Nucleated Dwarf Galaxies Research Articles

MgAl burning chain in ω Centauri

In this study, we report the results of the analysis of Fe, Mg, Al, and Si abundances analysis for a sample of 439 stars in ω Centauri, using high-resolution spectra obtained with the VLT/FLAMES multi-object spectrograph. Our analysis reveals the presence of four distinct Fe populations, with the main peak occurring at a low metallicity, consistent with previous literature findings. We observe a discrete and pronounced Mg–Al anticorrelation, which exhibits variations in shape and extension as a function of metallicity. Specifically, this anticorrelation is present in stars with metallicities lower than approximately −1.3 dex, while it becomes less evident or absent for higher [Fe/H] values. Additionally, we detect (anti)correlations between Mg and Si, and between Al and Si, whose extensions also vary with metallicity, similar to the Mg–Al anticorrelation. These results suggest that the MgAl cycle plays a crucial role in the formation of multiple populations in ω Centauri, with the presence of all (anti)correlations at metallicities lower than –1.3 dex, providing evidence for the burning of Mg at very high temperatures (> 108 K), at least in the metal-poor regime. Furthermore, we observe a clear trend of stars with [Al/Fe] > +0.5 dex as a function of metallicity, confirming for the first time the existence of the two channels of Al production and destruction. This evidence can help to provide further constraints on the potential nature of the polluters responsible for the observed chemical anomalies in this stellar system. Finally, we find that the two most metal-poor populations identified in our sample are compatible with null or very small metallicity dispersion and we discuss how this result fits into a scenario where ω Centauri is the remnant of a disrupted nucleated dwarf galaxy.

An evolutionary continuum from nucleated dwarf galaxies to star clusters.

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* ≈ 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.

Extremely efficient mergers of intermediate-mass black hole binaries in nucleated dwarf galaxies

ABSTRACT Gravitational waves emitted by merging black holes between ∼104 and 107 M⊙ will be detectable by the Laser Interferometer Space Antenna (LISA) with signal-to-noise ratios of several hundred out to redshift 20. Supermassive black hole (107–1010 M⊙) binary formation, coalescence, and merger within massive galaxies is well-studied. However, intermediate-mass black holes (IMBHs) between ∼104 and 106 M⊙ are hosted by low mass and dwarf galaxies; it is not trivial to extrapolate black hole merger time-scales to this IMBH binary regime, due to the starkly different host galaxy structure, kinematics, and morphology compared to massive galaxy hosts. We perform ultrahigh resolution N-body simulations to study IMBH dynamics in nucleated dwarf galaxies whose structural parameters are obtained from observations of nearby dwarf galaxies. Starting from 50 pc, an IMBH quickly forms a binary. Thereafter, the binary orbit shrinks rapidly due to the high central stellar densities furnished by nuclear star clusters (NSCs). We find high eccentricities (e ∼ 0.4–0.99) in our suite of IMBH binaries, and residual eccentricity may persist to the LISA regime. IMBH merger times are typically a few hundred million years, with a few exceptionally short merger times for high eccentricities. We find that IMBH-stellar encounters originate pre-dominantly from NSCs, if the NSC-to-IMBH binary mass ratio is greater than 10; otherwise, bulge stars contribute significantly. As the IMBH binary ejects stars, however, the NSCs are disrupted. We conclude that comparable-mass IMBHs merge very efficiently in nucleated dwarf galaxies, making them promising LISA sources, as well as a channel for IMBH growth.

Metal-poor nuclear star clusters in two dwarf galaxies near Centaurus A suggesting formation from the in-spiraling of globular clusters

Studies of nucleated dwarf galaxies can constrain the scenarios for the formation and evolution of nuclear star clusters (NSC) in low-mass galaxies and give us insights on the origin of ultra compact dwarf galaxies (UCDs). We report the discovery of a NSC in the dwarf galaxy KKs58 and investigate its properties together with those of another NSC in KK197. Both NSCs are hosted by dwarf elliptical galaxies of the Centaurus group. Combining ESO VLT MUSE data with photometry from VLT FORS2, CTIO Blanco DECam, and HST ACS, as well as high-resolution spectroscopy from VLT UVES, we analyse the photometric, kinematic and stellar population properties of the NSCs and their host galaxies. We confirm membership of the NSCs based on their radial velocities and location close to the galaxy centres. We also confirm the membership of two globular clusters (GCs) and detect oblate rotation in the main body of KK197. Based on high signal-to-noise spectra taken with MUSE of the NSCs of both KKs58 and KK197 we measure low metallicities, [Fe/H] = −1.75 ± 0.06 dex and [Fe/H] = −1.84 ± 0.05 dex, and stellar masses of 7.3 × 105 M⊙ and 1.0 × 106 M⊙, respectively. Both NSCs are more metal-poor than their hosts that have metallicities of −1.35 ± 0.23 dex (KKs58) and −0.84 ± 0.12 dex (KK197). This can be interpreted as NSC formation via the in-spiral of GCs. The masses, sizes and metallicities of the two NSCs place them among other NSCs, but also among the known UCDs of the Centaurus group. This indicates that NSCs might constitute the progenitors of a part of the low-mass UCDs, although their properties are almost indistinguishable from typical GCs.

Chandra Detection of Intracluster X-Ray Sources in Fornax

Abstract Based on archival Chandra observations with a total exposure of 1.3 Ms, we study X-ray point sources in the Fornax cluster of galaxies, with the primary aim of searching for intracluster X-ray source populations. We detect 1177 point sources out to a projected radius of ∼30 arcmin (∼180 kpc) from the cluster center and down to a limiting 0.5–8 keV luminosity of ∼3 × 1037 erg s−1. We construct the source surface density profile, after excluding sources associated with foreground stars, known globular clusters, ultra-compact dwarfs, and galactic nuclei. From this profile we statistically identify ∼183 excess sources that are not associated with the bulk stellar content of the individual member galaxies of Fornax, nor with the cosmic X-ray background. Taking into account Poisson error and cosmic variance, the cumulative significance of this excess is at the ≳2σ level (with a maximum of 3.6σ) outside three effective radii of the central giant elliptical, NGC 1399. The luminosity function of the excess sources is found to be significantly steeper than that of the GC-hosting sources (presumably low-mass X-ray binaries (LMXBs)), disfavoring the possibility that unidentified GCs are primarily responsible for the excess. We show that a large fraction of the excess can be related to the extended stellar halo of NGC 1399 and/or the diffuse intracluster light, thus providing strong evidence for the presence of intracluster X-ray sources in Fornax, the second unambiguous case for a galaxy cluster after Virgo. Other possible origins of the excess, including supernova-kicked LMXBs and stripped nucleated dwarf galaxies are discussed.

The Next Generation Fornax Survey (NGFS). IV. Mass and Age Bimodality of Nuclear Clusters in the Fornax Core Region

Abstract We present the analysis of 61 nucleated dwarf galaxies in the central regions (≲R vir/4) of the Fornax galaxy cluster. The galaxies and their nuclei are studied as part of the Next Generation Fornax Survey using optical imaging obtained with the Dark Energy Camera mounted at Blanco/Cerro Tololo Inter-American Observatory and near-infrared data obtained with VIRCam at VISTA/ESO. We decompose the nucleated dwarfs in nucleus and spheroid, after subtracting the surface brightness profile of the spheroid component and studying the nucleus using point source photometry. In general, nuclei are consistent with colors of confirmed metal-poor globular clusters, but with significantly smaller dispersion than other confirmed compact stellar systems in Fornax. We find a bimodal nucleus mass distribution with peaks located at and ∼6.3. These two nucleus subpopulations have different stellar population properties: the more massive nuclei are older than ∼2 Gyr and have metal-poor stellar populations (Z ≤ 0.02 Z ⊙), while the less massive nuclei are younger than ∼2 Gyr with metallicities in the range 0.02 < Z/Z ⊙ ≤ 1. We find that the nucleus mass ( ) versus galaxy mass ( ) relation becomes shallower for less massive galaxies starting around 108 M ⊙, and the mass ratio shows a clear anticorrelation with for the lowest masses, reaching 10%. We test current theoretical models of nuclear cluster formation and find that they cannot fully reproduce the observed trends. A likely mixture of in situ star formation and star cluster mergers seems to be acting during nucleus growth over cosmic time.

Chandra Detection of Intracluster X-Ray sources in Virgo

Abstract We present a survey of X-ray point sources in the nearest and dynamically young galaxy cluster, Virgo, using archival Chandra observations that sample the vicinity of 80 early-type member galaxies. The X-ray source populations at the outskirts of these galaxies are of particular interest. We detect a total of 1046 point sources (excluding galactic nuclei) out to a projected galactocentric radius of ∼40 kpc and down to a limiting 0.5–8 keV luminosity of ∼ 2 × 10 38 erg s − 1 . Based on the cumulative spatial and flux distributions of these sources, we statistically identify ∼120 excess sources that are not associated with the main stellar content of the individual galaxies, nor with the cosmic X-ray background. This excess is significant at a 3.5σ level, when Poisson error and cosmic variance are taken into account. On the other hand, no significant excess sources are found at the outskirts of a control sample of field galaxies, suggesting that at least some fraction of the excess sources around the Virgo galaxies are truly intracluster X-ray sources. Assisted with ground-based and HST optical imaging of Virgo, we discuss the origins of these intracluster X-ray sources, in terms of supernova-kicked low-mass X-ray binaries (LMXBs), globular clusters, LMXBs associated with the diffuse intracluster light, stripped nucleated dwarf galaxies and free-floating massive black holes.

THE NEXT GENERATION VIRGO CLUSTER SURVEY (NGVS). XIII. THE LUMINOSITY AND MASS FUNCTION OF GALAXIES IN THE CORE OF THE VIRGO CLUSTER AND THE CONTRIBUTION FROM DISRUPTED SATELLITES*

ABSTRACT We present measurements of the galaxy luminosity and stellar mass function in a 3.71 deg2 (0.3 Mpc2) area in the core of the Virgo Cluster, based on griz data from the Next Generation Virgo Cluster Survey (NGVS). The galaxy sample—which consists of 352 objects brighter than M g = −9.13 mag, the 50% completeness limit of the survey—reaches 2.2 mag deeper than the widely used Virgo Cluster Catalog and at least 1.2 mag deeper than any sample previously used to measure the luminosity function in Virgo. Using a Bayesian analysis, we find a best-fit faint-end slope of α = −1.33 ± 0.02 for the g-band luminosity function; consistent results are found for the stellar mass function and the luminosity function in the other four NGVS bandpasses. We discuss the implications for the faint-end slope of adding 92 ultracompact dwarfs (UCDs)—previously compiled by the NGVS in this region—to the galaxy sample, assuming that UCDs are the stripped remnants of nucleated dwarf galaxies. Under this assumption, the slope of the luminosity function (down to the UCD faint magnitude limit, M g = −9.6 mag) increases dramatically, up to α = −1.60 ± 0.06 when correcting for the expected number of disrupted non-nucleated galaxies. We also calculate the total number of UCDs and globular clusters that may have been deposited in the core of Virgo owing to the disruption of satellites, both nucleated and non-nucleated. We estimate that ∼150 objects with M g ≲ −9.6 mag and that are currently classified as globular clusters might, in fact, be the nuclei of disrupted galaxies. We further estimate that as many as 40% of the (mostly blue) globular clusters in the Virgo core might once have belonged to such satellites; these same disrupted satellites might have contributed ∼40% of the total luminosity in galaxies observed in the core region today. Finally, we use an updated Local Group galaxy catalog to provide a new measurement of the luminosity function of Local Group satellites, α = −1.21 ± 0.05, which is only 1.7σ shallower than measured in the core of the Virgo Cluster.

Globular clusters as tracers of the halo assembly of nearby central cluster galaxies

AbstractThe properties of globular cluster systems (GCSs) in the core of the nearby galaxy clusters Fornax and Hydra I are presented. In the Fornax cluster we have gathered the largest radial velocity sample of a GCS system so far, which enables us to identify photometric and kinematic sub-populations around the central galaxy NGC 1399. Moreover, ages, metallicities and [α/Fe] abundances of a sub-sample of 60 bright globular clusters (GCs) with high S/N spectroscopy show a multi-modal distribution in the correlation space of these three parameters, confirming heterogeneous stellar populations in the halo of NGC 1399. In the Hydra I cluster very blue GCs were identified. They are not uniformly distributed around the central galaxies. 3-color photometry including the U-band reveals that some of them are of intermediate age. Their location coincides with a group of dwarf galaxies under disruption. This is evidence of a structurally young stellar halo “still in formation”, which is also supported by kinematic measurements of the halo light that point to a kinematically disturbed system. The most massive GCs divide into generally more extended ultra-compact dwarf galaxies (UCDs) and genuine compact GCs. In both clusters, the spatial distribution and kinematics of UCDs are different from those of genuine GCs. Assuming that some UCDs represent nuclei of stripped galaxies, the properties of those UCDs can be used to trace the assembly of nucleated dwarf galaxies into the halos of central cluster galaxies. We show via semi-analytical approaches within a cosmological simulation that only the most massive UCDs in Fornax-like clusters can be explained by stripped nuclei, whereas the majority of lower mass UCDs belong to the star cluster family.

Stellar population properties of the most massive globular clusters and ultra-compact dwarf galaxies of the Fornax cluster

AbstractMost ultra-compact dwarf galaxies (UCDs) and very massive globular clusters reside in nearby galaxy clusters or around nearby giant galaxies. Due to their distance (> 4 Mpc) and compactness (reff < 100 pc) they are barely resolved, and thus it is difficult to obtain their internal properties. Here I present our most recent attempts to constrain the mass function, stellar content and dynamical state of UCDs in the Fornax cluster. Thanks to radial velocity membership assignment of ~ 950 globular clusters (GCs) and UCDs in the core of Fornax, the shape of their mass function is well constrained. It is consistent with the ‘standard’ Gaussian mass function of GCs. Our recent simulations on the disruption process of nucleated dwarf galaxies in cluster environments showed that ~ 40% of the most massive UCDs should originate from nuclear star clusters. Some Fornax UCDs actually show evidence for this scenario, as revealed by extended low surface brightness disks around them and onsets of tidal tails. Multi-band UV to optical imaging as well as low to medium resolution spectroscopy revealed that there exist UCDs with youngish ages, (sub-)solar [α/Fe] abundances, and probably He-enriched populations.

An extended star formation history in an ultra-compact dwarf

There has been significant controversy over the mechanisms responsible for forming compact stellar systems like ultra-compact dwarfs (UCDs), with suggestions that UCDs are simply the high-mass extension of the globular cluster population, or alternatively, the liberated nuclei of galaxies tidally stripped by larger companions. Definitive examples of UCDs formed by either route have been difficult to find, with only a handful of persuasive examples of stripped-nucleus-type UCDs being known. In this paper, we present very deep Gemini/GMOS spectroscopic observations of the suspected stripped-nucleus UCD NGC 4546-UCD1 taken in good seeing conditions (<0.7 arcsec). With these data we examine the spatially resolved kinematics and star formation history of this unusual object. We find no evidence of a rise in the central velocity dispersion of the UCD, suggesting that this UCD lacks a massive central black hole like those found in some other compact stellar systems, a conclusion confirmed by detailed dynamical modelling. Finally, we are able to use our extremely high signal-to-noise spectrum to detect a temporally extended star formation history for this UCD. We find that the UCD was forming stars since the earliest epochs until at least 1–2 Gyr ago. Taken together these observations confirm that NGC 4546-UCD1 is the remnant nucleus of a nucleated dwarf galaxy that was tidally destroyed by NGC 4546 within the last 1–2 Gyr.

An extended star formation history in an ultra-compact dwarf

There has been significant controversy over the mechanisms responsible for forming compact stellar systems like ultra compact dwarfs (UCDs), with suggestions that UCDs are simply the high mass extension of the globular cluster (GC) population, or alternatively, the liberated nuclei of galaxies tidally stripped by larger companions. Definitive examples of UCDs formed by either route have been difficult to find, with only a handful of persuasive examples of stripped-nucleus type UCDs being known. In this paper we present very deep Gemini/GMOS spectroscopic observations of the suspected stripped nucleus UCD NGC 4546-UCD1 taken in good seeing conditions (< 0.7"). With these data we examine the spatially resolved kinematics and star formation history of this unusual object. We find no evidence of a rise in the central velocity dispersion of the UCD, suggesting that this UCD lacks a massive central black hole like those found in some other compact stellar systems, a conclusion confirmed by detailed dynamical modelling. Finally we are able to use our extremely high signal to noise spectrum to detect a temporally extended star formation history for this UCD. We find that the UCD was forming stars since the earliest epochs until at least 1-2 Gyr ago. Taken together these observations confirm that NGC 4546-UCD1 is the remnant nucleus of a nucleated dwarf galaxy that was tidally destroyed by NGC 4546 within the last 1-2 Gyr.

Contribution of stripped nuclear clusters to globular cluster and ultracompact dwarf galaxy populations

We use the Millennium II cosmological simulation combined with the semi-analytic galaxy formation model of Guo et al. (2011) to predict the contribution of galactic nuclei formed by the tidal stripping of nucleated dwarf galaxies to globular cluster (GC) and ultra-compact dwarf galaxy (UCD) populations of galaxies. We follow the merger trees of galaxies in clusters back in time and determine the absolute number and stellar masses of disrupted galaxies. We assume that at all times nuclei have a distribution in nucleus-to-galaxy mass and nucleation fraction of galaxies similar to that observed in the present day universe. Our results show stripped nuclei follow a mass function $N(M) \sim M^{-1.5}$ in the mass range $10^6 < M/M_\odot < 10^8$, significantly flatter than found for globular clusters. The contribution of stripped nuclei will therefore be most important among high-mass GCs and UCDs. For the Milky Way we predict between 1 and 3 star clusters more massive than $10^5 M_\odot$ come from tidally disrupted dwarf galaxies, with the most massive cluster formed having a typical mass of a few times $10^6 M_\odot$, like omega Centauri. For a galaxy cluster with a mass $7 \times 10^{13} M_\odot$, similar to Fornax, we predict $\sim$19 UCDs more massive than $2\times10^6 M_\odot$ and $\sim$9 UCDs more massive than $10^7 M_\odot$ within a projected distance of 300 kpc come from tidally stripped dwarf galaxies. The observed number of UCDs are $\sim$200 and 23, respectively. We conclude that most UCDs in galaxy clusters are probably simply the high mass end of the GC mass function.

The AIMSS Project – I. Bridging the star cluster–galaxy divide★†‡§¶

We describe the structural and kinematic properties of the first compact stellar systems discovered by the Archive of Intermediate Mass Stellar Systems project. These spectroscopically confirmed objects have sizes (∼6 < Re [pc] < 500) and masses (∼2 × 106 < M*/M⊙ < 6 × 109) spanning the range of massive globular clusters, ultracompact dwarfs (UCDs) and compact elliptical galaxies (cEs), completely filling the gap between star clusters and galaxies. Several objects are close analogues to the prototypical cE, M32. These objects, which are more massive than previously discovered UCDs of the same size, further call into question the existence of a tight mass–size trend for compact stellar systems, while simultaneously strengthening the case for a universal ‘zone of avoidance’ for dynamically hot stellar systems in the mass–size plane. Overall, we argue that there are two classes of compact stellar systems (1) massive star clusters and (2) a population closely related to galaxies. Our data provide indications for a further division of the galaxy-type UCD/cE population into two groups, one population that we associate with objects formed by the stripping of nucleated dwarf galaxies, and a second population that formed through the stripping of bulged galaxies or are lower mass analogues of classical ellipticals. We find compact stellar systems around galaxies in low- to high-density environments, demonstrating that the physical processes responsible for forming them do not only operate in the densest clusters.

Ultra-compact dwarf galaxy formation by tidal stripping of nucleated dwarf galaxies

Ultra Compact Dwarf Galaxies (UCDs) and dwarf galaxy nuclei have many common properties, such as internal velocity dispersions and colour-magnitude trends, suggesting tidally stripped dwarf galaxies as a possible UCD origin. However, UCDs typically have sizes more than twice as large as nuclei at the same luminosity. We use a GPU-enabled version of the particle-mesh code \textsc{superbox} to study the possibility of turning nucleated dwarf galaxies into UCDs by tidally stripping them in a Virgo-like galaxy cluster. We find that motion in spherical potentials, where close passages happen many times, leads to the formation of compact ($r_h \lesssim 20$ pc) star clusters/UCDs. In contrast, orbital motion where close passages happen only once or twice leads to the formation of extended objects which are large enough to account for the full range of observed UCD sizes. For such motion, we find that dwarf galaxies need close pericentre passages with distances less than 10 kpc to undergo strong enough stripping so that UCD formation is possible. As tidal stripping produces objects with similar properties to UCDs, and our estimates suggest dwarf galaxies have been destroyed in sufficient numbers to explain the observed number of UCDs in M87, we consider tidal stripping to be a likely origin of UCDs. However, comparison with cosmological simulations is needed to determine if the number and spatial distribution of UCDs formed by tidal stripping matches the observations of UCDs in galaxy clusters.

On the origin of the stellar halo and multiple stellar populations in the globular cluster NGC 1851

We propose that the observed stellar halo around the globular cluster (GC) NGC 1851 is evidence for its formation in the central region of its defunct host dwarf galaxy. We numerically investigate the long-term dynamical evolution of a nucleated dwarf galaxy embedded in a massive dark matter halo under the strong tidal field of the Galaxy. The dwarf galaxy is assumed to have a stellar nucleus (or a nuclear star cluster) that could be the progenitor for NGC 1851. We find that although the dark matter halo and the stellar envelope of the host dwarf of NGC 1851 can be almost completely stripped during its orbital evolution around the Galaxy, a minor fraction of stars in the dwarf can remain trapped by the gravitational field of the nucleus. The stripped nucleus can be observed as NGC 1851 with no/little dark matter whereas stars around the nucleus can be observed as a diffuse stellar halo around NGC 1851. The simulated stellar halo has a symmetric distribution with a power-law density slope of ~ -2 and shows no tidal tails within ~200pc from NGC 1851. We show that two GCs can merge with each other to form a new nuclear GC embedded in field stars owing to the low stellar velocity dispersion of the host dwarf. This result makes no assumption on the ages and/or chemical abundances of the two merging GCs. Thus the observed stellar halo and characteristic multiple stellar populations in NGC 1851 suggest that NGC 1851 could have formed initially in the central region of an ancient dwarf galaxy.

Dynamical versus stellar masses of ultracompact dwarf galaxies in the Fornax cluster★

The origin of ultracompact dwarf (UCD) galaxies, compact extragalactic stellar systems, is still a puzzle for present galaxy formation models. We present the comprehensive analysis of high resolution multi-object spectroscopic data for a sample of 24 Fornax cluster UCDs obtained with VLT FLAMES. It comprises previously published data for 19 objects (Mieske et al. 2008) which we re-analysed, including 13 with available HST photometric data. Using Virtual Observatory technologies we found archival HST images for two more UCDs and then determined their structural properties. For all objects we derived internal velocity dispersions, stellar population parameters, and stellar mass-to-light ratios (M/L)* by fitting individual simple stellar population (SSP) synthetic spectra convolved with a Gaussian against the observed spectra using the NBursts full spectral fitting technique. For 14 objects we estimated dynamical masses suggesting no dark matter (DM) in 12 of them and no more than 40 per cent DM mass fraction in the remaining two, in contrast to findings for several UCDs in the Virgo cluster. Some Fornax UCDs even have too high values of (M/L)* estimated using the Kroupa stellar initial mass function (IMF) resulting in negative formally computed DM mass fractions. The objects with too high (M/L)* ratios compared to the dynamical ones have relatively short dynamical relaxation timescales, close to the Hubble time or below. We therefore suggest that their lower dynamical ratios (M/L)dyn are caused by low-mass star depletion due to dynamical evolution. Overall, the observed UCD characteristics suggest at least two formation channels: tidal threshing of nucleated dwarf galaxies for massive UCDs (~10^8 M_sun), and a classical scenario of red globular cluster formation for lower-mass UCDs (< 10^7 M_sun).

Origin of multiple stellar populations in globular clusters and their helium enrichment

The various scenarios proposed for the origin of the multiple, helium-enriched populations in massive globular clusters are critically compared to the relevant constraining observations. Among accretion of helium-rich material by pre-existing stars, star formation out of ejecta from massive asymptotic giant branch (AGB) stars or from fast rotating massive stars, and pollution by Population III stars, only the AGB option appears to be viable. Accretion or star formation out of outflowing discs would result in a spread of helium abundances, thus failing to produce the distinct, chemically homogeneous subpopulations such as those in the clusters ω Cen and NGC 2808. Pollution by Population III stars would fail to produce subpopulations selectively enriched in helium, but maintaining the same abundance of heavy elements. Still, it is argued that for the AGB option to work, two conditions should be satisfied: (i) AGB stars experiencing the hot bottom burning process (i.e. those more massive than ∼ 3M � ) should rapidly eject their envelope upon arrival on the AGB, thus experiencing just a few third dredge-up episodes; and (ii) clusters with multiple, helium-enriched populations should be the remnants of much more massive systems, such as nucleated dwarf galaxies, as indeed widely assumed.

The formation of ultra-compact dwarf galaxies and nucleated dwarf galaxies

Ultra compact dwarf galaxies (UCDs) have similar properties as massive globular clusters or the nuclei of nucleated galaxies. Recent observations suggesting a high dark matter content and a steep spatial distribution within groups and clusters provide new clues as to their origins. We perform high-resolution N-body/smoothed particle hydrodynamics simulations designed to elucidate two possible formation mechanisms for these systems: the merging of globular clusters in the centre of a dark matter halo, or the massively stripped remnant of a nucleated galaxy. Both models produce density profiles as well as the half light radii that can fit the observational constraints. However, we show that the first scenario results to UCDs that are underluminous and contain no dark matter. This is because the sinking process ejects most of the dark matter particles from the halo centre. Stripped nuclei give a more promising explanation, especially if the nuclei form via the sinking of gas, funneled down inner galactic bars, since this process enhances the central dark matter content. Even when the entire disk is tidally stripped away, the nucleus stays intact and can remain dark matter dominated even after severe stripping. Total galaxy disruption beyond the nuclei only occurs on certain orbits and depends on the amount of dissipation during nuclei formation. By comparing the total disruption of CDM subhaloes in a cluster potential we demonstrate that this model also leads to the observed spatial distribution of UCDs which can be tested in more detail with larger data sets.

Origin of lower velocity dispersions of ultra-compact dwarf galaxy populations in clusters of galaxies

Recent observations have revealed that velocity dispersions of 'ultra-compact dwarf' (UCD) galaxies are significantly smaller than those of other galaxy populations in the Fornax and the Virgo clusters of galaxies. In order to understand the origin of the observed lower velocity dispersions of UCDs, we numerically investigate line-of-sight velocity dispersion (σ los ) of galaxy populations with variously different orbits in clusters of galaxies with the total masses of M cl . We particularly investigate radial velocity dispersion profiles [σ los (R)] and velocity dispersions within the central 200 kpc of a cluster model (σ m ) for galaxies with different pericentre distances (r p ) and orbital eccentricities (e) in the model with M cl = 7.0 x 10 13 M ⊙ reasonable for the Fornax cluster. We find that σ los (R) and σ m of galaxies with smaller r p are steeper and smaller, respectively, for a given initial e distribution of galaxies. For example, we find that σ m is ∼260 km s- 1 for galaxies with r p < 50 kpc and ∼336 km s _1 for all galaxies in the model with the mean e of 0.6. These results imply that the observed lower velocity dispersion of UCD population is consistent with the UCDs having significantly smaller r p than other galaxy populations in the Fornax. We discuss these results in the context of the 'galaxy threshing' scenario in which UCDs originate from nuclei of nucleated dwarf galaxies. We suggest that the observed differences in kinematical properties between UCDs and other dwarf galaxy populations in clusters of galaxies can be understood in terms of the differences in orbital properties between UCDs and the dwarf populations.