arXiv subjects

Myung Gyoon Lee

Publications and source records attributed to Myung Gyoon Lee.

At least 37 records · Page 2Linked to original sources

arXiv2020

The Carnegie-Chicago Hubble Program. IX. Calibration of the Tip of the Red Giant Branch Method in the Mega-Maser Host Galaxy, NGC4258 (M106)

In the nearby galaxy NGC 4258, the well-modeled orbital motion of H$_2$O masers about its supermassive black hole provides the means to measure a precise geometric distance. As a result, NGC 4258 is one of a few "geometric anchors" available to calibrate the true luminosities of stellar distance indicators such as the Tip of the Red Giant Branch (TRGB) or the Cepheid Leavitt law. In this paper, we present a detailed study of the apparent magnitude of the TRGB within NGC 4258 using publicly-available HST observations optimally situated in the gas- and dust-free halo along the minor axis, spanning distances ranging from 8 to 22 kpc in projected galactocentric radius. We undertake a systematic evaluation of the uncertainties associated with measuring the TRGB in this galaxy, based on an analysis of 54 arcmin$^2$ of HST/ACS imaging. After quantifying these uncertainties, we measure the TRGB in NGC 4258 to be F814W$_0$ = 25.347 $\pm$ 0.014(stat) $\pm$ 0.042(sys) mag. Combined with a recent 1.5% megamaser distance to NGC 4258, we determine the absolute luminosity of the TRGB to be $M_{F814W}^{TRGB}$ = -4.050 $\pm$ 0.028(stat) $\pm$ 0.048(sys) mag. This new calibration agrees to better than 1% with an independent calibration presented in Freedman et al. (2019, 2020) that was based on detached eclipsing binaries (DEBs) located in the LMC.

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arXiv2020

Tracing Dark Matter Halos with Satellite Kinematics and the Central Stellar Velocity Dispersion of Galaxies

It has been suggested that the central stellar velocity dispersion of galaxies can trace dark matter halo mass directly. We test this hypothesis using a complete spectroscopic sample of isolated galaxies surrounded by faint satellite galaxies from the Sloan Digital Sky Survey Data Release 12. We apply a friends-of-friends algorithm with projected linking length $ΔD < 100$ kpc and radial velocity linking length $ΔV < 1000$ km s$^{-1}$ to construct our sample. Our sample includes 2807 isolated galaxies with 3417 satellite galaxies at $0.01 < z < 0.14$. We divide the sample into two groups based on the primary galaxy color: red and blue primary galaxies separated at $(g-r)_{0} = 0.85$. The central stellar velocity dispersions of the primary galaxies are proportional to the luminosities and stellar masses of the same galaxies. Stacking the sample based on the central velocity dispersion of the primary galaxies, we derive the velocity dispersions of their satellite galaxies, which trace the dark matter halo mass of the primary galaxies. The system velocity dispersion of the satellite galaxies shows a remarkably tight correlation with the central velocity dispersion of the primary galaxies for both red and blue samples. In particular, the slope of the relation is identical to 1 for red primary systems. This tight relation suggests that the central stellar velocity dispersion of galaxies is indeed an efficient and robust tracer for dark matter halo mass. We provide empirical relations between the central stellar velocity dispersion and the dark matter halo mass.

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arXiv2020

Mysterious Globular Cluster System of the Peculiar Massive Galaxy M85

We present a study on stellar population and kinematics of globular clusters (GCs) in the peculiar galaxy M85. We obtain optical spectra of 89 GCs at 8 kpc $< R <$ 160 kpc using the MMT/Hectospec. We divide them into three groups, blue/green/red GCs (B/G/RGCs), with their $(g-i)_0$ colors. All GC subpopulations have mean ages of 10 Gyr, but showing differences in metallicities. The BGCs and RGCs are the most metal-poor ([Z/H] $\sim -1.49$) and metal-rich ([Z/H] $\sim -0.45$), respectively, and the GGCs are in between. We find that the inner GC system exhibits a strong overall rotation that is entirely due to a disk-like rotation of the RGC system. The BGC system shows little rotation. The GGCs show kinematic properties clearly distinct among the GC subpopulations, having higher mean velocities than the BGCs and RGCs and being aligned along the major axis of M85. This implies that the GGCs have an origin different from the other GC subpopulations. The rotation-corrected velocity dispersion of the RGC system is much lower than that of the BGC system, indicating the truncation of the red halo of M85. The BGCs have a flat velocity dispersion profile out to $R$ = 67 kpc, reflecting the dark matter extent of M85. Using the velocity dispersion of the BGC system, we estimate the dynamical mass of M85 to be $3.8 \times 10^{12} M_{\odot}$. We infer that M85 has undergone merging events lately, resulting in the peculiar kinematics of the GC system.

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arXiv2020

A New Metal-poor Globular Cluster and Resolved Stars in the Outer Disk of the Black Eye Galaxy M64: Implication for the Origin of the Type III Disk Break

M64 is a nearby spiral galaxy with a Type III anti-truncation component. To trace the origin of the Type III component, we present Hubble Space Telescope/Advanced Camera for Surveys $F606W/F814W$ photometry of resolved stars in a field located in the outer disk ($2.5' \lesssim r \lesssim 6.5'$) of M64. At $r\approx 5.5'$ (7 kpc) to the east, we discover a new metal-poor globular cluster ($R_{\rm eff}=5.73\pm0.02$ pc and $M_V=-9.54\pm0.09$ mag), M64-GC1. This is the first globular cluster found in M64. The color-magnitude diagram (CMD) of the resolved stars in M64-GC1 is well matched by 12 Gyr isochrones with [Fe/H] $=-1.5\pm0.2$, showing that this cluster belongs to a halo. The CMD of the resolved stars in the entire ACS field shows two distinguishable red giant branches (RGBs): a curved metal-rich RGB and a vertical metal-poor RGB. The metal-rich RGB represents an old metal-rich ([Fe/H] $\approx -0.4$) disk population. In contrast, the CMD of the metal-poor RGB stars is very similar to the CMD of M64-GC1, showing that the metal-poor RGB represents a halo population. The radial number density profile of the metal-rich RGB stars is described by an exponential disk law, while the profile of the metal-poor RGB stars is described by a de Vaucouleurs's law. From these, we conclude that the origin of the Type III component in M64 is a halo which has a much lower metallicity than a disk or bulge population.

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arXiv2020

The Nature of Ultra-diffuse Galaxies in Distant Massive Galaxy Clusters: Abell 370 in the Hubble Frontier Fields

We report the discovery of ultra-diffuse galaxies (UDGs) in Abell 370 ($z=0.375$). We find 46 UDGs in Abell 370 from the images of the Hubble Frontier Fields (HFF). Most UDGs are low-luminosity red sequence galaxies, while a few of them are blue UDGs. We estimate the abundance of UDGs in Abell 370, $N(\rm UDG)=644\pm104$. Combining these results with those of Abell S1063 ($z=0.348$) and Abell 2744 ($z=0.308$) \citep{Lee17}, we derive a mean radial number density profile of UDGs in the three clusters. The number density profiles of UDGs and bright galaxies show a discrepancy in the central region of the clusters: the profile of UDGs shows a flattening as clustercentric distance decreases, while that of bright galaxies shows a continuous increase. This implies that UDGs are prone to disruption in the central region of the clusters. The relation between the abundance of UDGs and virial masses of their host systems is described by a power-law with an index nearly one: $N({\rm UDG})\propto M_{200}^{0.99\pm0.05}$ for $M_{200}>10^{13}~M_{\odot}$. We estimate approximately dynamical masses of UDGs using the fundamental manifold method, and find that most UDGs have dwarf-like masses $(M_{200}<10^{11}$ $M_{\odot})$. This implies that most UDGs have a dwarf-like origin and a small number of them could be failed $L^{*}$ galaxies. These results suggest that multiple origins may contribute to the formation and evolution of UDGs in massive galaxy clusters.

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arXiv2020

What Makes Ly$α$ Nebulae Glow? Mapping the Polarization of LABd05

"Ly$α$ nebulae" are giant ($\sim$100 kpc), glowing gas clouds in the distant universe. The origin of their extended Ly$α$ emission remains a mystery. Some models posit that Ly$α$ emission is produced when the cloud is photoionized by UV emission from embedded or nearby sources, while others suggest that the Ly$α$ photons originate from an embedded galaxy or AGN and are then resonantly scattered by the cloud. At least in the latter scenario, the observed Ly$α$ emission will be polarized. To test these possibilities, we are conducting imaging polarimetric observations of seven Ly$α$ nebulae. Here we present our results for LABd05, a cloud at $z$ = 2.656 with an obscured, embedded AGN to the northeast of the peak of Ly$α$ emission. We detect significant polarization. The highest polarization fractions $P$ are $\sim$10-20% at $\sim$20-40 kpc southeast of the Ly$α$ peak, away from the AGN. The lowest $P$, including upper-limits, are $\sim$5% and lie between the Ly$α$ peak and AGN. In other words, the polarization map is lopsided, with $P$ increasing from the Ly$α$ peak to the southeast. The measured polarization angles $θ$ are oriented northeast, roughly perpendicular to the $P$ gradient. This unique polarization pattern suggests that 1) the spatially-offset AGN is photoionizing nearby gas and 2) escaping Ly$α$ photons are scattered by the nebula at larger radii and into our sightline, producing tangentially-oriented, radially-increasing polarization away from the photoionized region. Finally we conclude that the interplay between the gas density and ionization profiles produces the observed central peak in the Ly$α$ emission. This also implies that the structure of LABd05 is more complex than assumed by current theoretical spherical or cylindrical models.

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arXiv2020

Calibration of the Tip of the Red Giant Branch (TRGB)

The Tip of the Red Giant (TRGB) method provides one of the most accurate and precise means of measuring the distances to nearby galaxies. Here we present a VIJHK absolute calibration of the TRGB based on observations of TRGB stars in the Large Magellanic Cloud (LMC),grounded on detached eclipsing binaries (DEBs). This paper presents a more detailed description of the method first presented in Freedman et al. (2019) for measuring corrections for the total line-of-sight extinction and reddening to the LMC. In this method, we use a differential comparison of the red giant population in the LMC, first with red giants in the Local Group galaxy, IC 1613, and then with those in the Small Magellanic Cloud. As a consistency check, we derive an independent calibration of the TRGB sequence using the SMC alone, invoking its geometric distance also calibrated by DEBs. An additional consistency check comes from near-infrared observations of Galactic globular clusters covering a wide range of metallicities. In all cases we find excellent agreement in the zero-point calibration. We then examine the recent claims by Yuan et al. (2019), demonstrating that, in the case of the SMC, they corrected for extinction alone while neglecting the essential correction for reddening as well. In the case of IC 1613, we show that their analysis contains an incorrect treatment of (over-correction for) metallicity. Using our revised (and direct) measurement of the LMC TRGB extinction, we find a value of Ho = 69.6 +/-0.8 (+/-1.1% stat) +/- 1.7 (+/-2.4% sys) km/s/Mpc.

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arXiv2019

The Next Generation Virgo Cluster Survey. XVII. A Search for Planetary Nebulae in Virgo Cluster Globular Clusters

The occurrence of planetary nebulae (PNe) in globular clusters (GCs) provides an excellent chance to study low-mass stellar evolution in a special (low-metallicity, high stellar density) environment. We report a systematic spectroscopic survey for the [O{\sc iii}] 5007 emission line of PNe in 1469 Virgo GCs and 121 Virgo ultra-compact dwarfs (UCDs), mainly hosted in the giant elliptical galaxies M87, M49, M86, and M84. We detected zero PNe in our UCD sample and discovered one PN ($M_{5007} = -4.1$ mag) associated with an M87 GC. We used the [O{\sc iii}] detection limit for each GC to estimate the luminosity-specific frequency of PNe, $α$, and measured $α$ in the Virgo cluster GCs to be $α\sim 3.9_{-0.7}^{+5.2}\times 10^{-8}\mathrm{PN}/L_\odot$. $α$ in Virgo GCs is among the lowest values reported in any environment, due in part to the large sample size, and is 5--6 times lower than that for the Galactic GCs. We suggest that $α$ decreases towards brighter and more massive clusters, sharing a similar trend as the binary fraction, and the discrepancy between the Virgo and Galactic GCs can be explained by the observational bias in extragalactic surveys toward brighter GCs. This low but non-zero efficiency in forming PNe may highlight the important role played by binary interactions in forming PNe in GCs. We argue that a future survey of less massive Virgo GCs will be able to determine whether PN production in Virgo GCs is governed by internal process (mass, density, binary fraction), or is largely regulated by external environment.

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arXiv2019

The Carnegie-Chicago Hubble Program. VII. The Distance to M101 via the Optical Tip of the Red Giant Branch Method

The Carnegie-Chicago Hubble Program (CCHP) is building a direct path to the Hubble constant (H0) using Population II stars as the calibrator of the SN Ia-based distance scale. This path to calibrate the SN Ia is independent of the systematics in the traditional Cepheid-based technique. In this paper, we present the distance to M101, the host to SN2011fe, using the I-band tip of the red giant branch (TRGB) based on observations from the ACS/WFC instrument on the Hubble Space Telescope. The CCHP targets the halo of M101 where there is little to no host-galaxy dust, the red giant branch is isolated from nearly all other stellar populations, and there is virtually no source confusion or crowding at the magnitude of the tip. Applying the standard procedure for the TRGB method from the other works in the CCHP series, we find a foreground-extinction-corrected M101 distance modulus of {μ_0}=29.07+/-0.04(stat)+/-0.05(sys) mag, which corresponds to a distance of D=6.52+/-0.12(stat)+/-0.15(sys) Mpc. This result is consistent with several recent Cepheid-based determinations, suggesting agreement between Population I and II distance scales for this nearby SN Ia-host galaxy. We further analyze four archival datasets for M101 that have targeted its outer disk to argue that targeting in the stellar halo provides much more reliable distance measurements from the TRGB method due to the combination of multiple structural components and heavily population contamination. Application of the TRGB in complex regions will have sources of uncertainty not accounted for in commonly used uncertainty measurement techniques.

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arXiv2019

The Carnegie Chicago Hubble Program VI: Tip of the Red Giant Branch Distances to M66 and M96 of the Leo I Group

We determine the distances to the Type Ia Supernova host galaxies M66 (NGC 3627) and M96 (NGC 3368) of the Leo I Group using the Tip of the Red Giant Branch (TRGB) method. We target the stellar halos of these galaxies using the Hubble Space Telescope ACS/WFC in the F606W and F814W bandpasses. By pointing to the stellar halos we sample RGB stars predominantly of Population II, minimize host-galaxy reddening, and significantly reduce the effects of source crowding. Our absolute calibration of the I-band TRGB is based on a recent detached eclipsing binary distance to the Large Magellanic Cloud. With this geometric zero point in hand, we find for M66 and M96, respectively, true distance moduli $ μ_0 = 30.23 \pm 0.04\text{ (stat)} \pm 0.06\text{ (sys)} $ mag and $ μ_0 = 30.29 \pm 0.02\text{ (stat)} \pm 0.06\text{ (sys)} $ mag.

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arXiv2019

The Carnegie-Chicago Hubble Program. VIII. An Independent Determination of the Hubble Constant Based on the Tip of the Red Giant Branch

We present a new and independent determination of the local value of the Hubble constant based on a calibration of the Tip of the Red Giant Branch (TRGB) applied to Type Ia supernovae (SNeIa). We find a value of Ho = 69.8 +/- 0.8 (+/-1.1\% stat) +/- 1.7 (+/-2.4\% sys) km/sec/Mpc. The TRGB method is both precise and accurate, and is parallel to, but independent of the Cepheid distance scale. Our value sits midway in the range defined by the current Hubble tension. It agrees at the 1.2-sigma level with that of the Planck 2018 estimate, and at the 1.7-sigma level with the SHoES measurement of Ho based on the Cepheid distance scale. The TRGB distances have been measured using deep Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging of galaxy halos. The zero point of the TRGB calibration is set with a distance modulus to the Large Magellanic Cloud of 18.477 +/- 0.004 (stat) +/-0.020 (sys) mag, based on measurement of 20 late-type detached eclipsing binary (DEB) stars, combined with an HST parallax calibration of a 3.6 micron Cepheid Leavitt law based on Spitzer observations. We anchor the TRGB distances to galaxies that extend our measurement into the Hubble flow using the recently completed Carnegie Supernova Project I sample containing about 100 well-observed SNeIa. There are several advantages of halo TRGB distance measurements relative to Cepheid variables: these include low halo reddening, minimal effects of crowding or blending of the photometry, only a shallow (calibrated) sensitivity to metallicity in the I-band, and no need for multiple epochs of observations or concerns of different slopes with period. In addition, the host masses of our TRGB host-galaxy sample are higher on average than the Cepheid sample, better matching the range of host-galaxy masses in the CSP distant sample, and reducing potential systematic effects in the SNeIa measurements.

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arXiv2019

A Wide-field Photometric Survey of Globular Clusters in the Peculiar Early-type Galaxy M85

We survey globular clusters (GCs) in M85 using $ugi$-band images of a $1^{\circ} \times 1^{\circ}$ field obtained with the MegaCam at the 3.6 m Canada-France-Hawaii Telescope. We identify 1318 GC candidates with 20.0 mag $< g_0 <$ 23.5 mag in the entire survey region. Their radial number density profile is well fit by a S{é}rsic profile with $n$ = 2.58$^{+0.43}_{-0.33}$ and effective radius $R_{\rm e,GCS}$ = 4$\rlap{.}{'}$14 (= 22 kpc), showing that the candidates at $R < 20'$ are mostly genuine GCs in M85. We estimate the total number of GCs, $N$(total) = $1216^{+82}_{-50}$, and the specific frequency, $S_N = 1.41^{+0.10}_{-0.06}$. The overall color distribution of the GCs in M85 is bimodal, but the GCs in the central region at $R < 2'$ do not show a bimodal distribution clearly. The radial number density profile and surface number density map of the blue GCs (BGCs) show more extended structures than those of the red GCs (RGCs). The spatial distributions of both BGCs and RGCs are elongated, similar to that of the galaxy stellar light. The number fraction of the RGCs in the central region is much smaller compared to those in other early-type galaxies of similar luminosity. The mean $(g-i)_0$ color of the RGCs in M85 is about 0.1 mag bluer than typical values for other Virgo early-type galaxies of similar luminosity, indicating that a significant fraction of the RGCs in M85 may be younger than typical GCs. These results indicate that M85 might have undergone a major wet merger recently.

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arXiv2018

The Carnegie-Chicago Hubble Program: Calibration of the Near-Infrared RR Lyrae Period-Luminosity Relation With HST

We present photometry of 30 Galactic RR Lyrae variables taken with HST WFC3/IR for the Carnegie-Chicago Hubble Program. These measurements form the base of the distance ladder measurements that comprise a pure Population II base to a measurement of Ho at an accuracy of 3%. These data are taken with the same instrument and filter (F160W) as our observations of RR Lyrae stars in external galaxies so as to to minimize sources of systematic error in our calibration of the extragalactic distance scale. We calculate mean magnitudes based on one to three measurements for each RR Lyrae star using star-by-star templates generated from densely time-sampled data at optical and mid-infrared wavelengths. We use four RR Lyrae stars from our sample with well-measured HST parallaxes to determine a zero point. This zero point will soon be improved with the large number of precise parallaxes to be provided by Gaia. We also provide preliminary calibration with the TGAS & Gaia DR2 data, and all three zero points are in agreement, to within their uncertainties.

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arXiv2018

Star Clusters in the Elliptical Galaxy NGC 4589 Hosting a Calcium-rich SN Ib (SN 2005CZ)

_NGC 4589, a bright E2 merger-remnant galaxy, hosts the peculiar fast and faint calcium-rich Type Ib supernova (SN) SN 2005cz. The progenitor of Ca-rich SNe Ib has been controversial: it could be a) a young massive star with 6-12 M$\odot$ in a binary system, or b) an old low-mass star in a binary system that was kicked out from the galaxy center. Moreover, previous distance estimates for this galaxy have shown a large spread, ranging from 20 Mpc to 60 Mpc. Thus, using archival $Hubble$ $Space$ $Telescope$/ACS $F435W$, $F555W$, and $F814W$ images, we search for star clusters in NGC 4589 in order to help resolve these issues. We find a small population of young star clusters with $25<V\leq27$ ($-7.1<M_V\leq-5.1$) mag and age $< 1$ Gyr in the central region at $R<0.5'$ ($<3.8$ kpc), thus supporting the massive-star progenitor scenario for SN 2005cz. In addition to young star clusters, we also find a large population of old globular clusters. In contrast to previous results in the literature, we find that the color distribution of the globular clusters is clearly bimodal. The turnover (Vega) magnitude in the $V$-band luminosity functions of the blue (metal-poor) globular clusters is determined to be $V_0{(\rm max)}=24.40\pm0.10$ mag. We derive the total number of globular clusters, $N_{\rm GC} =640\pm50$, and the specific frequency, $S_N =1.7\pm0.2$. Adopting a calibration for the metal-poor globular clusters, $M_V({\rm max})=-7.66\pm0.14$ mag, we derive a distance to this galaxy: $(m-M)_0=32.06\pm0.10({\rm ran})\pm0.15({\rm sys})$ ($d=25.8\pm2.2$ Mpc).

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arXiv2018

The Carnegie-Chicago Hubble Program. IV. The Distances to NGC 4424, NGC 4526, and NGC 4536 via the Tip of the Red Giant Branch

The Carnegie-Chicago Hubble Program (CCHP) is undertaking a re-calibration of the extragalactic distance scale, using Type Ia supernovae that are tied to Tip of the Red Giant Branch (TRGB) distances to local galaxies. We present here deep Hubble Space Telescope (HST) ACS/WFC imaging of the resolved stellar populations in the metal-poor halos of the SN Ia host galaxies NGC 4424, NGC 4526, and NGC 4536. These three Virgo constellation galaxies are prime targets for calibrating the extragalactic distance scale given their relative proximity in the local Universe and their low line-of-sight reddenings. Anchoring the TRGB zero-point to the geometric distance to the Large Magellanic Cloud via detached eclipsing binaries, we measure extinction-corrected distance moduli of 31.00 +/- 0.03 (stat) +/- 0.06 (sys) mag, 30.98 +/- 0.03 (stat) +/- 0.06 (sys) mag, and 30.99 +/- 0.03 (stat) +/- 0.06 (sys) mag for NGC 4424, NGC 4526, and NGC 4536, respectively, or 15.8 +/- 0.2 (stat) +/- 0.4 (sys) Mpc, 15.7 +/- 0.2 (stat) +/- 0.4 (sys) Mpc, and 15.8 +/- 0.2 (stat) +/- 0.4 (sys) Mpc. For these three galaxies, the distances are the first based on the TRGB, and for NGC 4424 and NGC 4526, they are the highest precision distances published to date, each measured to 3%. Finally, we report good agreement between our TRGB distances and the available Cepheid distances for NGC 4424 and NGC 4536, demonstrating consistency between the distance scales currently derived from stars of Population I and II.

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arXiv2018

The Carnegie-Chicago Hubble Program. V. The Distances to NGC 1448 and NGC 1316 via the Tip of the Red Giant Branch

The Carnegie-Chicago Hubble Program (CCHP) is re-calibrating the extragalactic SN Ia distance scale using exclusively Population II stars. This effort focuses on the Tip of the Red Giant Branch (TRGB) method, whose systematics are entirely independent of the Population I Cepheid-based determinations that have long served as calibrators for the SN Ia distance scale. We present deep Hubble Space Telescope imaging of the low surface-density and low line-of-sight reddening halos of two galaxies, NGC 1448 and NGC 1316, each of which have been hosts to recent SN Ia events. Provisionally anchoring the TRGB zero-point to the geometric distance to the Large Magellanic Cloud derived from detached eclipsing binaries, we measure extinction-corrected distance moduli of 31.23 +/-0.04 (stat) +/- 0.06 (sys) mag for NGC 1448 and 31.37 +/- 0.04 (stat) and +/- 0.06 (sys) mag for NGC 1316, respectively, giving metric distances of 17.7 +/- 0.3 (stat) +/- 0.5 (sys) Mpc, and 18.8 +/- 0.3 (stat) +/- 0.5 (sys) Mpc. We find agreement between our result and the available Cepheid distance for NGC 1448; for NGC 1316, where there are relatively few published distances based on direct measurements, we find that our result is consistent with the published SN Ia distances whose absolute scales are set from other locally-determined methods such as Cepheids. For NGC 1448 and NGC 1316, our distances are some of the most precise (and systematically accurate) measurements with errors at 1.7 (2.8) % and 1.6 (2.7) % levels, respectively.

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arXiv2018

Discovery of Two New Globular Clusters in the Milky Way

The spatial distribution of known globular clusters (GCs) in the Milky Way shows that the current census of GCs is incomplete in the direction of the Galactic plane. We present the discovery of two new GCs located close to the Galactic plane in the sky. These two GCs, RLGC 1 and RLGC 2, were discovered serendipitously during our new cluster survey (Ryu & Lee 2018) based on near-Infrared and mid-Infrared survey data. The two GCs show a grouping of resolved stars in their $K$ band images and a presence of faint diffuse light in their outer regions in the WISE $W1$ band images. They also show prominent red giant branches (RGBs) in their $K$ vs. $(J-K)$ color-magnitude diagrams (CMDs). We determine structural parameters of the two GCs using King profile fitting on their $K$ band radial number density profiles. The determined values are consistent with those of known GCs. Finally, we determine the distances, metallicities, and reddenings of the two GCs using the isochrone fitting on their CMDs. For the fitting, we assume that the ages of the two GCs are 12.6 Gyr and the brightest RGB stars of each cluster correspond to the tip of the RGB. Distances and metallicities of the two GCs are estimated to be $d=28.8\pm4.3$ kpc and $\textrm{[Fe/H]}=-2.2\pm0.2$ for RLGC 1 and $d=15.8\pm2.4$ kpc and $\textrm{[Fe/H]}=-2.1\pm0.3$ for RLGC 2. These results show that the two GCs are located at the far-half region of the Milky Way and they may belong to the halo of the Milky Way.

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arXiv2018

Nuclear starburst activity induced by elongated bulges in spiral galaxies

We study the effects of bulge elongation on the star formation activity in the centers of spiral galaxies using the data from the Sloan Digital Sky Survey Data Release 7. We construct a volume-limited sample of face-on spiral galaxies with $M_r < -$19.5 mag at 0.02 $\leq z <$ 0.055 by excluding barred galaxies, where the aperture of the SDSS spectroscopic fibre covers the bulges of the galaxies. We adopt the ellipticity of bulges measured by Simard et al. (2011) who performed two-dimensional bulge+disc decompositions using the SDSS images of galaxies, and identify nuclear starbursts using the fibre specific star formation rates derived from the SDSS spectra. We find a statistically significant correlation between bulge elongation and nuclear starbursts in the sense that the fraction of nuclear starbursts increases with bulge elongation. This correlation is more prominent for fainter and redder galaxies, which exhibit higher ratios of elongated bulges. We find no significant environmental dependence of the correlation between bulge elongation and nuclear starbursts. These results suggest that non-axisymmetric bulges can efficiently feed the gas into the centre of galaxies to trigger nuclear starburst activity.

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