arXiv subjects

Youkyung Ko

Publications and source records attributed to Youkyung Ko.

16 recordsLinked to original sources

arXiv2025

A JWST View of the Overmassive Black Hole in NGC 4486B

We present a new stellar dynamical measurement of the supermassive black hole (SMBH) in the compact elliptical galaxy NGC 4486B, based on integral field spectroscopy with JWST/NIRSpec. The two-dimensional kinematic maps reveal a resolved double nucleus and a velocity dispersion peak offset from the photometric center. Utilizing two independent methods-Schwarzschild orbit-superposition and Jeans Anisotropic Modeling-we tightly constrain the black hole mass by fitting the full line-of-sight velocity distribution. Our axisymmetric Schwarzschild models yield a best-fit black hole mass of $M_{BH} = 3.6^{+0.7}_{-0.7} \times 10^8 \, M_{\odot}$, slightly lower but significantly more precise than previous estimates. However, since our models do not account for the non-equilibrium nature of the double nucleus, this value may represent a lower limit. Across all tested dynamical models, the inferred $M_{BH}/M_*$ ratio ranges from ~ 4-13%, providing robust evidence for an overmassive SMBH in NGC 4486B. Combined with the galaxy's location deep within the Virgo Cluster, our results support the interpretation that NGC 4486B is the tidally stripped remnant core of a formerly massive galaxy. As the JWST/NIRSpec field of view is insufficient to constrain the dark matter halo, we incorporate archival ground-based long-slit kinematics extending to 5 arcsec. While this provides some leverage on the dark matter content, the constraints remain relatively weak. We place only an upper limit on the dark matter fraction, with $M_{DM}/M_{*} < 0.5$ within 1 kpc-well beyond the effective radius. The inferred black hole mass remains unchanged with or without a dark matter halo.

astro-ph.GA↗

arXiv2025

A Supermassive Black Hole in a Diminutive Ultra-compact Dwarf Galaxy Discovered with JWST/NIRSpec+IFU

The integral-field unit mode of the Near-Infrared Spectrograph (NIRSpec+IFU) mounted on the James Webb Space Telescope has now enabled kinematic studies of smaller and less massive compact stellar systems in which to search for central massive black holes (BHs) than ever before. We present here the first such detection using NIRSpec+IFU in its highest resolution (R~2700) mode. We report a $3\sigma$ detection of a central black hole with mass ${\cal M}_{BH}=2.2\pm1.1\times10^6\,M_\odot$ in UCD736 orbiting within the Virgo galaxy cluster based on Schwarzschild's modeling of the 1D kinematic profile. The presence of such a massive BH strongly argues against a globular cluster origin of this UCD, and rather suggests a tidally stripped formation route from a former $\gtrsim10^9\,M_\odot$ dwarf galaxy host. Two other methods produce results consistent with Schwarzschild's modelling, but can only provide upper-limits on ${\cal M}_{BH}$. This represents the detection of a BH in the most compact ($r_h\approx15\,{\rm pc}$) stellar system to date, with a ${\cal M}_{BH}$ corresponding to ~9 percent of the system's stellar mass, roughly in line with previously reported UCD BH detections and comparable to the BH detected in the compact elliptical galaxy NGC4486B.

astro-ph.GA↗

arXiv2023

An evolutionary continuum from nucleated dwarf galaxies to star clusters

Systematic studies have revealed hundreds of ultra-compact dwarf galaxies (UCDs) in the nearby Universe. With half-light radii $r_h$ of approximately 10-100 parsecs and stellar masses $M_*$ $\approx$ $10^6-10^8$ solar masses, UCDs are among the densest known stellar systems. Although similar in appearance to massive globular clusters, the detection of extended stellar envelopes, complex star formation histories, elevated mass-to-light ratio, and supermassive black holes suggest that some UCDs are remnant nuclear star clusters of tidally-stripped dwarf galaxies, or even ancient compact galaxies. However, only a few objects have been found in the transient stage of tidal stripping, and this assumed evolutionary path has never been fully traced by observations. Here we show that 106 galaxies in the Virgo cluster have morphologies that are intermediate between normal, nucleated dwarf galaxies and single-component UCDs, revealing a continuum that fully maps this morphological transition, and fills the `size gap' between star clusters and galaxies. Their spatial distribution and redder color are also consistent with stripped satellite galaxies on their first few pericentric passages around massive galaxies. The `ultra-diffuse' tidal features around several of these galaxies directly show how UCDs are forming through tidal stripping, and that this evolutionary path can include an early phase as a nucleated ultra-diffuse galaxy (UDG). These UCDs represent substantial visible fossil remnants of ancient dwarf galaxies in galaxy clusters, and more low-mass remnants probably remain to be found.

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arXiv2022

Tracing the Giant Outer Halo of the Mysterious Massive Disk Galaxy M104 I. Photometry of the Extended Globular Cluster Systems

M104 (NGC 4594, the Sombrero galaxy) is a mysterious massive early-type galaxy that shows a dominant bulge and a prominent disk. However, the presence of a halo in M104 has been elusive, and it is not yet known how M104 has acquired such a peculiar structure. Using wide ($\sim2$ deg$^2$) and deep $ugi$ images of M104 obtained with the CFHT/MegaCam, we detect a large number of globular clusters (GCs) found out to $R\approx35'$ ($\sim100$ kpc). The color distribution of these GCs shows two subpopulations: a blue (metal-poor) system and a red (metal-rich) system. The total number of GCs is estimated to be $N_{GC}=1610\pm30$ and the specific frequency to be $S_{N}=1.8\pm0.1$. The radial number density profile of the GCs is steep in the inner region at $R<20'$, and becomes shallow in the outer region at $20'<R<35'$. The outer region is dominated by blue GCs and is extended out to $R\approx35'$. This shows clearly the existence of a giant metal-poor halo in M104. The inner region is composed of a bulge hosting a disk, corresponding to a metal-rich halo as seen in early-type galaxies. At least two clumps of blue GCs are found in the outer region. One clump is overlapped with a faint stellar stream located in the south west, indicating that it may be a remnant of a disrupted dwarf galaxy. Our results imply that the metal-rich inner halo of M104 formed first via major mergers, and the metal-poor outer halo grew via numerous minor mergers.

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arXiv2022

The Next Generation Virgo Cluster Survey. XXXIII. Stellar Population Gradients in the Virgo Cluster Core Globular Cluster System

We present a study of the stellar populations of globular clusters (GCs) in the Virgo Cluster core with a homogeneous spectroscopic catalog of 692 GCs within a major axis distance $R_{\rm maj} = $ 840 kpc from M87. We investigate radial and azimuthal variations in the mean age, total metallicity, [Fe/H], and $α$-element abundance, of blue (metal-poor) and red (metal-rich) GCs using their co-added spectra. We find that the blue GCs have a steep radial gradient in [Z/H] within $R_{\rm maj} =$ 165 kpc, with roughly equal contributions from [Fe/H] and [$α$/Fe], and flat gradients beyond. By contrast, the red GCs show a much shallower gradient in [Z/H], which is entirely driven by [Fe/H]. We use GC-tagged Illustris simulations to demonstrate an accretion scenario where more massive satellites (with more metal- and $α$-rich GCs) sink further into the central galaxy than less massive ones, and where the gradient flattening occurs because of the low GC occupation fraction of low-mass dwarfs disrupted at larger distances. The dense environment around M87 may also cause the steep [$α$/Fe] gradient of the blue GCs, mirroring what is seen in the dwarf galaxy population. The progenitors of red GCs have a narrower mass range than those of blue GCs, which makes their gradients shallower. We also explore spatial inhomogeneity in GC abundances, finding that the red GCs to the northwest of M87 are slightly more metal-rich. Future observations of GC stellar population gradients will be useful diagnostics of halo merger histories.

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arXiv2021

The Color Gradients of the Globular Cluster Systems in M87 and M49

Combining data from the ACS Virgo Cluster Survey (ACSVCS) and the Next Generation Virgo cluster Survey (NGVS), we extend previous studies of color gradients of the globular cluster (GC) systems of the two most massive galaxies in the Virgo cluster, M87 and M49, to radii of $\sim 15~R_e$ ($\sim 200$ kpc for M87 and $\sim 250$ kpc for M49). We find significant negative color gradients, i.e., becoming bluer with increasing distance, out to these large radii. The gradients are driven mainly by the outwards decrease of the ratio of red to blue GC numbers. The color gradients are also detected out to $\sim 15~R_e$ in the red and blue sub-populations of GCs taken separately. In addition, we find a negative color gradient when we consider the satellite low-mass elliptical galaxies as a system, i.e., the satellite galaxies closer to the center of the host galaxy usually have redder color indices, both for their stars and GCs. According to the "two phase" formation scenario of massive early-type galaxies, the host galaxy accretes stars and GCs from low-mass satellite galaxies in the second phase. So the accreted GC system naturally inherits the negative color gradient present in the satellite population. This can explain why the color gradient of the GC system can still be observed at large radii after multiple minor mergers.

astro-ph.GA↗

arXiv2021

Fresh Insights on the Kinematics of M49's Globular Cluster System with MMT/Hectospec Spectroscopy

We present the first results of an MMT/Hectospec campaign to measure the kinematics of globular clusters (GCs) around M49 -- the brightest galaxy in the Virgo galaxy cluster, which dominates the Virgo B subcluster. The data include kinematic tracers beyond 95 kpc (~5.2 effective radii) for M49 for the first time, enabling us to achieve three key insights reported here. First, beyond ~20'-30' (~100-150 kpc), the GC kinematics sampled along the minor photometric axis of M49 become increasingly hotter, indicating a transition from GCs related to M49 to those representing the Virgo B intra-cluster medium. Second, there is an anomaly in the line-of-sight radial velocity dispersion ($σ_{r,los}$) profile in an annulus ~10-15' (~50-90 kpc) from M49 in which the kinematics cool by $Δσ_{r,los}~150$ km s$^{-1}$ relative to those in- or outward. The kinematic fingerprint of a previous accretion event is hinted at in projected phase-space, and we isolate GCs that both give rise to this feature, and are spatially co-located with two prominent stellar shells in the halo of M49. Third, we find a subsample of GCs with velocities representative of the dwarf galaxy VCC1249 that is currently interacting with M49. The spatial distribution of these GCs closely resembles the morphology of VCC1249's isophotes, indicating that several of these GCs are likely in the act of being stripped from the dwarf during its passage through M49's halo. Taken together, these results point toward the opportunity of witnessing on-going giant halo assembly in the depths of a cluster environment.

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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.

astro-ph.GA↗

arXiv2020

The Next Generation Virgo Cluster Survey. XXXIV. Ultra-Compact Dwarf (UCD) Galaxies in the Virgo Cluster

We present a study of ultra compact dwarf (UCD) galaxies in the Virgo cluster based mainly on imaging from the Next Generation Virgo Cluster Survey (NGVS). Using $\sim$100 deg$^{2}$ of $u^*giz$ imaging, we have identified more than 600 candidate UCDs, from the core of Virgo out to its virial radius. Candidates have been selected through a combination of magnitudes, ellipticities, colors, surface brightnesses, half-light radii and, when available, radial velocities. Candidates were also visually validated from deep NGVS images. Subsamples of varying completeness and purity have been defined to explore the properties of UCDs and compare to those of globular clusters and the nuclei of dwarf galaxies with the aim of delineating the nature and origins of UCDs. From a surface density map, we find the UCDs to be mostly concentrated within Virgo's main subclusters, around its brightest galaxies. We identify several subsamples of UCDs -- i.e., the brightest, largest, and those with the most pronounced and/or asymmetric envelopes -- that could hold clues to the origin of UCDs and possible evolutionary links with dwarf nuclei. We find some evidence for such a connection from the existence of diffuse envelopes around some UCDs, and comparisons of radial distributions of UCDs and nucleated galaxies within the cluster.

astro-ph.GA↗

arXiv2020

A discrete chemo-dynamical model of M87's globular clusters: Kinematics extending to ~ 400 kpc

We study the mass distribution and kinematics of the giant elliptical galaxy M87 (NGC 4486) using discrete chemo-dynamical, axisymmetric Jeans equation modelling. Our catalogue comprises 894 globular clusters (GCs) extending to a projected radius of $\sim 430$ kpc with line-of-sight velocities and colours, and Multi Unit Spectroscopic Explorer (MUSE) integral field unit data within the central $2.4$ kpc of the main galaxy. The gravitational potential for our models is a combination of a luminous matter potential with a varying mass-to-light ratio for the main galaxy, a supermassive black hole and a dark matter (DM) potential with a cusped or cored DM halo. The best-fitting models with either a cusped or a cored DM halo show no significant differences and both are acceptable. We obtain a total mass of $(2.16 \pm 0.38) \times 10^{13} M_{\odot}$ within $\sim$ 400 kpc. By including the stellar mass-to-light ratio gradient, the DM fraction increases from $\sim$ 26 percent (with no gradient) to $\sim$ 73 percent within $1\,R_e^{\rm maj}$ (major axis of half-light isophote, 14.2 kpc), and from $\sim$ 84 percent to $\sim$ 94 percent within $5\,R_e^{\rm maj}$ (71.2 kpc). Red GCs have moderate rotation with $V_{\rm max}/σ\sim$ 0.4, and blue GCs have weak rotation with $V_{\rm max}/σ\sim$ 0.1. Red GCs have tangential velocity dispersion anisotropy, while blue GCs are consistent with being nearly isotropic. Our results suggest that red GCs are more likely to be born in-situ, while blue GCs are more likely to be accreted.

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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

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

Gemini/GMOS Spectroscopy of Globular Clusters in the Merger Remnant Galaxy M85

M85 is a peculiar S0 galaxy in Virgo and is a well-known merger remnant. In this paper, we present the first spectroscopic study of globular clusters (GCs) in M85. We obtain spectra for 21 GC candidates and the nucleus of M85 using the Gemini Multi-Object Spectrograph on the Gemini North 8.1 m telescope. From their radial velocities, 20 of the GCs are found to be members of M85. We find a strong rotation signal of the M85 GC system with a rotation amplitude of 235 km s$^{-1}$. The rotation axis of the GC system has a position angle of about 161$^{\circ}$, which is 51$\rlap{.}{^\circ}$5 larger than that of the stellar light. The rotation-corrected radial velocity dispersion of the GC system is estimated to be $σ_{\rm r,cor} = $ 160 km s$^{-1}$. The rotation parameter $ΩR_{\rm icor}/σ_{\rm r,cor}$ of the GC system is derived to be 1.47$^{+1.05}_{-0.48}$, which is one of the largest among known early-type galaxies. The ages and metallicities of the GCs, which show the same trend as the results based on Lick indices, are derived from full spectrum fitting (ULySS). About a half of the GCs are an intermediate-age population of which the mean age is $\sim$ 3.7 $\pm$ 1.9 Gyr, having a mean [Fe/H] value of --0.26. The other half are old and metal-poor. These results suggest that M85 experienced a wet merging event about 4 Gyr ago, forming a significant population of star clusters. The strong rotational feature of the GC system can be explained by an off-center major merging.

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arXiv2016

To the Edge of M87 and Beyond: Spectroscopy of Intracluster Globular Clusters and Ultra Compact Dwarfs in the Virgo Cluster

We present the results from a wide-field spectroscopic survey of globular clusters (GCs) in the Virgo Cluster. We obtain spectra for 201 GCs and 55 ultracompact dwarfs (UCDs) using the Hectospec on the Multiple Mirror Telescope, and derive their radial velocities. We identify 46 genuine intracluster GCs (IGCs), not associated with any Virgo galaxies, using the 3D GMM test on the spatial and radial velocity distribution.They are located at the projected distance 200 kpc $\lesssim$ R $\lesssim$ 500 kpc from the center of M87. The radial velocity distribution of these IGCs shows two peaks, one at $v_{\rm r}$ = 1023 km s$^{-1}$ associated with the Virgo main body, and another at $v_{\rm r}$ = 36 km s$^{-1}$ associated with the infalling structure. The velocity dispersion of the IGCs in the Virgo main body is $σ_{\rm{GC}} \sim$ 314 km s$^{-1}$, which is smoothly connected to the velocity dispersion profile of M87 GCs, but much lower than that of dwarf galaxies in the same survey field, $σ_{\rm{dwarf}} \sim$ 608 km s$^{-1}$. The UCDs are more centrally concentrated on massive galaxies, M87, M86, and M84. The radial velocity dispersion of the UCD system is much smaller than that of dwarf galaxies. Our results confirm the large-scale distribution of Virgo IGCs indicated by previous photometric surveys. The color distribution of the confirmed IGCs shows a bimodality similar to that of M87 GCs. This indicates that most IGCs are stripped off from dwarf galaxies and some from massive galaxies in the Virgo.

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arXiv2015

Optical Spectroscopy of Supernova Remnants in M81 and M82

We present spectroscopy of 28 SNR candidates as well as one H II region in M81, and two SNR candidates in M82. Twenty six out of the M81 candidates turn out to be genuine SNRs, and two in M82 may be shocked condensations in the galactic outflow or SNRs. The distribution of [N II]/Hα ratios of M81 SNRs is bimodal. M81 SNRs are divided into two groups in the spectral line ratio diagrams: an [O III]-strong group and an [O III]-weak group. The latter have larger sizes, and may have faster shock velocity. [N II]/Hα ratios of the SNRs show a strong correlation with [S II]/Hα ratios. They show a clear radial gradient in [N II]/Hα and [S II]/Hα ratios: dLog ([N II]/Hα)/dLog R = -0.018 {\pm} 0.008 dex/kpc and dLog ([S II]/Hα)/dLog R = -0.016 {\pm} 0.008 dex/kpc where R is a deprojected galactocentric distance. We estimate the nitrogen and oxygen abundance of the SNRs from the comparison with shock-ionization models. We obtain a value for the nitrogen radial gradient, dLog(N/H)/dLogR = -0.023 {\pm} 0.009 dex/kpc, and little evidence for the gradient in oxygen. This nitrogen abundance shows a few times flatter gradient than those of the planetary nebulae and H II regions. We find that five SNRs are matched with X-ray sources. Their X-ray hardness colors are consistent with thermal SNRs.

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arXiv2013

Ks Band Luminosity Evolution of the Asymptotic Giant Branch Population Based on Star Clusters in the Large Magellanic Cloud

We present a study of Ks band luminosity evolution of the asymptotic giant branch (AGB) population in simple stellar systems using star clusters in the Large Magellanic Cloud (LMC). We determine physical parameters of LMC star clusters including center coordinates, radii, and foreground reddenings. Ages of 83 star clusters are derived from isochrone fitting with the Padova models, and those of 19 star clusters are taken from the literature. The AGB stars in 102 star clusters with log(age) = 7.3 - 9.5 are selected using near-infrared color magnitude diagrams based on 2MASS photometry. Then we obtain the Ks band luminosity fraction of AGB stars in these star clusters as a function of ages. The Ks band luminosity fraction of AGB stars increases, on average, as age increases from log(age) ~ 8.0, reaching a maximum at log(age) ~ 8.5, and it decreases thereafter. There is a large scatter in the AGB luminosity fraction for given ages, which is mainly due to stochastic effects. We discuss this result in comparison with five simple stellar population models. The maximum Ks band AGB luminosity fraction for bright clusters is reproduced by the models that expect the value of 0.7 - 0.8 at log(age) = 8.5 - 8.7. We discuss the implication of our results with regard to the study of size and mass evolution of galaxies.

astro-ph.CO↗