arXiv subjects

Sang Chul Kim

Publications and source records attributed to Sang Chul Kim.

At least 55 records · Page 3Linked to original sources

arXiv2014

The Early Chemical Enrichment Histories of Two Sculptor Group Dwarf Galaxies as Revealed by RR Lyrae Variables

We present the results of our analysis of the RR Lyrae (RRL) variable stars detected in two transition-type dwarf galaxies (dTrans), ESO294-G010 and ESO410-G005 in the Sculptor group, which is known to be one of the closest neighboring galaxy groups to our Local Group. Using deep archival images from the Advanced Camera for Surveys (ACS) onboard the Hubble Space Telescope (HST), we have identified a sample of RR Lyrae candidates in both dTrans galaxies [219 RRab (RR0) and 13 RRc (RR1) variables in ESO294-G010; 225 RRab and 44 RRc stars in ESO410-G005]. The metallicities of the individual RRab stars are calculated via the period-amplitude-[Fe/H] relation derived by Alcock et al. This yields mean metallicities of <[Fe/H]>_{ESO294} = -1.77 +/- 0.03 and <[Fe/H]>_{ESO410} = -1.64 +/- 0.03. The RRL metallicity distribution functions (MDFs) are investigated further via simple chemical evolution models; these reveal the relics of the early chemical enrichment processes for these two dTrans galaxies. In the case of both galaxies, the shapes of the RRL MDFs are well-described by pre-enrichment models. This suggests two possible channels for the early chemical evolution for these Sculptor group dTrans galaxies: 1) The ancient stellar populations of our target dwarf galaxies might have formed from the star forming gas which was already enriched through "prompt initial enrichment" or an "initial nucleosynthetic spike" from the very first massive stars, or 2) this pre-enrichment state might have been achieved by the end products from more evolved systems of their nearest neighbor, NGC 55.

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arXiv2014

Spectroscopic Study of Extended Star Clusters in Dwarf Galaxy NGC 6822

We present a spectroscopic study of the four extended star clusters (ESCs) in NGC 6822 based on the data obtained with Gemini Multi-Object Spectrograph (GMOS) on the Gemini-South 8.1m telescope. Their radial velocities derived from the spectra range from $-61.2 \pm 20.4$ km s$^{-1}$ (for C1) to $-115.34 \pm 57.9$ km s$^{-1}$ (for C4) and, unlike the intermediate age carbon stars, they do not display any sign of systematic rotation around NGC 6822. The ages and metallicities derived using the Lick indices show that the ESCs are old ($\geq 8$ Gyr) and metal poor ([Fe/H]$\lesssim -1.5$). NGC 6822 is found to have both metal poor ([Fe/H]$\approx -2.0$) and metal rich ([Fe/H]$\approx -0.9$) star clusters within 15 arcmin (2 kpc) from the center, while only metal poor clusters are observed in the outer halo with $r \geq 20$ arcmin (2.6 kpc). The kinematics, old ages, and low metallicities of ESCs suggest that ESCs may have accreted into the halo of NGC 6822. Based on the velocity distribution of ESCs, we have determined the total mass and the mass-to-light ratio of NGC 6822: $M_{N6822} = 7.5^{+4.5}_{-0.1} \times 10^{9} M_{\odot}$ and $(M/L)_{N6822} = 75^{+45}_{-1} (M/L)_{\odot}$. It shows that NGC 6822 is one of the most dark matter dominated dwarf galaxies in the Local Group.

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arXiv2013

Do quiescent and active galaxies have different M_BH-sigma relations?

To investigate the validity of the assumption that quiescent galaxies and active galaxies follow the same black hole mass - stellar velocity dispersion relation, as required for the calibration of M_BH estimators for broad line AGNs, we determine and compare the M-sigma relations, respectively, for quiescent and active galaxies. For the quiescent galaxy sample, composed of 72 dynamical M_BH measurements, we update sigma for 28 galaxies using homogeneous H-band measurements that are corrected for galaxy rotation. For active galaxies, we collect 25 reverberation-mapped AGNs and improve sigma measurement for two objects . Combining the two samples, we determine the virial factor $f$, first by scaling the active galaxy sample to the M-sigma relation of quiescent galaxies, and second by simultaneously fitting the quiescent and active galaxy samples, as f=5.1_{-1.1}^{+1.5} and f=5.9_{-1.5}^{+2.1}, respectively. The M-sigma relation of active galaxies appears to be shallower than that of quiescent galaxies. However, the discrepancy is caused by a difference in the accessible M_BH distribution at given sigma, primarily due to the difficulty of measuring reliable stellar velocity dispersion for the host galaxies of luminous AGNs. Accounting for the selection effects, we find that active and quiescent galaxies are consistent with following intrinsically the same M-sigma relation.

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arXiv2013

Calibrating stellar velocity dispersions based on spatially-resolved h-band spectra for improving the m-sigma relation

To calibrate stellar velocity dispersion measurements from optical and near-IR stellar lines, and to improve the black hole mass (MBH)- stellar velocity dispersion (sigma) relation, we measure stellar velocity dispersions based on high quality H-band spectra for a sample of 31 nearby galaxies, for which dynamical MBH is available in the literature. By comparing velocity dispersions measured from stellar lines in the H-band with those measured from optical stellar lines, we find no significant difference, suggesting that optical and near-IR stellar lines represent the same kinematics and that dust effect is negligible for early-type galaxies. Based on the spatially-resolved rotation and velocity dispersion measurements along the major axis of each galaxy, we find that a rotating stellar disk is present for 80% of galaxies in the sample. For galaxies with a rotation component, velocity dispersions measured from a single aperture spectrum can vary by up to ~20%, depending on the size of the adopted extraction aperture. To correct for the rotational broadening, we derive luminosity-weighted velocity dispersion within the effective radius of each galaxy, providing uniformly measured velocity dispersions to improve the MBH-sigma relation.

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arXiv2013

Wide-Field Multiband Photometry of Globular Cluster Systems in the Fornax Galaxy Cluster

We present wide-field multiband photometry of globular cluster (GC) systems in NGC 1399, NGC 1404, and NGC 1387 located at the central region of the Fornax galaxy cluster. Observation was carried out through U, B, V, and I bands, which marks one of the widest and deepest U-band studies on extragalactic GC systems. The present U-band photometry enables us to significantly reduce the contamination by a factor of two for faint sources (Vo~23.5). The main results based on some 2000 GC candidates around NGC 1399, NGC 1404, and NGC 1387 are as follows: (1) the GC system in each galaxy exhibits bimodal color distributions in all colors examined, but the shape of color histograms varies systematically depending on colors; (2) NGC 1399 shows that the mean colors of both blue and red GCs become bluer with increasing galactocentric radius; (3) NGC 1399 shows overabundance of GCs in the directions of NGC 1404 and NGC 1387, indicating their ongoing interactions; and (4) NGC 1399 also exhibits a ~0.5' offset between the center of the inner GC distribution and the galaxy's optical center, suggesting that NGC 1399 is not yet dynamically relaxed and may be undergoing merger events.

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arXiv2012

The chemical properties of low-redshift QSOs

We investigate the chemical properties of low-z QSOs, using archival UV spectra obtained with the HST and IUE for a sample of 70 Palomar-Green QSOs at z < 0.5. By utilizing the flux ratio of UV emission lines (i.e., NV /CIV, (SiIV+OIV])/CIV, and NV/HeII) as metallicity indicators, we compare broad-line region (BLR) gas metallicity with AGN properties, i.e., black hole mass, luminosity, and Eddington ratio. We find that BLR metallicity correlates with Eddington ratio while the dependency on black hole mass is much weaker. Although these trends of low-z AGNs appear to be different from those of high-z QSOs, the difference between low-z and high-z samples is partly caused by the limited dynamical range of the samples. We find that metal enrichment at the center of galaxies is closely connected to the accretion activity of black holes and that the scatter of metallicity correlations with black hole mass increases over cosmic time.

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arXiv2012

What determines the sizes of red early-type galaxies?

The sizes of galaxies are known to be closely related with their masses, luminosities, redshifts and morphologies. However, when we fix these quantities and morphology, we still find large dispersions in the galaxy size distribution. We investigate the origin of these dispersions for red early-type galaxies, using two SDSS-based catalogs. We find that the sizes of faint galaxies (log(M_dyn/M_sun) < 10.3 or M_r > -19.5, where M_r is the r-band absolute magnitude, k-corrected to z = 0.1) are affected more significantly by luminosity, while the sizes of bright galaxies (log(M_dyn/M_sun) > 11.4 or M_r < -21.4) are by dynamical mass. At fixed mass and luminosity, the sizes of low-mass galaxies (log(M_dyn/M_sun) ~ 10.45 and M_r ~ -19.8) are relatively less sensitive to their colors, color gradients and axis ratios. On the other hand, the sizes of intermediate-mass (log(M_dyn/M_sun) ~ 10.85 and M_r ~ -20.4) and high-mass (log(M_dyn/M_sun) ~ 11.25 and M_r ~ -21.0) galaxies significantly depend on those parameters, in the sense that larger red early-type galaxies have bluer colors, more negative color gradients (bluer outskirts) and smaller axis ratios. These results indicate that the sizes of intermediate- and high-mass red early-type galaxies are significantly affected by their recent minor mergers or rotations, whereas the sizes of low-mass red early-type galaxies are affected by some other mechanisms. Major dry mergers also seem to have influenced on the size growth of high-mass red early-type galaxies.

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arXiv2012

WIYN Open Cluster Study : Wide-Field CCD Photometry of the Old Open Cluster NGC 6819

We present a comprehensive photometric study of the old open cluster, NGC 6819 using 1x1 degree field VI MOSAIC CCD imaging taken with the WIYN 0.9m telescope. The resultant color-magnitude diagram (CMD) shows a well developed main sequence (MS) extending from V~14.5 mag down to our photometric limit of V~21 mag. Fitting theoretical isochrones with adopted values of the reddening and metallicity (E(B-V)=0.14, [Fe/H]=+0.09 dex) to the observed CMD yields a distance modulus of (m-M)_{0}=11.93+/-0.10 and an age of ~2.6 Gyr for NGC 6819. Our wide-field imaging reveals that NGC 6819 is larger in areal extent (R=13') than previously thought. The wide-field also benefits our estimate of the degree of field star contamination, and ultimately yields improved measurements of the structural parameters (r_c=2.80', r_t=38.2', and r_h=7') and tidal mass of the cluster (M_{tid}=3542.4 M(sun)). The flattened luminosity and mass functions indicate that NGC 6819 has experienced mass segregation as a result of its dynamical evolution. Our variability study of the cluster blue straggler star (BSS) population using the Welch-Stetson variability index (I_{WS}) has revealed a number of variable BSS candidates.

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arXiv2012

The Globular Cluster System of NGC 4636 and Formation of Globular Clusters in Giant Elliptical Galaxies

We present a spectroscopic analysis of the metallicities, ages, and alpha-elements of the globular clusters (GCs) in the giant elliptical galaxy (gE) NGC 4636 in the Virgo cluster. Line indices of the GCs are measured from the integrated spectra obtained with Faint Object Camera and Spectrograph (FOCAS) on the Subaru 8.2 m telescope. We derive [Fe/H] values of 59 GCs based on the Brodie & Huchra method, and [Z/H], age, and [a/Fe] values of 33 GCs from the comparison of the Lick line indices with single stellar population models. The metallicity distribution of NGC 4636 GCs shows a hint of a bimodality with two peaks at [Fe/H]=-1.23 (sigma=0.32) and -0.35 (sigma=0.19). The age spread is large from 2 Gyr to 15 Gyr and the fraction of young GCs with age < 5 Gyr is about 27%. The [a/Fe] of the GCs shows a broad distribution with a mean value [a/Fe]~0.14 dex. The dependence of these chemical properties on the galactocentric radius is weak. We also derive the metallicities, ages, and [a/Fe] values for the GCs in other nearby gEs (M87, M49, M60, NGC 5128, NGC 1399, and NGC 1407) from the line index data in the literature using the same methods as used for NGC 4636 GCs. The metallicity distribution of GCs in the combined sample of seven gEs including NGC 4636 is found to be bimodal, supported by the KMM test with a significance level of >99.9%. All these gEs harbor some young GCs with ages less than 5 Gyr. The mean age of the metal-rich GCs ([Fe/H] > -0.9) is about 3 Gyr younger than that of the metal-poor GCs. The mean value of [a/Fe] of the gE GCs is smaller than that of the Milky Way GCs. We discuss these results in the context of GC formation in gEs.

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arXiv2012

Accretion Properties of High- and Low-Excitation Young Radio Galaxies

Young radio galaxies (YRGs) provide an ideal laboratory to explore the connection between accretion disk and radio jet thanks to their recent jet formation. We investigate the relationship between the emission-line properties, the black hole accretion rate, and the radio properties using a sample of 34 low-redshift (z < 0.4) YRGs. We classify YRGs as high-excitation galaxies (HEGs) and low-excitation galaxies (LEGs) based on the flux ratio of high-ionization to low-ionization emission lines. Using the Hα luminosities as a proxy of accretion rate, we find that HEGs in YRGs have \sim1 dex higher Eddington ratios than LEGs in YRGs, suggesting that HEGs have higher mass accretion rate or higher radiative efficiency than LEGs. In agreement with previous studies, we find that the luminosities of emission lines, in particular Hα, are correlated with radio core luminosity, suggesting that accretion and young radio activities are fundamentally connected.

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arXiv2012

The Development of WIFIS: a Wide Integral Field Infrared Spectrograph

We present the current results from the development of a wide integral field infrared spectrograph (WIFIS). WIFIS offers an unprecedented combination of etendue and spectral resolving power for seeing-limited, integral field observations in the 0.9-1.8 um range and is most sensitive in the 0.9-1.35 um range. Its optical design consists of front-end re-imaging optics, an all-reflective image slicer-type, integral field unit (IFU) called FISICA, and a long-slit grating spectrograph back-end that is coupled with a HAWAII 2RG focal plane array. The full wavelength range is achieved by selecting between two different gratings. By virtue of its re-imaging optics, the spectrograph is quite versatile and can be used at multiple telescopes. The size of its field-of-view is unrivalled by other similar spectrographs, offering a 4.5" x 12" integral field at a 10-meter class telescope (or 20" x 50" at a 2.3-meter telescope). The use of WIFIS will be crucial in astronomical problems which require wide-field, two-dimensional spectroscopy such as the study of merging galaxies at moderate redshift and nearby star/planet-forming regions and supernova remnants. We discuss the final optical design of WIFIS, and its predicted on-sky performance on two reference telescope platforms: the 2.3-m Steward Bok telescope and the 10.4-m Gran Telescopio Canarias. We also present the results from our laboratory characterization of FISICA. IFU properties such as magnification, field-mapping, and slit width along the entire slit length were measured by our tests. The construction and testing of WIFIS is expected to be completed by early 2013. We plan to commission the instrument at the 2.3-m Steward Bok telescope at Kitt Peak, USA in Spring 2013.

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arXiv2012

Trends of Papers Published from 2006 to 2010 in Journals Nature and Science

We present an analysis of the papers published in the journals Nature and Science in the years from 2006 to 2010. During this period, a total of 7788 papers were published in the two journals. This includes 544 astronomy papers that comprise 7.0% of the papers in `all' research fields and 18.9% of those in the fields of `physical sciences'. The sub-fields of research of the astronomy papers are distributed, in descending order of number of papers, in Solar System, stellar astronomy, galaxies and the universe, the Milky Way Galaxy, and exoplanets. The observational facilities used for the studies are mainly ground-based telescopes (31.1%), spacecrafts (27.0%), and space telescopes (22.8%), while 16.0% of papers did not use any noticeable facilities and 1.7% used other facilities. Korean scientists have published 86 papers (33 in Nature and 53 in Science), which is 1.10% of all the papers (N=7788) in the two journals. The share of papers by Korean astronomers among the scientific papers by Koreans is 8.14%, slightly higher than the contribution of astronomy papers (7.0%) in both journals.

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arXiv2012

Hubble Space Telescope Pixel Analysis of the Interacting S0 Galaxy NGC 5195 (M51B)

We report the properties of the interacting S0 galaxy NGC 5195 (M51B), revealed in a pixel analysis using the HST/ACS images in the F435W, F555W and F814W (BVI) bands. We analyze the pixel color-magnitude diagram (pCMD) of NGC 5195, focusing on the properties of its red and blue pixel sequences and the difference from the pCMD of NGC 5194 (M51A; the spiral galaxy interacting with NGC 5195). The red pixel sequence of NGC 5195 is redder than that of NGC 5194, which corresponds to the difference in the dust optical depth of 2<Δτ_V<4 at fixed age and metallicity. The blue pixel sequence of NGC 5195 is very weak and spatially corresponds to the tidal bridge between the two interacting galaxies. This implies that the blue pixel sequence is not an ordinary feature in the pCMD of an early-type galaxy, but that it is a transient feature of star formation caused by the galaxy-galaxy interaction. We also find a difference in the shapes of the red pixel sequences on the pixel color-color diagrams (pCCDs) of NGC 5194 and NGC 5195. We investigate the spatial distributions of the pCCD-based pixel stellar populations. The young population fraction in the tidal bridge area is larger than that in other areas by a factor > 15. Along the tidal bridge, young populations seem to be clumped particularly at the middle point of the bridge. On the other hand, the dusty population shows a relatively wide distribution between the tidal bridge and the NGC 5195 center.

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arXiv2011

Ultraviolet color-color relation of early-type galaxies at 0.05 < z < 0.12

We present the ultraviolet (UV) color-color relation of early-type galaxies (ETGs) in the nearby universe (0.05 < z < 0.12) to investigate the properties of hot stellar populations responsible for the UV excess (UVX). The initial sample of ETGs is selected by the spectroscopic redshift and the morphology parameter from the Sloan Digital Sky Survey (SDSS) DR7, and then cross-matched with the Galaxy Evolution Explorer (GALEX) Far-UV (FUV) and Near-UV (NUV) GR6 data. The cross-matched ETG sample is further classified by their emission line characteristics in the optical spectra into quiescent, star-forming, and AGN categories. Contaminations from early-type spiral galaxies, mergers, and morphologically disturbed galaxies are removed by visual inspection. By drawing the FUV - NUV (as a measure of UV spectral shape) vs. FUV - r (as a measure of UVX strength) diagram for the final sample of ~3700 quiescent ETGs, we find that the "old and dead" ETGs consist of a well-defined sequence in UV colors, the "UV red sequence", so that the stronger UVX galaxies should have a harder UV spectral shape systematically. However, the observed UV spectral slope is too steep to be reproduced by the canonical stellar population models in which the UV flux is mainly controlled by age or metallicity parameters. Moreover, 2 mag of color spreads both in FUV - NUV and FUV - r appear to be ubiquitous among any subsets in distance or luminosity. This implies that the UVX in ETGs could be driven by yet another parameter which might be even more influential than age or metallicity.

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arXiv2011

Hubble Space Telescope Pixel Analysis of the Interacting Face-on Spiral Galaxy NGC 5194 (M51A)

A pixel analysis is carried out on the interacting face-on spiral galaxy NGC 5194 (M51A), using the HST/ACS images in the F435W, F555W and F814W (BVI) bands. After 4x4 binning of the HST/ACS images to secure a sufficient signal-to-noise ratio for each pixel, we derive several quantities describing the pixel color-magnitude diagram (pCMD) of NGC 5194: blue/red color cut, red pixel sequence parameters, blue pixel sequence parameters and blue-to-red pixel ratio. The red sequence pixels are mostly older than 1 Gyr, while the blue sequence pixels are mostly younger than 1 Gyr, in their luminosity-weighted mean stellar ages. The color variation in the red pixel sequence from V = 20 mag arcsec^(-2) to V = 17 mag arcsec^(-2) corresponds to a metallicity variation of Δ[Fe/H] ~ 2 or an optical depth variation of Δτ_V ~ 4 by dust, but the actual sequence is thought to originate from the combination of those two effects. At V < 20 mag arcsec^(-2), the color variation in the blue pixel sequence corresponds to an age variation from 5 Myr to 300 Myr under the assumption of solar metallicity and τ_V = 1. To investigate the spatial distributions of stellar populations, we divide pixel stellar populations using the pixel color-color diagram and population synthesis models. As a result, we find that the pixel population distributions across the spiral arms agree with a compressing process by spiral density waves: dense dust \rightarrow newly-formed stars. The tidal interaction between NGC 5194 and NGC 5195 appears to enhance the star formation at the tidal bridge connecting the two galaxies. We find that the pixels corresponding to the central active galactic nucleus (AGN) area of NGC 5194 show a tight sequence at the bright-end of the pCMD, which are in the region of R ~ 100 pc and may be a photometric indicator of AGN properties.

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arXiv2011

Extended Star Clusters in the Remote Halo of the Intriguing Dwarf Galaxy NGC 6822

We present a study on four new star clusters discovered in the halo of the intriguing dwarf irregular galaxy NGC 6822 from a wide field survey covering 3 deg x 3 deg area carried out with MegaCam at Canada-France-Hawaii Telescope (CFHT). The star clusters have extended structures with half-light radii R_h ~ 7.5 -- 14.0 pc, larger than typical Galactic globular clusters and other known globular clusters in NGC 6822. The integrated colors and color magnitude diagrams (CMD) of resolved stars suggest that the new star clusters are 2 -- 10 Gyr old and relatively metal poor with Z=0.0001--0.004 based on the comparison with theoretical models. The projected distance of each star cluster from the galaxy center ranges from 10.7 arcmin (~ 1.5 kpc) to 77 arcmin (~ 11 kpc), far beyond the optical body of the galaxy. Interestingly, the new star clusters are aligned along the elongated old stellar halo of NGC 6822, which is almost perpendicular to the HI gas distribution where young stellar populations exist. We also find that the colors and half-light radii of the new clusters are correlated with the galactocentric distance: clusters farther from the galaxy center are larger and bluer than those closer to the galaxy center. We discuss the stellar structure and evolution of NGC 6822 implied by these new extended star clusters in the halo. We also discuss the current status of observational and theoretical understandings regarding the origin of extended star clusters in NGC 6822 and other galaxies.

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arXiv2011

Optical spectral classification of southern ultraluminous infrared galaxies

We present a study of the optical spectral properties of 115 ultraluminous infrared galaxies (ULIRGs) in the southern sky. Using the optical spectra obtained at CTIO 4 m and provided by the 2dF Galaxy Redshift Survey and the 6dF Galaxy Survey, we measure emission line widths and fluxes for spectral classification. We determine the spectral types of ULIRGs with H_alpha measurement using the standard diagnostic diagrams. For ULIRGs without H_alpha measurement, we determine their spectral types using the plane of flux ratio between [OIII]_lambda5007 and H_beta versus [OIII] line width based on our new empirical criterion. This criterion is efficient to distinguish active galactic nuclei (AGNs) from non-AGN galaxies with completeness and reliability of about 90 per cent. The sample of 115 ULIRGs is found to consist of 8 broad-line AGNs, 49 narrow-line AGNs, and 58 non-AGNs. The AGN fraction is on average 50 per cent and increases with infrared luminosity and IRAS 25-60 micron colour, consistent with previous studies. The IRAS 25-60 micron colour distributions are significantly different between AGN and non-AGN ULIRGs, while their IRAS 60-100 micron colour distributions are similar.

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arXiv2009

Near-Infrared Photometric Study of the Old Open Cluster Trumpler 5

We present JHK near-infrared photometric study for the old open cluster (OC) Trumpler 5 (Tr 5), based on the 2MASS data. From the color-magnitude diagrams of Tr 5, we have located the position of the red giant clump (RGC) stars, and used the mean magnitude of the RGC stars in K-band to estimate the distance to Tr 5, d = 3.1 +/- 0.1 kpc ((m-M)_0 = 12.46 +/- 0.04). From fitting the theoretical isochrones of Padova group, we have estimated the reddening, metallicity, and age : E(B-V) = 0.64 +/- 0.05, [Fe/H] = -0.4 +/- 0.1 dex, and t =2.8 +/- 0.2 Gyr (log t=9.45 +/- 0.04), respectively. These parameters generally agree well with those obtained from the previous studies on Tr 5 and confirms that this cluster is an old OC with metallicity being metal-poorer than solar abundance, located in the anti-Galactic center region.

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