Ultra-compact Dwarf Research Articles

Early-type galaxy speciation: elliptical (E) and ellicular (ES) galaxies in the Mbh–M*,sph diagram, and a merger-driven explanation for the origin of ES galaxies, antitruncated stellar discs in lenticular (S0) galaxies, and the Sérsicification of E galaxy light profiles

ABSTRACT In a recent series of papers, supermassive black holes were used to discern pathways in galaxy evolution. By considering the black holes’ coupling with their host galaxy’s bulge/spheroid, the progression of mass within each component has shed light on the chronological sequence of galaxy speciation. Offsets between the galaxy-morphology-dependent $M_{\rm bh}$–$M_{\rm \star ,sph}$ scaling relations trace a pattern of ‘punctuated equilibrium’ arising from merger-driven transitions between galaxy types, such as from spirals to dust-rich lenticulars and further to ‘ellicular’ and elliptical galaxies. This study delves deeper into the distinction between the ellicular galaxies – characterised by their intermediate-scale discs – and elliptical galaxies. Along the way, it is shown how some antitruncated large-scale discs in lenticular galaxies can arise from the coexistence of a steep intermediate-scale disc and a relatively shallow large-scale disc. This observation undermines application of the popular exponential-disc plus Sérsic-bulge model for lenticular galaxies and suggests some past bulge mass measurements have been overestimated. Furthermore, it is discussed how merger-driven disc-heating and blending likely leads to the spheroidalisation of discs and the conglomeration of multiple discs leads to the (high-n) Sérsicification of light profiles. The ellicular and elliptical galaxy distribution in the $M_{\rm bh}$–$M_{\rm \star ,sph}$ diagram is explored relative to major-merger-built lenticular galaxies and brightest cluster galaxies. The super-quadratic $M_{\rm bh}$–$M_{\rm \star }$ relations, presented herein, for merger-built systems should aid studies of massive black hole collisions and the gravitational wave background. Finally, connections to dwarf compact elliptical and ultracompact dwarf galaxies, with their 100–1000 times higher $M_{\rm bh}/M_{\rm \star ,sph}$ ratios, are presented.

The Lower Limit of Dynamical Black Hole Masses Detectable in Virgo Compact Stellar Systems Using the JWST/NIRSpec IFU

Due to observational challenges, the mass function of black holes (BH) at lower masses is poorly constrained in the local universe. Understanding the occupation fraction of BHs in low-mass galaxies is crucial for constraining the origins of supermassive BH seeds. Compact stellar systems (CSSs), including ultracompact dwarf galaxies (UCDs) and compact elliptical galaxies (cEs), are potential intermediate-mass BH hosts. Despite the difficulties posed by their limited spheres of influence, stellar dynamical modeling has been effective in estimating central BH masses in CSSs. Some CSSs may harbor a BH constituting up to 20% of their host stellar mass, while others might not have a central BH. In support of our ongoing efforts to determine the BH masses in select CSSs in the Virgo cluster using JWST/NIRSpec IFU observations and orbit-superposition dynamical models, we create mock kinematic data mimicking the characteristics of observed cEs/UCDs in the Virgo cluster with different BH masses. We then construct a series of dynamical models using the orbit-superposition code FORSTAND and explore the accuracy of recovering the BH mass. We find that the mass of BHs comprising 1% or more of the total host stellar mass can be accurately determined through kinematic maps featuring higher-order velocity moments. We also assess how BH mass measurement is affected by deprojection methods, regularization factors, anisotropy parameters, orbit initial conditions, the absence of higher-order velocity moments, the spatial resolution, and the signal-to-noise ratio.

Efficiency of black hole formation via collisions in stellar systems

Context. This paper explores the theoretical relation between star clusters and black holes within them, focusing on the potential role of nuclear star clusters (NSCS), globular clusters (GCS), and ultra-compact dwarf galaxies (UCDS) as environments that allow for black hole formation via stellar collisions. Aims. This study aims to identify the optimal conditions for stellar collisions across a range of stellar systems, leading to the formation of very massive stars that subsequently collapse into black holes. We analyze data from numerical simulations and observations of diverse stellar systems, encompassing various initial conditions, initial mass functions, and evolution scenarios. Methods. We computed a critical mass, determined by the interplay of the collision time, system age, and initial properties of the star cluster. The efficiency of black hole formation (ϵBH) is defined as the ratio of initial stellar mass divided by the critical mass. Results. We find that stellar systems with a ratio of initial stellar mass over critical mass above 1 exhibit a high efficiency in terms of black hole formation, ranging from 30 − 100%. While there is some scatter, potentially attributed to complex system histories and the presence of gas, the results highlight the potential for achieving high efficiencies via a purely collisional channel in black hole formation. Conclusions. In conclusion, this theoretical exploration elucidates the connection between star clusters and black hole formation. The study underscores the significance of UCDS, GCS, and NSCS as environments conducive to the black hole formation scenario via stellar collisions. The defined black hole formation efficiency (ϵBH) is shown to be influenced by the ratio of the initial stellar mass to the critical mass.

Formation Pathways of the Compact Stellar Systems

The formation pathways of compact stellar systems (CSSs) are still under debate. We utilize the NewHorizon simulation to investigate the origins of such objects in the field environment. We identified 55 CSS candidates in the simulation whose properties are similar to those of the observed ultracompact dwarfs and compact ellipticals (cEs). All but two most massive objects (cE candidates) are a result of a short starburst. Sixteen are formed by tidal stripping, while the other 39 are intrinsically compact from their birth. The stripped objects originate from dwarf-like galaxies with a dark halo, but most of their dark matter is stripped through their orbital motion around a more massive neighbor galaxy. The 39 intrinsically compact systems are further divided into associated or isolated groups, depending on whether they were born near a massive dark halo or not. The isolated intrinsically compact objects (seven) are born in a dark halo and their stellar properties are older and metal-poor compared to the associated counterparts (32). The stripped compact objects occupy a distinct region in the age–metallicity plane from the intrinsically compact objects. The associated intrinsically compact objects in our sample have never had a dark halo; they are the surviving star clumps of a massive galaxy.

A search for intermediate-mass black holes in compact stellar systems through optical emissions from tidal disruption events

ABSTRACT Intermediate-mass black holes (IMBHs) are expected to exist in globular clusters (GCs) and compact stellar systems (CSSs) in general, but none have been conclusively detected. Tidal disruption events (TDEs), where a star is tidally disrupted by the gravitational field of a black hole, have been observed to occur around the supermassive black holes (SMBHs) found at the centres of galaxies, and should also arise around IMBHs, especially in the dense stellar cores of CSSs. However, to date none have been observed in such environments. Using data from the Zwicky Transient Facility, we search for TDEs associated with CSSs, but none are found. This non-detection allows us to set an upper limit on the TDE rate in CSSs of $n_\text{TDE,Total}\lessapprox 10^{-7} \, \mathrm{ CSS}^{-1}\, \text{yr}^{-1}$, which is 2 dex below the observed TDE rate involving SMBHs interacting with 1 M⊙ main-sequence stars in the nuclei of massive galaxies. We also consider ultracompact dwarfs (UCDs) formed through a tidal stripping process in the surveyed volume. On the assumption that these CSSs contain SMBHs and TDE rates are comparable to current observed optical rates in galactic nuclei ($\approx 3.2 \times 10^{-5}\, \text{gal}^{-1}\, \text{yr}^{-1}$), we determine an upper limit for the number of UCDs formed through a tidal stripping process in the surveyed volume to be NGC,Strip < 1.4 × 104, which we estimate represents $\lt 6~{{\ \rm per\ cent}}$ of the population of GCs >106 M⊙.

Four new eclipsing accreting ultracompact white dwarf binaries found with the Zwicky Transient Facility

Accreting ultracompact white dwarf binaries contain a white dwarf that is accreting from a degenerate object. They have orbital periods shorter than 65 minutes. We report the discovery and the orbital period of four new eclipsing accreting ultracompact white dwarf binaries found using the Zwicky Transient Facility (ZTF) and discuss their photometric properties. We searched through a list of 4171 dwarf novae compiled using the ZTF and used the box least square method to search for periodic signals in the data. We found four eclipsing accreting ultracompact binaries with orbital periods between 25.9 and 56 minutes.\ One had previously been published as an AM Canum Venaticorum (AM CVn), and the other three systems are new discoveries. The two shorter-period systems are likely also AM CVn systems, while the longest-period system, with a period of 56 minutes, showed multiple super-outbursts over two years, which is more consistent with it being a helium CV.

Formation of Transitional cE/UCD Galaxies through Massive/Dwarf Disc Galaxy Mergers

The dynamics of the merger of a dwarf disc galaxy with a massive spiral galaxy of the Milky Way type were studied in detail. The remnant of such interaction after numerous crossings of the satellite through the disc of the main galaxy was a compact stellar core, the characteristics of which were close to small compact elliptical galaxies (cEs) or large ultra-compact dwarfs (UCDs). Such transitional cE/UCD objects with an effective radius of 100–200 pc arise as a result of stripping the outer layers of the stellar core during the destruction of a dwarf disc galaxy. Numerical models of the satellite before interaction included baryonic matter (stars and gas) and dark mass. We used N-body to describe the dynamics of stars and dark matter, and we used smoothed-particle hydrodynamics to model the gas components of both galaxies. The direct method of calculating the gravitational force between all particles provided a qualitative resolution of spatial structures up to 10 pc. The dwarf galaxy fell onto the gas and stellar discs of the main galaxy almost along a radial trajectory with a large eccentricity. This ensured that the dwarf crossed the disc of the main galaxy at each pericentric approach over a time interval of more than 9 billion years. We varied the gas mass and the initial orbital characteristics of the satellite over a wide range, studying the features of mass loss in the core. The presence of the initial gas component in a dwarf galaxy significantly affects the nature of the formation and evolution of the compact stellar core. The gas-rich satellite gives birth to a more compact elliptical galaxy compared to the merging gas-free dwarf galaxy. The initial gas content in the satellite also affects the internal rotation in the stripped nucleus. The simulated cE/UCD galaxies contained very little gas and dark matter at the end of their evolution.

The isolated elliptical galaxy NGC 5812–MOND or dark matter?

AbstractThere exist isolated elliptical galaxies, whose dynamics can be modeled without resorting to dark matter or MOND, for example, NGC 7507. Such objects lack understanding within the current framework of galaxy formation. The isolated elliptical NGC 5812 is another object to investigate a possible role of isolation. We use globular clusters (GCs) and the galaxy light itself as dynamical tracers to constrain its mass profile. We employ Gemini/GMOS mask spectroscopy, apply the GMOS reduction procedures provided within IRAF, measure GC velocities by cross correlation methods and extract the line‐of‐sight kinematics of galaxy spectra using the tool pPXF. We identify 28 GCs with an outermost galactocentric distance of 20 kpc, for which velocities could be obtained. Furthermore, 16 spectra of the integrated galaxy light out to 6 kpc have been used to model the central kinematics. These spectra provide evidence for a disturbed velocity field, which is plausible given the disturbed morphology of the galaxy. We construct spherical Jeans models for the galaxy light and apply tracer mass estimators for the globular clusters. With the assumptions inherent to the mass estimators, MOND is compatible with the mass out to 20 kpc. However, a dark matter free galaxy is not excluded, given the uncertainties related to a possible nonsphericity and a possible nonequilibrium state. We find one globular cluster with an estimated mass of , the first Ultra Compact Dwarf in an isolated elliptical. We put NGC 5812 into the general context of dark matter or alternative ideas in elliptical galaxies. The case for a MONDian phenomenology also among early‐type galaxies has become so strong that deviating cases appear astrophysically more interesting than agreements. The baryonic Tully Fisher relation (BTFR) as predicted by MOND is observed in some samples of early‐type galaxies, in others not. However, in cases of galaxies that deviate from the MONDian prediction, data quality and data completeness are often problematic.

The contribution of supermassive black holes in stripped nuclei to the supermassive black hole population of UCDs and galaxy clusters

ABSTRACT We use the hydrodynamic EAGLE simulation to predict the numbers and masses of supermassive black holes in remnant nuclei of disrupted galaxies (stripped nuclei) and compare these to confirmed measurements of black holes in observed ultra-compact dwarf galaxies (UCDs). We find that black holes in stripped nuclei are consistent with the numbers and masses of those in observed UCDs. Approximately 50 per cent of stripped nuclei with M > 2 × 106 M⊙ should contain supermassive black holes. We further calculate how the presence of a black hole increases the dynamical mass of a stripped nucleus via the mass elevation ratio, Ψ defined as the ratio of the kinematically derived mass to the expected mass from stellar population synthesis. We find Ψsim$= 1.51^{+0.06}_{-0.04}$ for M > 107 M⊙ stripped nuclei, consistent with that of observed UCDs, which have Ψobs = 1.7 ± 0.2 above M > 107 M⊙. We also find that the mass elevation ratios of stripped nuclei with supermassive black holes can explain the observed number of UCDs with elevated mass-to-light ratios. Finally, we predict the relative number of massive black holes in stripped nuclei and galaxy nuclei and find that stripped nuclei should increase the number of black holes in galaxy clusters by 30 − 100 per cent, depending on the black hole occupation fraction of low-mass galaxies. We conclude that the population of supermassive black holes in UCDs represents a large and unaccounted-for portion of supermassive black holes in galaxy clusters.

Initial mass function variability from the integrated light of diverse stellar systems

ABSTRACT We present a uniform analysis of the stellar initial mass function (IMF) from integrated light spectroscopy of 15 compact stellar systems (11 globular clusters in M31 and 4 ultra compact dwarfs in the Virgo cluster, UCDs) and two brightest Coma cluster galaxies (BCGs), covering a wide range of metallicities (−1.7 < [Fe/H] < 0.01) and velocity dispersions (7.4 km s−1 <σ < 275 km s−1). The S/N ∼100 Å−1 Keck LRIS spectra are fitted over the range 4000 < λ/Å < 10 000 with flexible full-spectrum stellar population synthesis models. We use the models to fit simultaneously for ages, metallicities, and individual elemental abundances of the population, allowing us to decouple abundance variations from variations in IMF slope. We show that compact stellar systems do not follow the same trends with physical parameters that have been found for early-type galaxies. Most globular clusters in our sample have an IMF consistent with that of the Milky Way, over a wide range of [Fe/H] and [Mg/Fe]. There is more diversity among the UCDs, with some showing evidence for a bottom-heavy IMF, but with no clear correlation with metallicity, abundance, or velocity dispersion. The two Coma BCGs have similar velocity dispersion and metallicity, but we find the IMF of NGC 4874 is consistent with that of the Milky Way while NGC 4889 presents evidence for a significantly bottom-heavy IMF. For this sample, the IMF appears to vary between objects in a way that is not explained by a single metallicity-dependent prescription.

JWST photometry of globular cluster populations in Abell 2744 at z = 0.3

ABSTRACT JWST imaging of the rich galaxy cluster Abell 2744 at z = 0.308 has been used by the UNCOVER team to construct mosaic images in the NIRCam filters. The exceptionally deep images in the (F115W, F150W, and F200W) bands reveal a large population of unresolved point-like sources across the field, the vast majority of which are globular clusters (GCs) or ultra-compact dwarfs. To the limits of our photometry, more than 10 000 such objects were measured, most of which are in the haloes of the five largest A2744 galaxies but which also include GCs around some satellite galaxies and throughout the intergalactic medium. The measured luminosity function covers almost 4 mag and follows a classic lognormal shape, though the GCLF turnover point still lies fainter than the photometric completeness limit. The colour index (F115W–F200W) in particular covers an intrinsic spread of 0.5 mag, clearly resolving the expected range of GC metallicities. The overall results are consistent with a large, normal GC population seen at a 3.5 Gyr earlier stage of dynamical evolution.

The creation of a massive UCD by tidal threshing from NGC 936

ABSTRACT We study a compact nucleus embedded in an early-type dwarf galaxy, MATLAS-167, which is in the process of disruption by the tidal force of the neighbouring giant S0 galaxy, NGC 936, in a group environment. Using the imaging data of the MATLAS survey, we analyse the stellar tidal tail of MATLAS-167 and its central compact nucleus, designated as NGC 936_UCD. We find that NGC 936_UCD has a luminosity of Mg = −11.43 ± 0.01 mag and a size of 66.5 ± 17 pc, sharing the global properties of ultra-compact dwarf galaxies (UCDs) but significantly larger and brighter compared to the typical UCD populations observed in the Virgo cluster. By integrating the total luminosity of both the tidal stream and MATLAS-167, we estimate that the disrupted dwarf progenitor possesses a luminosity of Mg = −15.92 ± 0.06 mag, a typical bright early-type dwarf galaxy luminosity. With the help of the optical spectrum observed by the SDSS survey, we derive the simple stellar population properties of NGC 936_UCD: a light-weighted age of 5.6 ± 0.7 Gyr and metallicity of [Z/H] = −0.83 ± 0.3 dex. Our findings suggest that tidal threshing is a possible formation mechanism of bright UCD populations in close proximity to giant galaxies.

A model for GN-z11: top-heavy stellar initial mass functions in forming galactic nuclei and ultra-compact dwarfs

ABSTRACT Recent JWST observations of the z = 10.6 galaxy GN-z11 have revealed a very high gas-phase nitrogen abundance (higher than four times the solar value), a very small half-light radius (≈60 pc), and a large stellar mass (Ms ≈ 109 M⊙) for its size. We consider that this object is a forming galactic nucleus or ultra-compact dwarf galaxy rather than a proto globular cluster, and thereby investigate the chemical abundance pattern using one-zone chemical evolution models. The principal results of the models are as follows. The observed log (N/O) > −0.24, log (C/O) > −0.78, and 12 + log (O/H) ≈ 7.8 can be self-consistently reproduced by the models both with very short star formation time-scales (<107 yr) and with top-heavy stellar initial mass functions. The adopted assumption of no chemical enrichment by massive (m > 25 M⊙) core collapse supernovae (CCSNe) is also important for the reproduction of high gas-phase log (N/O), because such CCSNe can decrease high log (N/O) of gas polluted by OB and Wolf–Rayet stars. GN-z11 can have a significant fraction (>0.5) of nitrogen-rich ([N/Fe] > 0.5) stars, which implies a possible link between nitrogen-rich stellar populations of the inner Galaxy and giant elliptical galaxies and high-z objects with high gas-phase log (N/O) like GN-z11.

The globular cluster system of the nearest Seyfert II galaxy Circinus

Context. The globular cluster (GC) system of Circinus galaxy has not been probed previously partly because of the location of the galaxy at −3.8° Galactic latitude, which suffers severely from interstellar extinction, stellar crowding, and Galactic foreground contamination. However, the deep near-infrared (NIR) photometry by the VISTA Variables in the Via Láctea Extended Survey (VVVX) in combination with the precise astrometry of Gaia EDR3 allow us to map GCs in this region. Aims. Our long-term goal is to study and characterise the distributions of GCs and ultra-compact dwarfs in Circinus galaxy, which is the nearest Seyfert II galaxy. Here we conduct the first pilot search for GCs in this galaxy. Methods. We used NIR VVVX photometry in combination with Gaia EDR3 astrometric features, such as astrometric excess noise and the ratio of the sum of the blue photometer (BP) and red photometer (RP) to the broad G passband (BRexcess), to build the first homogeneous catalogue of GCs in Circinus galaxy. A robust combination of selection criteria allows us to effectively clean interlopers from our sample. Results We report the detection of ∼70 GC candidates in this galaxy at a 3σ confidence level. They show a bimodal colour distribution with the blue peak at (G − Ks)0 = 0.985±0.127 mag with a dispersion of 0.211±0.091 mag and the red peak at (G − Ks)0 = 1.625±0.177 mag with a dispersion of 0.482±0.114 mag. A GC specific frequency (S​N) of 1.3±0.2 was derived for the galaxy, and we estimated a total population of 120±40 GCs. Based on the projected radial distribution it appears that Circinus has a different distribution of GC candidates than MW and M 31. Conclusions. We demonstrate that Circinus galaxy hosts a sizeable number of cluster candidates. This result is the first leap towards understanding the evolution of old stellar clusters in this galaxy.

A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE)

Using a compilation of Hα fluxes for 384 star-forming galaxies detected during the Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE), we study several important scaling relations linking the star formation rate, specific star formation rate, stellar mass, stellar mass surface density, and atomic gas depletion timescale for a complete sample of galaxies in a rich environment. The extraordinary sensitivity of the narrow-band imaging data allows us to sample the whole dynamic range of the Hα luminosity function, from massive galaxies (Mstar ≃ 1011 M⊙) to dwarf systems (Mstar ≃ 106 M⊙), where the ionised gas emission is due to the emission of single O-early B stars. This extends previous works to a dynamic range in stellar mass and star formation rate (10−4 ≲ SFR ≲ 10 M⊙ yr−1) that has never been explored so far. The main-sequence relation derived for all star-forming galaxies within one virial radius of the Virgo cluster has a slope comparable to that observed in other nearby samples of isolated objects, but its dispersion is about three times larger (∼1 dex). The dispersion is tightly connected to the available amount of HI gas, with gas-poor systems located far below objects of similar stellar mass, but with a normal HI content. When measured on unperturbed galaxies with a normal HI gas content (HI-def ≤ 0.4), the relation has a slope a = 0.92 ± 0.06, an intercept b = −1.57 ± 0.06 (at a pivot point of log Mstar = 8.451 M⊙), and a scatter σ ≃ 0.40, and it has a constant slope in the stellar mass range 106 ≲ Mstar ≲ 3 × 1011 M⊙. The specific star formation rate of HI-poor galaxies is significantly lower than that of HI-rich systems of similar stellar mass, while their atomic gas consumption timescale τHI is fairly similar, in particular, for objects of stellar mass 107 ≲ Mstar ≲ 109 M⊙. We compare these observational results to the prediction of models expressly tuned to reproduce the effects induced by the interaction of galaxies with their surrounding environment. The observed scatter in the main-sequence relation can be reproduced only after a violent and active stripping process such as ram-pressure stripping that removes gas from the disc (outer parts first) and quenches star formation on short (< 1 Gyr) timescales. This rules out milder processes such as starvation. This interpretation is also consistent with the position of galaxies of different star formation activity and gas content within the phase-space diagram. We also show that the star-forming regions that formed in the stripped material outside perturbed galaxies are located well above the main-sequence relation drawn by unperturbed systems. These extraplanar HII regions, which might be at the origin of ultra-compact dwarf galaxies (UCDs) and other compact sources typical in rich environments, are living a starburst phase lasting only ≲50 Myr. They later become quiescent systems.

Outer disc edge: properties of low-frequency aperiodic variability in ultracompact interacting binaries

ABSTRACT Flickering, and more specifically aperiodic broad-band variability, is an important phenomenon used in understanding the geometry and dynamics of accretion flows. Although the inner regions of accretion flows are known to generate variability on relatively fast time-scales, the broad-band variability generated in the outer regions has mostly remained elusive due to its long intrinsic variability time-scales. Ultracompact AM CVn systems are relatively small when compared to other accreting binaries and are well suited to search and characterize low-frequency variability. Here, we present the first low-frequency power spectral analysis of the ultracompact accreting white dwarf system SDSS J1908+3940. The analysis reveals a low-frequency break at ∼6.8 × 10−7 Hz in the time-averaged power spectrum as well as a second higher frequency component with characteristic frequency of ∼1.3 × 10−4 Hz. We associate both components with the viscous time-scales within the disc through empirical fits to the power spectrum as well as analytical fits using the fluctuating accretion disc model. Our results show that the low-frequency break can be associated with the outer disc regions of a geometrically thin accretion flow. The detection of the low-frequency break in SDSS J1908+3940 provides a precedent for further detection of similar features in other ultracompact accreting systems. More importantly, it provides a new observable that can help constrain simulations of accretion flows.

Detection of Intracluster Globular Clusters in the First JWST Images of the Gravitational Lens Cluster SMACS J0723.3–7327 at z = 0.39

We present a survey of globular clusters (GCs) in the massive gravitational lens cluster SMACS J0723.3–7327 at z = 0.39 based on the early released JWST/NIRCam images. In the color–magnitude diagrams of the point sources, we clearly find a rich population of intracluster GCs that are spread over a wide area of the cluster. Their ages, considering the cluster redshift, are younger than 9.5 Gyr. The F200W (AB) magnitudes of these GCs, 26.5 mag < F200W0 < 29.5 mag, correspond to −15.2 mag < M F200W < −12.2 mag, showing that they belong to the brightest GCs (including ultracompact dwarfs). The spatial distributions of these GCs show a megaparsec-scale structure elongated along the major axis of the brightest cluster galaxy. In addition, they show a large number of substructures, some of which are consistent with the substructures seen in the map of diffuse intracluster light. The GC number density map is, in general, consistent with the dark matter mass density map based on the strong lensing analysis in the literature. The radial number density profile of the GCs in the outer region is steeper than the dark matter mass profile obtained from lensing models. These results are consistent with those for the GCs found in the deep HST images of A2744, another massive cluster at z = 0.308, and in simulated galaxy clusters. This shows that the intracluster GCs are an excellent independent tool to probe the dark matter distribution in galaxy clusters, as well as reveal the cluster assembly history in the JWST era.

Photometric properties of nuclear star clusters and their host galaxies in the Fornax cluster

Aims. We aim to investigate the relations between nuclear star clusters (NSCs) and their host galaxies and to offer a comparison between the structural properties of nucleated and non-nucleated galaxies. We also address the environmental influences on the nucleation of galaxies in the Fornax main cluster and the Fornax A group. Methods. We selected 557 galaxies (105.5 M⊙ &lt; M*, galaxy &lt; 1011.5 M⊙) for which structural decomposition models and non-parametric morphological measurements are available from our previous work. We determined the nucleation of galaxies based on a combination of visual inspection of galaxy images and residuals from multi-component decomposition models, as well as using a model selection statistic, the Bayesian information criterion (BIC), to avoid missing any faint nuclei. We also tested the BIC as an unsupervised method to determine the nucleation of galaxies. We characterised the NSCs using the nucleus components from the multi-component models conducted in the g′, r′, and i′ bands. Results. Overall, we find a dichotomy in the properties of nuclei that reside in galaxies more or less massive than M*, galaxy ≈ 108.5 M⊙. In particular, we find that the nuclei tend to be bluer than their host galaxies and follow a scaling relation of $ M_{\mathrm{*,nuc}} \propto {M_{\mathrm{*,galaxy}}}^{0.5} $ for M*, galaxy &lt; 108.5 M⊙. In galaxies with M*, galaxy &gt; 108.5 M⊙, we find redder nuclei compared to the host galaxy, which follows M*, nuc ∝ M*, galaxy. Comparing the properties of nucleated and non-nucleated early-type galaxies, we find that nucleated galaxies tend to be redder in global (g′−r′) colour, have redder outskirts relatively to their own inner regions (Δ(g′−r′)), are less asymmetric (A), and exhibit less scatter in the brightest second-order moment of light (M20) than their non-nucleated counterparts at a given stellar mass. However, with the exception of Δ(g′−r′) and the Gini coefficient (G), we do not find any significant correlations with cluster-centric distance. Yet, we find the nucleation fractions to be typically higher in the Fornax main cluster than in the Fornax A group, and that the nucleation fraction is highest towards the centre of their respective environments. Additionally, we find that the observed ultra-compact dwarf (UCD) fraction (i.e. the number of UCDs over the number of UCDs and nucleated galaxies) in Fornax and Virgo peaks at the cluster centre and is consistent with the predictions from simulations. Lastly, we find that the BIC can recover our labels of nucleation up to an accuracy of 97% without interventions. Conclusions. The different trends in NSC properties suggest that different processes are at play at different host stellar masses. A plausible explanation is that the combination of globular cluster in-spiral and in situ star formation play a key role in the build-up of NSCs. In addition, the environment is clearly another important factor in the nucleation of galaxies, particularly at the centre of the cluster where the nucleation and UCD fractions peak. Nevertheless, the lack of significant correlations with the structures of the host galaxies is intriguing. Finally, our exploration of the BIC as a potential method of determining nucleation have applications for large-scale future surveys, such as Euclid.

A Population of Luminous Globular Clusters and Stripped Nuclei with Elevated Mass to Light Ratios around NGC 5128* * This paper includes data gathered with the 6.5 m Magellan Telescope at Las Campanas Observatory, Chile.

The dense central regions of tidally disrupted galaxies can survive as ultracompact dwarfs (UCDs) that hide among the luminous globular clusters (GCs) in the halo of massive galaxies. An exciting confirmation of this model is the detection of overmassive black holes in the centers of some UCDs, which also lead to elevated dynamical mass-to-light ratios (M/L dyn). Here we present new high-resolution spectroscopic observations of 321 luminous GC candidates in the massive galaxy NGC 5128/Centaurus A. Using these data we confirm 27 new luminous GCs, and measure velocity dispersions for 57 luminous GCs (with g-band luminosities between 2.5 × 105 and 2.5 × 107 L ⊙), of which 48 are new measurements. Combining these data with size measurements from Gaia, we determine the M/L dyn for all 57 luminous GCs. We see a clear bimodality in the M/L dyn distribution, with a population of normal GCs with mean M/L dyn = 1.51 ± 0.31, and a second population of ∼20 GCs with elevated mean M/L dyn = 2.68 ± 0.22. We show that black holes with masses ∼4%–18% of the luminous GCs can explain the elevated mass-to-light ratios. Hence, it is plausible that the NGC 5128 sources with elevated M/L dyn are mostly stripped galaxy nuclei that contain massive central black holes, though future high spatial resolution observations are necessary to confirm this hypothesis for individual sources. We also present a detailed discussion of an extreme outlier, VHH81-01, one of the largest and most massive GC in NGC 5128, making it an exceptionally strong candidate to be a tidally stripped nucleus.

Warm Dark Matter Galaxies with Central Supermassive Black Holes

We generalize the Thomas–Fermi approach to galaxy structure to include central supermassive black holes and find, self-consistently and non-linearly, the gravitational potential of the galaxy plus the central black hole (BH) system. This approach naturally incorporates the quantum pressure of the fermionic warm dark matter (WDM) particles and shows its full power and clearness in the presence of supermassive black holes. We find the main galaxy and central black hole magnitudes as the halo radius rh, halo mass Mh, black hole mass MBH, velocity dispersion σ, and phase space density, with their realistic astrophysical values, masses and sizes over a wide galaxy range. The supermassive black hole masses arise naturally in this framework. Our extensive numerical calculations and detailed analytic resolution of the Thomas–Fermi equations show that in the presence of the central BH, both DM regimes—classical (Boltzmann dilute) and quantum (compact)—do necessarily co-exist generically in any galaxy, from the smaller and compact galaxies to the largest ones. The ratio R(r) of the particle wavelength to the average interparticle distance shows consistently that the transition, R≃1, from the quantum to the classical region occurs precisely at the same point rA where the chemical potential vanishes. A novel halo structure with three regions shows up: in the vicinity of the BH, WDM is always quantum in a small compact core of radius rA and nearly constant density; in the region rA&lt;r&lt;ri until the BH influence radius ri, WDM is less compact and exhibits a clear classical Boltzmann-like behavior; for r&gt;ri, the WDM gravity potential dominates, and the known halo galaxy shows up with its astrophysical size. DM is a dilute classical gas in this region. As an illustration, three representative families of galaxy plus central BH solutions are found and analyzed: small, medium and large galaxies with realistic supermassive BH masses of 105M⊙, 107M⊙ and 109M⊙, respectively. In the presence of the central BH, we find a minimum galaxy size and mass Mhmin≃107M⊙, larger (2.2233×103 times) than the one without BH, and reached at a minimal non-zero temperature Tmin. The supermassive BH heats up the DM and prevents it from becoming an exactly degenerate gas at zero temperature. Colder galaxies are smaller, and warmer galaxies are larger. Galaxies with a central black hole have large masses Mh&gt;107M⊙&gt;Mhmin; compact or ultracompact dwarf galaxies in the range 104M⊙&lt;Mh&lt;107M⊙ cannot harbor central BHs. We find novel scaling relations MBH=DMh38 and rh=CMBH43, and show that the DM galaxy scaling relations Mh=bΣ0rh2 and Mh=aσh4/Σ0 hold too in the presence of the central BH, Σ0 being the constant surface density scale over a wide galaxy range. The galaxy equation of state is derived: pressure P(r) takes huge values in the BH vicinity region and then sharply decreases entering the classical region, following consistently a self-gravitating perfect gas P(r)=σ2ρ(r) behavior.