can expect constraints on both angular diameter distance $D_A$ and expansion rate $H^{-1}$. But, future surveys will probe a larger part of the sky and go to deeper redshifts, which correspond to more number of galaxies. Analysing each galaxy using spectroscopy, which is a time consuming task, will not be practically possible. So, photometry will be the most convenient way to measure redshifts for future surveys such as LSST, Euclid, etc. The advantage of photometry is measuring the redshift of vast number of galaxies in a single exposure, but the disadvantage are the errors associated with the measured redshifts. Using a wedge approach, wherein the clustering is split into different wedges along the line-of-sight ${\pi}$ and across the line-of-sight ${\sigma}$, we show that the BAO information can be recovered even for photometric catalogues with errors along the line-of-sight. This means that we can get cosmological distance constraints even if we don't have spectroscopic information.

• Journal of The Korean Astronomical Society
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• v.46 no.3
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• pp.103-123
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• 2013

Star clusters are superb astrophysical laboratories containing cospatial and coeval samples of stars with similar chemical composition. We initiate the Sejong Open cluster Survey (SOS) - a project dedicated to providing homogeneous photometry of a large number of open clusters in the SAAO Johnson-Cousins' UBV I system. To achieve our main goal, we pay much attention to the observation of standard stars in order to reproduce the SAAO standard system. Many of our targets are relatively small sparse clusters that escaped previous observations. As clusters are considered building blocks of the Galactic disk, their physical properties such as the initial mass function, the pattern of mass segregation, etc. give valuable information on the formation and evolution of the Galactic disk. The spatial distribution of young open clusters will be used to revise the local spiral arm structure of the Galaxy. In addition, the homogeneous data can also be used to test stellar evolutionary theory, especially concerning rare massive stars. In this paper we present the target selection criteria, the observational strategy for accurate photometry, and the adopted calibrations for data analysis such as color-color relations, zero-age main sequence relations, Sp - MV relations, Sp - $T_{eff}$ relations, Sp - color relations, and $T_{eff}$ - BC relations. Finally we provide some data analysis such as the determination of the reddening law, the membership selection criteria, and distance determination.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.72.2-72.2
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• 2013

We present a UBVI and $H{\alpha}$ photometric study of the young open cluster NGC 1893 in the outer Galaxy as part of "Sejong Open cluster Survey (SOS)" project. Using the properties of early-type stars in the photometric diagrams 65 early-type members were selected. More than 120 $H{\alpha}$ emission stars and candidates were found by $H{\alpha}$ photometry. Together with the published young stellar object catalogue and X-ray source list for this cluster, a total of 837 pre-main sequence (PMS) stars were identified in our photometric data. We obtained the mean reddening of < E(B-V) > = $0.56{\pm}0.08$ mag from the (U-B, B-V) diagram and confirmed the normal reddening law ($R_V=3.1$) toward NGC 1893 based on color excess ratios from optical to mid-infrared wavelengths. The zero-age main sequence fitting to the reddening-corrected color-magnitude diagrams gives a distance modulus of $V_0-M_V=12.8{\pm}0.1$. The age of the cluster inferred from stellar evolution models is about 1-2 Myr. We also found the Salpeter/Kroupa type initial mass function for this cluster. Finally, the mass accretion rate of 80 PMS stars with UV excess emission was estimated for the stars with masses from $0.6M_{\odot}$ to $5M_{\odot}$.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.68.1-68.1
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• 2014

M74 is a nearby face-on spiral galaxy that hosts three core-collapse supernovae (SNe) : SN Ic 2002ap, SN II-P 2003gd, and SN II-P 2013ej. Therefore it is an ideal target to investigate the properties of the core-collapse SNe and to improve the calibration of Type II-P SNe as a standardizable candle. However, its distance is not well known. We present a new distance estimate to M74 based on the tip of the red giant branch (TRGB). From the photometry of archival F555W and F814W images taken with the Hubble Space Telescope, we derive the TRGB to be at $ITRGB=26.13{\pm}0.02$ and the distance modulus to be $30.04{\pm}0.04$ (random) ${\pm}0.12$ (systematic) (corresponding to a linear distance, $10.19{\pm}0.14{\pm}0.56Mpc$). With this result, we calibrate the standardized candle method of SNe II-P. From the absolute magnitude of SN 2003gd corrected for its expansion velocity and reddening, we derive the value of the Hubble constant, $H0=72{\pm}6{\pm}7km\;s-1\;Mpc-1$. It is in agreement with the uncertainty with the recent estimates based on the luminosity calibration of Type Ia SNe.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.86.2-86.2
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• 2017

Blue stragglers (BSs) are the objects that are brighter and bluer than the stars at main-sequence turn-off point. In this study, we present the Ca-by and VI photometry for Galactic dwarf spheroidal galaxies using Subaru/Suprime-Cam and investigate the spatial distribution characteristics of BS stars using the hk index as a photometric metallicity indicator. We compare the cumulative radial distribution of the BS stars with those of two groups of red-giant-branch (RGB) stars divided by the hk-index strength, and find that the spatial distribution of all BS stars is closer to that of hk-weak (i.e. metal-poor) RGB stars. We also find that the hk-strong BS stars are more centrally concentrated than the hk-weak ones. We will discuss the use of hk-index as a metallicity indicator for the hot BS stars and suggest possible explanations for the results in terms of the origin of BS stars in the dwarf gal.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.1
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• pp.55.2-55.2
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• 2010

We present a photometric and spectroscopic study of the globular clusters (GCs) in the giant elliptical galaxy NGC 4636 in the Virgo cluster. The photometry of the GCs is based on HST/WFPC2 images for the inner region and deep, wide field Washington $CT_1$ CCD images for the outer region. We investigated the color distribution, the radial number density profile, the spatial distribution, and the mean color of the bright blue GCs about the GCs in NGC 4636. We obtained spectra of the GC candidates in NGC 4636 using the Multi-Object Spectroscopy (MOS) mode of Faint Object Camera and Spectrograph (FOCAS) on the SUBARU 8.2m Telescope. We measured the velocities for 105 GCs in NGC 4636. Using the 238 GCs obtained from combining our results with data in the literature, we investigated the kinematics of the GC system of NGC 4636: the rotation, the velocity dispersion, the radial variation, and the orbit. We also investigated the distribution and the radial variation of the metallicities, ages, and alpha-elements of the GCs in NGC 4636 derived using the absorption lines. We compared the GC kinematics of NGC 4636 with those of other six gEs, finding that the kinematic properties of the GCs are diverse among gEs. We found several correlations between the kinematics of the GCs and the global parameters of their host galaxies. Finally we discuss the implication of the results for the formation models of the GC system in gEs, and suggest a mixture scenario for the origin of the GCs in gEs.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.36-36
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• 2013

Globular clusters (GCs) are among the oldest stellar objects in the universe and provide valuable constraints on many aspects of galaxy evolution. GC systems typically exhibit bimodal color distributions the phenomenon of which has been a major topic in the area of GC research. GC color bimodality established a paradigm where scenarios to explain its origin require two GC groups with different formation origins. The GC division, asserted mainly by photometric color bimodality so far, has been viewed as the presence of two distinct metallicity subgroups within individual galaxies. In this study, we make use of spectroscopy of GC systems associated with two giant galaxies, M31 (the Andromeda) and M87 (NGC 4486), to investigate the GC bimodality and the underlying metallicity distributions. Recent spectroscopy on the globular cluster (GC) system of M31 with unprecedented precision witnessed a clear bimodality in absorption-line index distributions of old GCs. Given that spectroscopy is a more detailed probe into stellar population than photometry; the discovery of index bimodality may point to the very existence of dual GC populations. However, here we show that the observed spectroscopic dichotomy of M31 GCs emerges due to the nonlinear nature of metallicity-to-index conversion and thus one does not necessarily have to invoke two separate GC subsystems. We present spectra of 130 old globular clusters (GCs) associated with the Virgo giant elliptical galaxy M87, obtained using the Multi-Object Spectrography (MOS) mode of Faint Object Camera and Spectrograph (FOCAS) on the Subaru telescope. M87 GCs with reliable metallicity measurements exhibit significant inflection along the color-metallicity relations, through which observed color bimodality is reproduced from a broad, unimodal metallicity distribution. Our findings lend further support to this new interpretation of the GC color bimodality, which could change much of the current thought on the formation of GC systems and their host galaxies.

• Journal of The Korean Astronomical Society
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• v.38 no.4
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• pp.429-435
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• 2005

We present BV CCD photometry for the open clusters Czernik 24 and Czernik 27. These clusters have never been studied before, and we provide, for the first time, the cluster parameters; reddening, distance, metallicity and age. Czernik 24 is an old open cluster with age $1.8{\pm}0.2Gyr$, metallicity $[Fe/H]=-0.41{\pm}0.15dex$, distance modulus $(m-M)_0=13.1{\pm}0.3mag$ ($d=4.1{\pm}0.5kpc$), and reddening $E(B-V)=0.54{\pm}0.12mag$. The parameters for Czernik 27 are estimated to be $age=0.63{\pm}0.07Gyr$, $[Fe/H]=-0.02{\pm}0.10dex$, $(m-M)_0=13.8{\pm}0.2mag$ ($d=5.8{\pm}0.5kpc$), and $E(B-V)=0.15{\pm}0.05mag$. The metallicity and distance values for Czernik 24 are consistent with the relation between the metallicity and the Galactocentric distance of other old open clusters. We find the metallicity gradient of 51 old open clusters including Czernik 24 to be ${\Delta}[Fe/H]/{\Delta}R_{gc}=-0.064 {\pm}0.009dex\;kpc^{-1}$.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.74.3-74.3
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• 2015

We present 2D- photometric decompositions of ~1,200 nearby dwarf galaxies. Our representative sample is derived from 'A catalog of Visually classified galaxies in the Local Universe'(Ann, Seo and Ha APJS,,,2015) of which galaxy morphological types are determined by visual inspection of color images using the Sloan Digital Sky Survey data release 7. In this catalog, dwarf galaxies were divided into 5 subtypes : dS0, dE, dSph, dEbc, dEblue with distinction of the presence of nucleation in dE, dSph, and dS0. The dSph types are less brighter than other types, and galaxies with nuclei are slightly brighter than those with no nuclei in the same types. Sersic index n have a range 1~1.5, and $dE_{un}$ and $dSph_{un}$ galaxies have n less than 1, and $dSph_n$ galaxies have largest values. We performed two-dimensional decomposition of galaxies using GALFIT, and analyzed their structural components, and residual features which are seen in the residual image.

• Publications of The Korean Astronomical Society
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• v.27 no.4
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• pp.141-144
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• 2012

We present the results of Spectral Energy Distribution (SED) fitting of far-infrared galaxies detected in the AKARI Deep Field-South (ADF-S) Survey and discuss their physical properties. Additionally, we perform a comparison between photometric redshifts estimated using only optical and both optical and infrared data. We conclude that our sample consists mostly of nearby galaxies rich in dust and young stars. We observe an improvement in the estimation of photometric redshifts when the IR data are included, comparing to a standard approach based mainly on the optical to UV photometry.