Browsing by Author "Ko, Youkyung"

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    An evolutionary continuum from nucleated dwarf galaxies to star clusters
    (2023) Wang, Kaixiang; Peng, Eric W.; Liu, Chengze; Mihos, J. Christopher; Cote, Patrick; Ferrarese, Laura; Taylor, Matthew A.; Blakeslee, John P.; Cuillandre, Jean-Charles; Duc, Pierre-Alain; Guhathakurta, Puragra; Gwyn, Stephen; Ko, Youkyung; Lancon, Ariane; Lim, Sungsoon; Macarthur, Lauren A.; Puzia, Thomas; Roediger, Joel; Sales, Laura V.; Sanchez-Janssen, Ruben; Spengler, Chelsea; Toloba, Elisa; Zhang, Hongxin; Zhu, Mingcheng
    Systematic studies1-4 have revealed hundreds of ultra-compact dwarf galaxies (UCDs5) in the nearby Universe. With half-light radii rh of approximately 10-100 parsecs and stellar masses M* approximate to 106-108 solar masses, UCDs are among the densest known stellar systems6. Although similar in appearance to massive globular clusters7, the detection of extended stellar envelopes4,8,9, complex star formation histories10, elevated mass-to-light ratio11,12 and supermassive black holes13-16 suggest that some UCDs are remnant nuclear star clusters17 of tidally stripped dwarf galaxies18,19, or even ancient compact galaxies20. However, only a few objects have been found in the transient stage of tidal stripping21,22, and this assumed evolutionary path19 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 colour are also consistent with stripped satellite galaxies on their first few pericentric passages around massive galaxies23. 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 galaxy24,25. 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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    Fresh Insights on the Kinematics of M49's Globular Cluster System with MMT/Hectospec Spectroscopy
    (2021) Taylor, Matthew A.; Ko, Youkyung; Cote, Patrick; Ferrarese, Laura; Peng, Eric W.; Zabludoff, Ann; Roediger, Joel; Sanchez-Janssen, Ruben; Hendel, David; Chilingarian, Igor; Liu, Chengze; Spengler, Chelsea; Zhang, Hongxin
    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 (similar to 5.2 effective radii) for M49 for the first time, enabling us to achieve three key insights reported here. First, beyond similar to 20 '-30 ' (similar to 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 (sigma ( r,los)) profile in an annulus similar to 10-15 ' (similar to 50-90 kpc) from M49 in which the kinematics cool by Delta sigma ( r,los) approximate to 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 VCC 1249 that is currently interacting with M49. The spatial distribution of these GCs closely resembles the morphology of VCC 1249'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 ongoing giant halo assembly in the depths of a cluster environment.
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    The Color Gradients of the Globular Cluster Systems in M87 and M49
    (2022) Wu, Yiming; Liu, Chengze; Peng, Eric W.; Ko, Youkyung; Cote, Patrick; Jain, Rashi; Ferrarese, Laura; Yang, Xiaohu; Lancon, Ariane; Puzia, Thomas; Lim, Sungsoon
    Combining data from the ACS Virgo Cluster Survey and the Next Generation Virgo cluster Survey, 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 similar to 15 R (e) (similar to 200 kpc for M87 and similar to 250 kpc for M49, where R (e) is the effective radius). 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 outward decrease in the ratio of red to blue GC numbers. The color gradients are also detected out to similar to 15 R (e) in the red and blue subpopulations 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, for both their stars and their 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 an 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.
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    The Next Generation Virgo Cluster Survey. XVII. A Search for Planetary Nebulae in Virgo Cluster Globular Clusters
    (2019) Sun, Weijia; Peng, Eric W.; Ko, Youkyung; Cote, Patrick; Ferrarese, Laura; Lee, Myung Gyoon; Liu, Chengze; Longobardi, Alessia; Chilingarian, Igor V.; Spengler, Chelsea; Zabludoff, Ann I.; Zhang, Hong-Xin; Cuillandre, Jean-Charles; Gwyn, Stephen D. J.
    The occurrence of planetary nebulae (PNe) in globular clusters (GCs) provides an excellent chance to study lowmass stellar evolution in a special (low-metallicity, high stellar density) environment. We report a systematic spectroscopic survey for the [O III] 5007 angstrom 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 III] detection limit for each GC to estimate the luminosity-specific frequency of PNe, a, and measured a in the Virgo cluster GCs to be alpha similar to 3.9(-0.7)(+5.2) x 10(-8) PN/(circle dot). The value of a in the Virgo GCs is among the lowest reported in any environment, due in part to the large sample size, and it is 5-6 times lower than that for the Galactic GCs. We suggest that a decreases toward 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 nonzero 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 the Virgo GCs is governed by an internal process (mass, density, binary fraction) or if it is largely regulated by the external environment.
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    The Next Generation Virgo Cluster Survey. XXXIII. Stellar Population Gradients in the Virgo Cluster Core Globular Cluster System
    (2022) Ko, Youkyung; Peng, Eric W.; Cote, Patrick; Ferrarese, Laura; Liu, Chengze; Longobardi, Alessia; Lancon, Ariane; Munoz, Roberto P.; Puzia, Thomas H.; Alamo-Martinez, Karla A.; Sales, Laura, V; Ramos-Almendares, Felipe; Abadi, Mario G.; Lee, Myung Gyoon; Hwang, Ho Seong; Caldwell, Nelson; Blakeslee, John P.; Boselli, Alessandro; Cuillandre, Jean-Charles; Duc, Pierre-Alain; Eyheramendy, Susana; Guhathakurta, Puragra; Gwyn, Stephen; Jordan, Andres; Lim, Sungsoon; Sanchez-Janssen, Ruben; Toloba, Elisa
    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 (maj) = 840 kpc from M87. We investigate radial and azimuthal variations in the mean age, total metallicity, [Fe/H], and alpha-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 (maj) = 165 kpc, with roughly equal contributions from [Fe/H] and [alpha/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 alpha-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 [alpha/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.