arXiv subjects

Myung Gyoon Lee

Publications and source records attributed to Myung Gyoon Lee.

At least 163 records · Page 9Linked to original sources

arXiv2006

Ultraluminous and Hyperluminous Infrared Galaxies in the SDSS, 2dFGRS and 6dFGS

We present a result of cross-correlating the Infrared Astronomical Satellite Faint Source Catalogue (IRAS FSC) with the spectroscopic catalogues of galaxies in the Fourth Data Release of Sloan Digital Sky Survey (SDSS), the Final Data Release of 2dF Galaxy Redshift Survey (2dFGRS) and the Second Data Release of 6dF Galaxy Survey (6dFGS). We have identified 324 ultraluminous infrared galaxies (ULIRGs) including 190 newly discovered ULIRGs, and 2 hyperluminous infrared galaxies (HLIRGs). Adding these new ULIRGs, we increase the number of known ULIRGs by about 30 per cent. The reliability of the cross-correlation is estimated using the likelihood ratio method. The incompleteness of our sample introduced by the identification procedure in this study is estimated to be about 5 per cent. Our sample covers the redshift range of z=0.037$-$0.517 with a median redshift of $\bar{z}$=0.223, which is larger than that ($\bar{z}$=0.184) of the sample of previously known ULIRGs.

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arXiv2006

The Nature of Blue Early-Type Galaxies in the Goods Fields

We present a study of the nature of the blue early-type galaxies (BEGs) in the GOODS north and south fields using the GOODS HST/ACS archival data. Using visual inspection, we have selected 58 BEGs and 113 normal red early-type galaxies (REGs) in the sample of 1,949 galaxies with spectroscopic redshifts. We find that the BEGs are generally bluer, fainter, and less-massive than the REGs, although a few BEGs are exceptionally bright and massive. The number fraction of the BEGs to total early-type galaxies is almost constant ($\sim0.3$) at $z \le 1.1$. In addition, we find that the size of the BEGs in given redshift bin decrease as redshift decreases. The BEGs look similar to the REGs in the images and surface brightness profiles. However, at least 27 BEGs show traces of tidal disturbances in their fine structures: elongated cores, off-centered cores, asymmetric internal color distributions, tidally distorted outer structures, collisional rings, or very nearby companions. Twenty-one BEGs are detected in the X-ray bands and eleven of them are as luminous as $L_{0.5-10 {\rm keV}} \ge 10^{43.5} {\rm erg~s}^{-1}$, indicating the existence of AGNs in their centers. These results show that at least a half of the BEGs may be descendants of mergers/interacting-galaxies and that at least a quarter of the BEGs may be AGN-host galaxies. The BEGs may evolve into REGs, and the size evolution of the BEGs is consistent with the galactic \emph{downsizing} scenario.

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arXiv2006

Deep Extragalactic Surveys around the Ecliptic Poles with AKARI (ASTRO-F)

AKARI (formerly ASTRO-F) is an infrared space telescope designed for an all-sky survey at 10-180 (mu)m, and deep pointed surveys of selected areas at 2-180 (mu)m. The deep pointed surveys with AKARI will significantly advance our understanding of galaxy evolution, the structure formation of the Universe, the nature of the buried AGNs, and the cosmic infrared background. Here we describe the important characteristics of the AKARI mission: the orbit, and the attitude control system, and investigate the optimum survey area based on the updated pre-flight sensitivities of AKARI, taking into account the cirrus confusion noise as well as the surface density of bright stars. The North Ecliptic Pole (NEP) is concluded to be the best area for 2-26 (mu)m deep surveys, while the low-cirrus noise regions around the South Ecliptic Pole (SEP) are worth considering for 50-180 (mu)m pointed surveys to high sensitivities limited by the galaxy confusion noise. Current observational plans of these pointed surveys are described in detail. Comparing these surveys with the deep surveys with the Spitzer Space Telescope, the AKARI deep surveys are particularly unique in respect of their continuous wavelength coverage over the 2-26 (mu)m range in broad-band deep imaging, and their slitless spectroscopy mode over the same wavelength range.

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arXiv2006

Theoretical Isochrones with Extinction in the K Band. II. J - K versus K

We calculate theoretical isochrones in a consistent way for five filter pairs near the J and K band atmospheric windows (J-K, J-K', J-Ks, F110W-F205W, and F110W-F222M) using the Padova stellar evolutionary models of Girardi et al. We present magnitude transformations between various K-band filters as a function of color. Isochrones with extinction of up to 6 mag in the K band are also presented. As found for the filter pairs composed of H & K band filters, we find that the reddened isochrones of different filter pairs behave as if they follow different extinction laws, and that the extinction curves of Hubble Space Telescope NICMOS filter pairs in the color-magnitude diagram are considerably nonlinear. Because of these problems, extinction values estimated with NICMOS filters can be in error by up to 1.3 mag. Our calculation suggests that the extinction law implied by the observations of Rieke et al for wavelengths between the J and K bands is better described by a power-law function with an exponent of 1.66 instead of 1.59, which is commonly used with an assumption that the transmission functions of J and K filters are Dirac delta functions.

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arXiv2006

Spatial Distribution of Faint Fuzzy Star Clusters in NGC 5195

We present a study of a faint fuzzy star cluster system in the nearby SB0 galaxy NGC 5195 interacting with the famous spiral galaxy NGC 5194 (M51), based on HST ACS BVI mosaic images taken by the Hubble Heritage Team. We have found about 50 faint fuzzy star clusters around NGC 5195 which are larger than typical globular clusters with effective radii r_eff > 7 pc and red with (V-I) > 1.0. They are mostly fainter than M_V ~ 8.3 mag. From the comparison of BVI photometry of these clusters with the simple stellar population models, we find that they are as massive as ~ 10^5 M_sun and older than 1 Gyr. Strikingly, most of these clusters are found to be scattered in an elongated region almost perpendicular to the northern spiral arm of NGC 5194, and the center of the region is slightly north of the NGC 5195 center, while normal compact red clusters of NGC 5195 are located around the bright optical body of the host galaxy. This is in contrast against the cases of NGC 1023 and NGC 3384 where spatial distribution of faint fuzzy clusters shows a ring structure around the host galaxy. We suggest that at least some faint fuzzy clusters are experiencing tidal interactions with the companion galaxy NGC 5194 and must be associated with the tidal debris in the western halo of NGC 5195.

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arXiv2005

BOAO Photometric Survey of Galactic Open Clusters. III. Czernik 24 and Czernik 27

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 +/- 0.2 Gyr, metallicity [Fe/H]=-0.41 +/- 0.15 dex, distance modulus (m-M)_0 = 13.1 +/- 0.3 mag (d=4.1 +/- 0.5 kpc), and reddening E(B-V) = 0.54 +/- 0.12 mag. The parameters for Czernik 27 are estimated to be age = 0.63 +/- 0.07 Gyr, [Fe/H]= -0.02 +/- 0.10 dex, (m-M)_0 = 13.8 +/- 0.2 mag (d=5.8 +/- 0.5 kpc), and E(B-V) = 0.15 +/- 0.05 mag. 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 +/- 0.009 dex/kpc.

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arXiv2005

Giant Halos in Dwarf Irregular Galaxies vs. Dwarf Elliptical Galaxies

We present a discovery of a giant stellar halo in NGC 6822, a dwarf irregular galaxy in the Local Group. This halo is mostly made of old red giants, showing striking features: 1) it is several times larger than the main body of the galaxy seen in the optical images, and 2) it is elongated in the direction almost perpendicular to the HI disk of NGC 6822. The structure of this stellar halo looks similar to the shape of dwarf elliptical galaxies, indicating that the halos of dwarf irregular galaxies share the same origin with those of the dwarf elliptical galaxies.

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arXiv2005

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.

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arXiv2005

Low Mass X-ray Binaries in 6 Elliptical Galaxies: Connection to Globular Clusters

We present a systematic study of the low mass X-ray binary (LMXB) populations of 6 elliptical galaxies. We utilize Chandra archival data to identify 665 X-ray point sources and HST archival data supplemented by ground observations to identify 6173 GCs. We associate 209 LMXBs with red GC (RGC) and 76 LMXBs with blue GCs (BGC), while we find no optical GC counterpart for 258 LMXBs. This is the largest GC-LMXB sample studied so far. We confirm previous reports suggesting that the fraction of GCs associated with LMXBs is 3 times larger in RGCs than in BGCs, indicating that metallicity is a primary factor in the GC-LMXB formation. While as already known, the brighter (and bigger) GCs have a higher probability to host LMXBs, we find that this optical luminosity (or mass) dependency is stronger in RGCs than in BGCs. We also find that GCs located near the galaxy center have a higher probability to harbor LMXBs compared to those in the outskirts. The radial distributions of GC-LMXBs (for both RGC and BGC) are steeper than those of the whole optical GC sample, but consistent with those of the optical halo light, suggesting that there must be another parameter (in addition to metallicity) governing LMXB formation in GCs. This second parameter must depend on the galacto-centric distance. We find no statistically significant difference in the X-ray properties among RGC-LMXBs, BGC-LMXBs and field-LMXBs. The similarity of the X-ray spectra of BGC-LMXBs and RGC-LMXBs is inconsistent with the irradiation-induced stellar wind model prediction. The similarity of the X-ray luminosity functions (XLFs) of GC-LMXBs and field-LMXBs indicates that there is no significant difference in the fraction of BH binaries present in these two populations, in contrast to Galactic LMXBs where BH binaries are not found in GCs.

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arXiv2005

Properties of Resolved Star Clusters in M51

We present a study of compact star clusters in the nearby pair of interacting galaxies NGC 5194/95 (M51), based on multifilter Hubble Space Telescope WFPC2 archival images. We have detected ~400 isolated clusters. Our requirement that clusters be detected based only on their morphology results in the selection of relatively isolated objects, and we estimate that we are missing the majority (by a factor 4-6) of <10 Myr clusters due to crowding. Hence we focus on the cluster population older than 10 Myr. An age distribution shows a broad peak between 100-500 Myr, which is consistent with the crossing times of NGC 5195 through the NGC 5194 disk estimated in both single and multiple-passage dynamical models. We estimate that the peak contains approximately 2.2-2.5 times more clusters than expected from a constant rate of cluster formation over this time interval. We estimate the effective radii of our sample clusters and find a median value of 3-4 pc. Additionally, we see correlations of increasing cluster size with cluster mass (with a best fit slope of 0.14\pm0.03) at the 4sigma level, and with cluster age (0.06\pm0.02) at the 3sigma level. Finally, we report for the first time the discovery of faint, extended star clusters in the companion, NGC 5195, an SB0 galaxy. These have red [(V-I)>1.0] colors, effective radii >7 pc, and are scattered over the disk of NGC 5195. Our results indicate that NGC 5195 is therefore currently the third known barred lenticular galaxy to have formed so-called "faint fuzzy" star clusters. (abridged)

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arXiv2005

Theoretical Isochrones with Extinction in the K Band

We calculate theoretical isochrones in a consistent way for five filters (K, K', Ks, F205W, and F222M) in the atmospheric window between 1.9 and 2.5 micron using the Padova stellar evolutionary models by Girardi et al. Even when displayed in the same Vega magnitude system, the near-infrared colors of the same isochrone can differ by up to 0.18 mag at its bright end, depending on the filter. We present magnitude transformations between K-band filters as a function of color from H and K band filters. Isochrones with extinction at K of up to 6 mag are also presented. We find that care is needed when comparing extinction values that are estimated using different filter sets in the K band, in particular when comparing those of atmospheric and space filter sets: extinction values for space filters can be in error by up to 0.3 mag. To reduce this error, we introduce an "effective extinction slope" for each filter set and isochrone model, which describes the extinction behavior of isochrones in the color-magnitude diagram more correctly than the actual extinction law. Our calculation also suggests that the extinction law implied by the observations of Rieke et al. for wavelengths between the H and K bands is better described by a power-law function with an exponent of 1.61 instead of 1.55, which is commonly used with an assumption that the transmission functions of H and K filters are Dirac delta functions.

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arXiv2004

The Globular Cluster Systems of Five Nearby Spiral Galaxies: New Insights from Hubble Space Telescope Imaging

We use available multifilter Hubble Space Telescope (HST) WFPC2 imaging of five (M81, M83, NGC 6946, M101, and M51) low inclination, nearby spiral galaxies to study ancient star cluster populations. M81 globular clusters (GC) have an intrinsic color distribution which is very similar to those in the Milky Way and M31, with ~40% of the clusters having colors expected for a metal-rich population. On the other hand, the GC system in M51 appears almost exclusively blue and metal poor. This lack of metal-rich GCs associated with the M51 bulge indicates that the bulge formation history of this Sbc galaxy may have differed significantly from that of our own. Ancient clusters in M101, and possibly in NGC 6946, appear to have luminosity distributions which continue to rise to our detection limit (M_V ~ -6.0), well beyond the expected turnover (M_V ~ -7.4) in the luminosity function. This is reminiscent of the situation in M33, a Local Group galaxy of similar Hubble type. The faint ancient cluster candidates in M101 and NGC 6946 have colors and radii similar to their more luminous counterparts, and we suggest that these are either intermediate age (3-9 Gyr) disk clusters or the low mass end of the original GC population. If the faint, excess GC candidates are excluded, we find that the specific frequency (S_N) of ancient clusters formed in later-type spirals is roughly constant, with S_N=0.5 +- 0.2. By combining the results of this study with literature values for other systems, we find that the total GC specific frequencies in spirals appear to correlate best with Hubble type and bulge/total ratio, rather than with galaxy luminosity or galaxy mass (abridged).

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arXiv2004

SX Phoenicis Stars in the Globular Cluster NGC 5466

Through time-series CCD photometry of the globular cluster NGC 5466, we have detected nine SX Phoenicis stars including three new ones. All the SX Phoenicis stars are located in the blue straggler region in the color-magnitude diagram of NGC 5466. Five of them show clearly double-radial mode features, the periods of which are well consistent with the theoretical ratio of the first overtone mode to the fundamental mode (P$_{1H}$/P$_F$). Normally, it has not been easy to secure a P-L relation of the SX Phoenicis stars, because determination of the pulsational mode of the SX Phoenicis stars has been difficult. The existence of five SX Phoenicis stars in NGC5466 with double-radial modes allows us to derive reliably a P-L relation for the fundamental mode of the SX Phoenicis stars. Using seven SX Phoenicis stars including five stars with double-radial modes, we derive a P-L relation for the fundamental mode in NGC 5466, $ = - 3.25 (\pm 0.46) Log P + 14.70 (\pm 0.06), (σ= \pm0.04) $, corresponding to $ = - 3.25 (\pm0.46)Log P - 1.30 (\pm0.06) $ for an adopted distance modulus of $(m-M)_0=16.00$ and zero reddening.

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arXiv2003

On the Formation of Giant Elliptical Galaxies and Globular Clusters

I review the current status of understanding when, how long, and how giant elliptical galaxies formed, focusing on the globular clusters. Several observational evidences show that massive elliptical galaxies formed at $z>2$ ($>10$ Gyr ago). Giant elliptical galaxies show mostly a bimodal color distribution of globular clusters, indicating a factor of $\approx 20$ metallicity difference between the two peaks. The red globular clusters (RGCs) are closely related with the stellar halo in color and spatial distribution, while the blue globular clusters (BGCs) are not. The ratio of the number of the RGCs and that of the BGCs varies depending on galaxies. It is concluded that the BGCs might have formed 12--13 Gyr ago, while the RGCs and giant elliptical galaxies might have formed similarly 10-11 Gyr ago. It remains now to explain the existence of a gap between the RGC formation epoch and the BGC formation epoch, and the rapid metallicity increase during the gap ($Δt \approx 2$ Gyr). If hierarchical merging can form a significant number of giant elliptical galaxies $>10$ Gyr ago, several observational constraints from stars and globular clusters in elliptical galaxies can be explained.

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arXiv2003

New SX Phoenicis Stars in the Globular Cluster M53

Through time-series CCD photometry of the metal-poor globular cluster M53, we have discovered eight new SX Phoenicis type stars (labeled from SXP1 to SXP8). All the new SX Phoenicis stars are located in the blue straggler star region of a color-magnitude diagram of M53. One of these stars (SXP2) is found to have very closely separated pulsation frequencies: $f_1/f_2 = 0.9595$ where $f_1$ and $f_2$ are primary and secondary frequencies. This may be due to excitation of non-radial modes. Six of these SX Phoenicis stars are considered to be pulsating in the fundamental mode. They show a tight linear correlation between the period and luminosity. We derive a period - luminosity relation for the fundamental mode for the period range of $-1.36 < Log P[d]< -1.15$ : $ =-3.010(\pm0.262)Log P + 15.310(\pm0.048)$ with an rms scatter of 0.038, corresponding to $ =-3.010 Log P - 1.070$ for an adopted distance modulus of $(m-M)_V=16.38$ (Harris 1996).

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arXiv2003

Radio/X-ray Offsets of Large Scale Jets Caused by Synchrotron Time Lags

In the internal shock scenario, we argue that electrons in most kpc (or even larger) scale jets can be accelerated to energies high enough to emit synchrotron X-rays, if shocks exist on these scales. These high energy electrons emit synchrotron radiation at high frequencies and cool as they propagate downstream along the jet, emitting at progressively lower frequencies and resulting in time lags and hence radio/X-ray (and optical/X-ray if the optical knot is detectable) offsets at bright knots, with the centroids of X-ray knots being closer to the core. Taking into account strong effects of jet expansion, the behaviour of radio/X-ray and optical/X-ray offsets at bright knots in M87, Cen A, 3C 66B, 3C 31, 3C 273, and PKS 1127-145 is consistent with that of synchrotron time lags due to radiative losses. This suggests that the large scale X-ray and optical jets in these sources are due to synchrotron emission.

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arXiv2002

Wide field CCD photometry of the globular cluster M92

We present wide field CCD photometry of a galactic globular cluster M92 obtained in the V and I bands with the CFH12K mosaic CCD at the Canada-France-Hawaii Telescope. A well-defined color-magnitude diagram is derived down to 5 magnitudes fainter than the cluster main sequence turn-off. After removing the background contribution, we obtain luminosity and mass functions, surface density profiles, and the surface number density maps of the stars belonging to the cluster. The surface density profile of all stars shows that the cluster's halo extends at least out to ~30' from the cluster center in agreement with previous study, but the profile of faint stars at the very outer region of the cluster shows a different gradient compared with that of bright stars. For a mass function of the cluster, we find that the inner region of the cluster has x = 1.2+-0.2, whereas the outer region has x = 1.8 +-0.3, clearly indicating a mass segregation of the cluster. An estimate of the photometric mass of the cluster implies that the remnant populations (white dwarfs and neutron stars) contribute at least 25% of the total cluster mass. The surface density map of M92 shows some evidence that the tidal tail of M92 may be oriented perpendicular to the direction toward the Galactic center.

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arXiv2002

Around the Clock Observations of the Q0957+561 A,B Gravitationally Lensed Quasar II: Results for the second observing season

We report on an observing campaign in March 2001 to monitor the brightness of the later arriving Q0957+561 B image in order to compare with the previously published brightness observations of the (first arriving) A image. The 12 participating observatories provided 3543 image frames which we have analyzed for brightness fluctuations. From our classical methods for time delay determination, we find a 417.09 +/- 0.07 day time delay which should be free of effects due to incomplete sampling. During the campaign period, the quasar brightness was relatively constant and only small fluctuations were found; we compare the structure function for the new data with structure function estimates for the 1995--6 epoch, and show that the structure function is statistically non-stationary. We also examine the data for any evidence of correlated fluctuations at zero lag. We discuss the limits to our ability to measure the cosmological time delay if the quasar's emitting surface is time resolved, as seems likely.

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