arXiv subjects

Suk-Jin Yoon

Publications and source records attributed to Suk-Jin Yoon.

At least 73 records · Page 4Linked to original sources

arXiv2013

Density distributions of outflow driven turbulence

Protostellar jets and outflows are signatures of star formation and promising mechanisms for driving supersonic turbulence in molecular clouds. We quantify outflow-driven turbulence through three-dimensional numerical simulations using an isothermal version of the robust total variation diminishing code. We drive turbulence in real-space using a simplified spherical outflow model, analyse the data through density probability distribution functions (PDF), and investigate the Core Formation Rate per free-fall time (CFR_ff). The real-space turbulence driving method produces a negatively skewed density PDF possessing an enhanced tail on the low-density side. It deviates from the log-normal distributions typically obtained from Fourier-space turbulence driving at low densities, but can provide a good fit at high-densities, particularly in terms of mass weighted rather than volume weighted density PDF. Due to this fact, we suggest that the CFR_ff determined from a Fourier-driven turbulence model could be comparable to that of our particular real-space driving model, which has a ratio of solenoidal to compressional components from the resulting turbulence velocity fields of ~0.6.

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arXiv2013

The effect of second generation populations on the integrated colors of metal-rich globular clusters in early-type galaxies

The mean color of globular clusters (GCs) in early-type galaxies is in general bluer than the integrated color of halo field stars in host galaxies. Metal-rich GCs often appear more associated with field stars than metal-poor GCs, yet show bluer colors than their host galaxy light. Motivated by the discovery of multiple stellar populations in Milky Way GCs, we present a new scenario in which the presence of second-generation (SG) stars in GCs is responsible for the color discrepancy between metal-rich GCs and field stars. The model assumes that the SG populations have enhanced helium abundance as evidenced by observations, and it gives a good explanation of the bluer optical colors of metal-rich GCs than field stars as well as strong Balmer lines and blue UV colors of metal-rich GCs. Ours may be complementary to the recent scenario suggesting the difference in stellar mass functions (MFs) as an origin for the GC-to-star color offset. A quantitative comparison is given between the SG and MF models.

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arXiv2013

Nonlinear Color-Metallicity Relations of Globular Clusters. IV. Testing the Nonlinearity Scenario for Color Bimodality via HST/WFC3 u-band Photometry of M84 (NGC 4374)

Color distributions of globular clusters (GCs) in most massive galaxies are bimodal. Assuming linear color-to-metallicity conversions, bimodality is viewed as the presence of merely two GC subsystems with distinct metallicities, which serves as a critical backbone of various galaxy formation theories. Recent studies, however, revealed that the color-metallicity relations (CMRs) often used to derive GC metallicities (e.g., CMRs of g-z, V-I and C-T1) are in fact inflected. Such inflection can create bimodal color distributions if the underlying GC metallicity spread is simply broad as expected from the hierarchical merging paradigm of galaxy formation. In order to test the nonlinear-CMR scenario for GC color bimodality, the u-band photometry is proposed because the u-related CMRs (e.g., CMRs of u-g and u-z) are theoretically predicted to be least inflected and most distinctive among commonly used optical CMRs. Here, we present Hubble Space Telescope (HST)/WFC3 F336W (u-band) photometry of the GC system in M84, a giant elliptical in the Virgo galaxy cluster. Combining the u data with the existing HST ACS/WFC g and z data, we find that the u-z and u-g color distributions are different from the g-z distribution in a very systematic manner and remarkably consistent with our model predictions based on the nonlinear-CMR hypothesis. The results lend further confidence to validity of the nonlinear-CMR scenario as an explanation for GC color bimodality. There are some GC systems showing bimodal spectroscopic metallicity, and in such systems the inflected CMRs often create stronger bimodality in the color domain.

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arXiv2013

Nonlinear Color-Metallicity Relations of Globular Clusters. V. Nonlinear Absorption-line Index versus Metallicity Relations and Bimodal Index Distributions of M31 Globular Clusters

Recent spectroscopy on the globular cluster (GC) system of M31 with unprecedented precision witnessed a clear bimodality in absorption-line index distributions of old GCs. Such division of extragalactic GCs, so far asserted mainly by photometric color bimodality, has been viewed as the presence of merely two distinct metallicity subgroups within individual galaxies and forms a critical backbone of various galaxy formation theories. Given that spectroscopy is a more detailed probe into stellar population than photometry, the discovery of index bimodality may point to the very existence of dual GC populations. However, here we show that the observed spectroscopic dichotomy of M31 GCs emerges due to the nonlinear nature of metallicity-to-index conversion and thus one does not necessarily have to invoke two separate GC subsystems. We take this as a close analogy to the recent view that metallicity-color nonlinearity is primarily responsible for observed GC color bimodality. We also demonstrate that the metallicity-sensitive magnesium line displays non-negligible metallicity-index nonlinearity and Balmer lines show rather strong nonlinearity. This gives rise to bimodal index distributions, which are routinely interpreted as bimodal metallicity distributions, not considering metallicity-index nonlinearity. Our findings give a new insight into the constitution of M31's GC system, which could change much of the current thought on the formation of GC systems and their host galaxies.

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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

Initial Size Distribution of the Galactic Globular Cluster System

Despite the importance of their size evolution in understanding the dynamical evolution of globular clusters (GCs) of the Milky Way, studies are rare that focus specifically on this issue. Based on the advanced, realistic Fokker-Planck (FP) approach, we predict theoretically the initial size distribution (SD) of the Galactic GCs along with their initial mass function and radial distribution. Over one thousand FP calculations in a wide parameter space have pinpointed the best-fit initial conditions for the SD, mass function, and radial distribution. Our best-fit model shows that the initial SD of the Galactic GCs is of larger dispersion than today's SD, and that typical projected half-light radius of the initial GCs is ~4.6 pc, which is 1.8 times larger than that of the present-day GCs (~2.5 pc). Their large size signifies greater susceptibility to the Galactic tides: the total mass of destroyed GCs reaches 3-5x10^8 M_sun$, several times larger than the previous estimates. Our result challenges a recent view that the Milky Way GCs were born compact on the sub-pc scale, and rather implies that (1) the initial GCs are generally larger than the typical size of the present-day GCs, (2) the initially large GCs mostly shrink and/or disrupt as a result of the galactic tides, and (3) the initially small GCs expand by two-body relaxation, and later shrink by the galactic tides.

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arXiv2012

Yonsei Evolutionary Population Synthesis (YEPS) Model. I. Spectroscopic Evolution of Simple Stellar Populations

We present a series of papers on the year-2012 version of Yonsei Evolutionary Population Synthesis (YEPS) model which is constructed on over 20 years of heritage. This first paper delineates the spectroscopic aspect of integrated light from stellar populations older than 1 Gyr. The standard YEPS is based on the most up-to-date Yonsei-Yale stellar evolutionary tracks and BaSel 3.1 flux libraries, and provides absorption line indices of the Lick/IDS system and high-order Balmer lines for simple stellar populations as functions of stellar parameters, such as metallicity, age and α-element mixture. Special care has been taken to incorporate systematic contribution from horizontal-branch stars which alters the temperature-sensitive Balmer lines significantly, resulting in up to 5 Gyr difference in age estimation of old, metal-poor stellar populations. We also find that the horizontal branches exert an appreciable effect not only on the Balmer lines but also on the metallicity-sensitive lines including the magnesium index. This is critical to explain the intriguing bimodality found in index distributions of globular clusters in massive galaxies and to derive spectroscopic metallicities accurately from various indices. A full set of the spectroscopic and photometric YEPS model data of the entire parameter space is currently downloadable at http://web.yonsei.ac.kr/cosmic/data/YEPS.htm

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arXiv2012

A Keck/LRIS Spatially-Resolved Spectroscopic Study of a LINER Galaxy SDSS J091628.05+420818.7

Using spatially-resolved spectra obtained with the Low Resolution Imaging Spectrometer at the Keck I telescope, we investigate the nature of ionizing sources and kinematic properties of emission-line gas in a LINER galaxy SDSS J091628.05+420818.7, which is a nearby (z = 0.0241) and bright (M_r = -20.2) early-type galaxy. After subtracting stellar absorption features using a combination of simple stellar population models, we measure the flux, line-of-sight velocity, and velocity dispersion of four emission lines, i.e., Hα, Hβ, [O III] λ5007, and [N II] λ6584, to study radial change of emission-line fluxes and velocities. Compared to the point-spread-function of the observation, the emission-line region is slightly extended but comparable to the seeing size. The central concentration of emission-line gas suggests that ionization is triggered by a nuclear source, excluding old stellar population as ionizing sources. We find that emission-line gas is counter-rotating with respect to stellar component and that the [O III] λ5007 line is blueshifted compared to other emission lines, possibly due to an outflow.

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arXiv2012

UV Properties of Galactic Globular Clusters with GALEX I. The Color-Magnitude Diagrams

We present GALEX data for 44 Galactic globular clusters obtained during 3 GALEX observing cycles between 2004 and 2008. This is the largest homogeneous data set on the UV photometric properties of Galactic globular clusters ever collected. The sample selection and photometric analysis are discussed, and color-magnitude diagrams are presented. The blue and intermediate-blue horizontal branch is the dominant feature of the UV color-magnitude diagrams of old Galactic globular clusters. Our sample is large enough to display the remarkable variety of horizontal branch shapes found in old stellar populations. Other stellar types that are obviously detected are blue stragglers and post core-He burning stars. The main features of UV color-magnitude diagrams of Galactic globular clusters are briefly discussed. We establish the locus of post-core He burning stars in the UV color-magnitude diagram and present a catalog of candidate AGB-manqu é, post early-AGB, and post-AGB stars within our cluster sample.

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arXiv2011

Nonlinear Color-Metallicity Relations of Globular Clusters. II. A Test on the Nonlinearity Scenario for Color Bimodality Using the u-band Colors: The Case of M87 (NGC 4486)

The optical color distributions of globular clusters (GCs) in most large elliptical galaxies are bimodal. Based on the assumed linear relationship between GC colors and their metallicities, the bimodality has been taken as evidence of two GC subsystems with different metallicities in each galaxy and led to a number of theories in the context of galaxy formation. More recent observations and modeling of GCs, however, suggest that the color-metallicity relations (CMRs) are inflected, and thus colors likely trace metallicities in a nonlinear manner. The nonlinearity could produce bimodal color distributions from a broad underlying metallicity spread, even if it is unimodal. Despite the far-reaching implications, whether CMRs are nonlinear and whether the nonlinearity indeed causes the color bimodality are still open questions. Given that the spectroscopic refinement of CMRs is still very challenging, we here propose a new photometric technique to probe the possible nonlinear nature of CMRs. In essence, a color distribution of GCs is a "projected" distribution of their metallicities. Since the form of CMRs hinges on which color is used, the shape of color distributions varies depending significantly on the colors. Among other optical colors, the u-band related colors (e.g., u - g and u - z) are theoretically predicted to exhibit significantly less inflected CMRs than other preferred CMRs (e.g., for g - z). As a case study, we performed the HST/WFPC2 archival u-band photometry for the M87 (NGC 4486) GC system with confirmed color bimodality. We show that the u-band color distributions are significantly different from that of g - z, and consistent with our model predictions. With more u-band measurements, this method will support or rule out the nonlinear-CMR scenario for the origin of GC color bimodality with high confidence. [Abridged]

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arXiv2011

Nonlinear Color-Metallicity Relations of Globular Clusters. III. On the Discrepancy in Metallicity between Globular Cluster Systems and their Parent Elliptical Galaxies

One of the conundrums in extragalactic astronomy is the discrepancy in observed metallicity distribution functions (MDFs) between the two prime stellar components of early-type galaxies-globular clusters (GCs) and halo field stars. This is generally taken as evidence of highly decoupled evolutionary histories between GC systems and their parent galaxies. Here we show, however, that new developments in linking the observed GC colors to their intrinsic metallicities suggest nonlinear color-to-metallicity conversions, which translate observed color distributions into strongly-peaked, unimodal MDFs with broad metal-poor tails. Remarkably, the inferred GC MDFs are similar to the MDFs of resolved field stars in nearby elliptical galaxies and those produced by chemical evolution models of galaxies. The GC MDF shape, characterized by a sharp peak with a metal-poor tail, indicates a virtually continuous chemical enrichment with a relatively short timescale. The characteristic shape emerges across three orders of magnitude in the host galaxy mass, suggesting a universal process of chemical enrichment among various GC systems. Given that GCs are bluer than field stars within the same galaxy, it is plausible that the chemical enrichment processes of GCs ceased somewhat earlier than that of field stellar population, and if so, GCs preferentially trace the major, vigorous mode of star formation events in galactic formation. We further suggest a possible systematic age difference among GC systems, in that the GC systems in more luminous galaxies are older. This is consistent with the downsizing paradigm of galaxies and supports additionally the similar nature shared by GCs and field stars. Our findings suggest that GC systems and their parent galaxies have shared a more common origin than previously thought, and hence greatly simplify theories of galaxy formation.

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arXiv2011

The effect of helium-enhanced stellar populations on the ultraviolet-upturn phenomenon of early-type galaxies

Recent observations and modeling of globular clusters with multiple populations strongly indicate the presence of super helium-rich subpopulations in old stellar systems. Motivated by this, we have constructed new population synthesis models with and without helium-enhanced subpopulations to investigate their impact on the UV-upturn phenomenon of quiescent early-type galaxies. We find that our models with helium- enhanced subpopulations can naturally reproduce the strong UV-upturns observed in giant elliptical galaxies assuming an age similar to that of old globular clusters in the Milky Way. The major source of far-UV (FUV) flux, in this model, is relatively metal- poor and helium-enhanced hot horizontal branch stars and their progeny. The Burstein et al. (1988) relation of the F U V - V color with metallicity is also explained either by the variation of the fraction of helium-enhanced subpopulations or by the spread in mean age of stellar populations in early-type galaxies.

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arXiv2009

The Presence of Two Distinct Red Giant Branches in the Globular Cluster NGC 1851

There is a growing body of evidence for the presence of multiple stellar populations in some globular clusters, including NGC 1851. For most of these peculiar globular clusters, however, the evidence for the multiple red giant-branches (RGBs) having different heavy elemental abundances as observed in Omega Centauri is hitherto lacking, although spreads in some lighter elements are reported. It is therefore not clear whether they also share the suggested dwarf galaxy origin of Omega Cen or not. Here we show from the CTIO 4m UVI photometry of the globular cluster NGC 1851 that its RGB is clearly split into two in the U - I color. The two distinct RGB populations are also clearly separated in the abundance of heavy elements as traced by Calcium, suggesting that the type II supernovae enrichment is also responsible, in addition to the pollutions of lighter elements by intermediate mass asymptotic giant branch stars or fast-rotating massive stars. The RGB split, however, is not shown in the V - I color, as indicated by previous observations. Our stellar population models show that this and the presence of bimodal horizontal-branch distribution in NGC 1851 can be naturally reproduced if the metal-rich second generation stars are also enhanced in helium.

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arXiv2009

Probing the Intermediate-Age Globular Clusters in NGC 5128 from Ultraviolet Observations

We explore the age distribution of the globular cluster (GC) system of the nearby elliptical galaxy NGC 5128 using ultraviolet (UV) photometry from Galaxy Evolution Explorer (GALEX) observations, with UV - optical colors used as the age indicator. Most GCs in NGC 5128 follow the general trends of GCs in M31 and Milky Way in UV - optical color-color diagram, which indicates that the majority of GCs in NGC 5128 are old similar to the age range of old GCs in M31 and Milky Way. A large fraction of spectroscopically identified intermediate-age GC (IAGC) candidates with ~ 3-8 Gyr are not detected in the FUV passband. Considering the nature of intermediate-age populations being faint in the far-UV (FUV) passband, we suggest that many of the spectroscopically identified IAGCs may be truly intermediate in age. This is in contrast to the case of M31 where a large fraction of spectroscopically suggested IAGCs are detected in FUV and therefore may not be genuine IAGCs but rather older GCs with developed blue horizontal branch stars. Our UV photometry strengthens the results previously suggesting the presence of GC and stellar subpopulation with intermediate age in NGC 5128. The existence of IAGCs strongly indicates the occurrence of at least one more major star formation episode after a starburst at high redshift.

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arXiv2009

The Fundamental Planes of E+A galaxies and GALEX UV-excess early-type galaxies: Revealing their intimate connection

Strong Balmer absorption lines and the lack of Ha and [OII] emission lines signify that E+As are post-starburst systems. Recent studies suggest that E+As may undergo the transition from the `blue cloud' to the `red sequence' and eventually migrate to red sequence ETGs. An observational validation of this scenario is to identify the intervening galaxy population between E+As and the red-sequence. Motivated by recent findings with GALEX that a large fraction of ETGs exhibit UV-excess as a sign of RSF, we investigate the possible connection of the UV-excess galaxies to E+As. In particular, we examine the FP scaling relations of the largest sample of ~1,000 E+As selected from the SDSS and ~20,000 morphologically-selected SDSS ETGs with GALEX UV data. The FP parameters, combined with stellar population indicators, reveal a certain group of UV-excess ETGs that bridges between E+As and quiescent red galaxies. The newly identified galaxies are the post-starburst systems characterized by UV-excess but no Ha emission. This is a conceptual generalisation of "E+A", in that the Balmer absorption line in the "E+A" definition is replaced with UV-optical colours that are far more sensitive to RSF than the Balmer lines. We refer to these UV-excess galaxies as "E+a" galaxies, which stands for elliptical ("E") galaxies with a minority of A-type ("a") young stars. The species are either (1) galaxies that experienced starbursts weaker than those observed in E+As (1~10% of E+As, "mild E+As") or (2) the products of passively evolved E+As after quenching star formation quite a while ago (~1 Gyr, "old E+As"). We suggest that the latter type of E+a galaxies represents the most recent arrival to the red sequence in the final phase of the "E+A" to "red early-type" transition. (Abridged)

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arXiv2008

Compact stellar systems in the Fornax cluster: a UV perspective

In recent years, increasing evidence for chemical complexity and multiple stellar populations in massive globular clusters (GCs) has emerged, including extreme horizontal branches (EHBs) and UV excess. Our goal is to improve our understanding of UV excess in the regime of both massive GCs and ultra-compact dwarf galaxies (UCDs). To this end, we use deep archival GALEX data of the central Fornax cluster to measure NUV and FUV magnitudes of UCDs and massive GCs. We obtain NUV photometry for a sample of 35 compact objects with -13.5<M_V<-10 mag. Of those, 21 objects also have FUV photometry. Roughly half of the sources fall into the UCD luminosity regime (M_V <=-11 mag). We find that seven out of 17 massive Fornax GCs exhibit a NUV excess with respect to expectations from stellar population models, even for models with enhanced Helium abundance. This suggests that not only He-enrichment has contributed to forming the EHB population of these GCs. The GCs extend to stronger UV excess than GCs in M31 and massive GCs in M87, at the 97% confidence level. Most of the UCDs with FUV photometry also show evidence for UV excess, but their UV colours can be matched by isochrones with enhanced Helium abundances and old ages 12-14 Gyrs. We find that Fornax compact objects with X-ray emission detected from Chandra images are almost disjunct in colour from compact objects with GALEX UV detection, with only one X-ray source among the 35 compact objects. However, since this source is one of the three most UV bright GCs, we cannot exclude that the physical processes causing X-ray emission also contribute to some of the observed UV excess.

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arXiv2007

On the Origin of Bimodal Horizontal-Branches in Massive Globular Clusters: The Case of NGC 6388 and NGC 6441

Despite the efforts of the past decade, the origin of the bimodal horizontal-branch (HB) found in some globular clusters (GCs) remains a conundrum. Inspired by the discovery of multiple stellar populations in the {\it most massive} Galactic GC, $ω$ Centauri, we investigate the possibility that two distinct populations may coexist and are responsible for the bimodal HBs in the {\it third} and {\it fifth} brightest GCs, NGC 6388 and NGC 6441. Using the population synthesis technique, we examine two different chemical ``self-enrichment'' hypotheses in which a primordial GC was sufficiently massive to contain two or more distinct populations as suggested by the populations found in $ω$ Cen: (1) the age-metallicity relation scenario in which two populations with different metallicity and age coexist, following an internal age-metallicity relation, and (2) the super-helium-rich scenario in which GCs contain a certain fraction of helium-enhanced stars, for instance, the second generation stars formed from the helium-enriched ejecta of the first. The comparative study indicates that the detailed color-magnitude diagram morphologies and the properties of the RR Lyrae variables in NGC 6388 and NGC 6441 support the latter scenario; i.e., the model which assumes a minor fraction ($\sim$ 15 %) of helium-excess (Y $\simeq$ 0.3) stars. The results suggest that helium content is the main driver behind the HB bimodality found most often in massive GCs. If confirmed, the GC-to-GC variation of helium abundance should be considered a {\it local} effect, further supporting the argument that age is the {\it global} second parameter of HB morphology.

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arXiv2007

The Look-back Time Evolution of Far-Ultraviolet Flux from the Brightest Cluster Elliptical Galaxies at z < 0.2

We present the GALEX UV photometry of the elliptical galaxies in Abell clusters at moderate redshifts (z < 0.2) for the study of the look-back time evolution of the UV upturn phenomenon. The brightest elliptical galaxies (M_r < -22) in 12 remote clusters are compared with the nearby giant elliptical galaxies of comparable optical luminosity in the Fornax and Virgo clusters. The sample galaxies presented here appear to be quiescent without signs of massive star formation or strong nuclear activity, and show smooth, extended profiles in their UV images indicating that the far-UV (FUV) light is mostly produced by hot stars in the underlying old stellar population. Compared to their counterparts in nearby clusters, the FUV flux of cluster giant elliptical galaxies at moderate redshifts fades rapidly with ~ 2 Gyrs of look-back time, and the observed pace in FUV - V color evolution agrees reasonably well with the prediction from the population synthesis models where the dominant FUV source is hot horizontal-branch stars and their progeny. A similar amount of color spread (~ 1 mag) in FUV - V exists among the brightest cluster elliptical galaxies at z ~ 0.1, as observed among the nearby giant elliptical galaxies of comparable optical luminosity.

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