Intermediate-mass Black Holes Research Articles

Three-dimensional radiation hydrodynamics simulations of wandering intermediate-mass black holes considering the anisotropic radiation and dust sublimation

ABSTRACT By performing three-dimensional radiation hydrodynamics simulations, we study Bondi–Hoyle–Lyttleton accretion on to intermediate-mass black holes (BHs) wandering in the dusty gas. Here, we take into account the anisotropic radiation feedback and the sublimation of dust grains. Our simulations show that when the relative velocity between the BH and the gas is small ($\sim 20\rm\, km~s^{-1}$) and gas density is $\sim 10^4 \rm cm^{-3}$, the gas mainly accretes from near the equatorial plane of the accretion disc at a time-averaged rate of 0.6 per cent of the Bondi–Hoyle–Lyttleton rate. An ionized region like two spheres glued together at the equatorial plane is formed, and the dense shock shell appears near the ionization front. The BH is accelerated at $\sim 10^{-8}\, \rm cm~s^{-2}$ due to the gravity of the shell. For denser gas ($\sim 10^6 \rm cm^{-3}$), the time-averaged accretion rate is also 0.6 per cent of the Bondi–Hoyle–Lyttleton rate. However, the BH is decelerated at $\sim 10^{-7}\, \rm cm~s^{-2}$ due to gravity of the dense downstream gas although the dense shock shell appears upstream. Our simulations imply that intermediate-mass BHs in the early universe keep floating at $\gtrsim {\rm several}\, 10\, \rm km~s^{-1}$ without increasing mass in interstellar gas with density of $\sim 10^4\, \rm cm^{-3}$, and slow down and grow into supermassive BHs in galaxies with the density of $\sim 10^6\, \rm cm^{-3}$.

Gravitational wave probes on self-interacting dark matter surrounding an intermediate mass black hole

The presence of dark matter overdensities surrounding a black hole can influence the evolution of a binary system. The gravitational wave signals emitted by a black hole binary offer a promising means to probe the dark matter environments near a black hole. The dense region of dark matter can lead to the dephasing of gravitational waveforms, which can be detected by upcoming experiments such as the Laser Interferometer Space Antenna (LISA). The dark matter density profile around the black hole can vary for different dark matter models. Our study specifically investigates the impact of the ultralight self-interacting scalar dark matter (SIDM) on the gravitational wave signals emitted by black hole binaries. A distinctive characteristic of SIDM surrounding a black hole, as opposed to collisionless dark matter, is the formation of a soliton core. We perform a Fisher matrix analysis to estimate the size of the soliton and the corresponding SIDM parameter space that future LISA-like gravitational wave experiments can explore. Published by the American Physical Society 2024

The dragon-II simulations – III. Compact binary mergers in clusters with up to 1 million stars: mass, spin, eccentricity, merger rate, and pair instability supernovae rate

ABSTRACT Compact binary mergers forming in star clusters may exhibit distinctive features that can be used to identify them among observed gravitational-wave sources. Such features likely depend on the host cluster structure and the physics of massive star evolution. Here, we dissect the population of compact binary mergers in the dragon-II simulation data base, a suite of 19 direct N-body models representing dense star clusters with up to 106 stars and $\lt 33~{{\ \rm per\ cent}}$ of stars in primordial binaries. We find a substantial population of black hole binary (BBH) mergers, some of them involving an intermediate-mass BH (IMBH), and a handful mergers involving a stellar BH and either a neutron star (NS) or a white dwarf (WD). Primordial binary mergers, $\sim 30~{{\ \rm per\ cent}}$ of the whole population, dominate ejected mergers. Dynamical mergers, instead, dominate the population of in-cluster mergers and are systematically heavier than primordial ones. Around 20 per cent of dragon-II mergers are eccentric in the Laser Interferometer Space Antenna (LISA) band and 5 per cent in the LIGO band. We infer a mean cosmic merger rate of $\mathcal {R}\sim 30(4.4)(1.2)$ yr−1 Gpc−3 for BBHs, NS–BH, and WD–BH binary mergers, respectively, and discuss the prospects for multimessenger detection of WD–BH binaries with LISA. We model the rate of pair-instability supernovae (PISNe) in star clusters and find that surveys with a limiting magnitude mbol = 25 can detect ∼1–15 yr−1 PISNe. Comparing these estimates with future observations could help to pin down the impact of massive star evolution on the mass spectrum of compact stellar objects in star clusters.

Statistical study of a large and cleaned sample of ultraluminous and hyperluminous X-ray sources

Context. Ultraluminous and hyperluminous X-ray (ULX and HLX) sources could constitute interesting laboratories to further improve our understanding of the supermassive black hole growth through super-Eddington accretion episodes and successive mergers of lighter holes. ULXs are thought to be powered by super-Eddington accretion onto stellar-mass compact objects, while HLXs are of an unknown nature, but they could be good candidates for accreting intermediate mass black holes (IMBHs). However, a significant portion of the sample of ULX and HLX candidates derived from catalogue searches are in fact background active galactic nuclei (AGN). Aims. Here we build samples of ULXs and HLXs from the three largest X-ray catalogues available, compiled from XMM-Newton, Swift-XRT, and Chandra detections, and the GLADE catalogue containing 1.7 million galaxies at D < 1000 Mpc. We aim to characterise the frequency, environment, hardness, and variability of ULXs and HLXs to better assess their differences and understand their populations. Methods. After a thorough classification of these X-ray sources, we were able to remove ∼42% of sources with a signal-to-noise ratio (S/N) > 3 which were shown to be contaminants, to obtain the cleanest sample of ULXs and HLXs to date. From a sample of 1342 ULXs and 191 HLXs detected with a S/N > 3σ, we study the occupation fraction, hardness, variability, radial distribution, and preferred environment of the sources. We built their Malmquist-corrected X-ray luminosity functions (XLFs) and compared them with previous studies. Thanks to the unprecedented size of the sample, we were able to statistically compare ULXs and HLXs and assess the differences in their nature. The interpretation of HLXs as IMBHs is investigated. Results. A significant break is seen in the XLF at ∼1040 erg s−1. With our ULX sample, containing ≲2% of contaminants, we are able to confirm that ULXs are located preferentially in spiral galaxies and galaxies with higher star formation rates. While X-ray binaries (XRBs), ULXs, and most HLXs share common hardness and variability distributions, a fraction of HLXs appear significantly softer. Unlike ULXs, HLXs seem to reside equally in spiral as well as lenticular and elliptical galaxies. We note that 35% of the HLX candidates have an optical counterpart, and we estimate the mass of 120 of them to be in the range of 2 × 103 − 105 M⊙. Most of the HLX population is found to be consistent with an accreting massive black hole in a dwarf galaxy satellite of the primary host. This diverse nature needs to be confirmed with deeper optical and infrared observations, as well as upcoming X-ray facilities.

Spectroscopic and Photometric Study of the Low-luminous AGN NGC 4395

The study of intermediate mass black holes (IMBHs) and estimating the black hole mass could potentially elucidate the origin of supermassive black hole (SMBH) seeds present at high redshifts and how they co-evolve with their host galaxy. The difficulty in detecting these IMBHs and the requirement of high spatial resolution to measure BHs mass makes this task challenging. In this work, we have performed photometric and spectroscopic analyses of an extremely low luminosity Seyfert 1 galaxy NGC4395. Our observations revealed strong emission lines in the spectra, and we observed a fractional variability in the V-band of 12%. Utilizing the broad-line region (BLR) size-luminosity relation for active galactic nuclei (AGNs), we estimated the size of the BLR to be 74±14 light minutes, based on a continuum luminosity of 1038 erg s−1 at 5100 Å. Additionally, the gas clouds are found to be revolving with a velocity dispersion of 384 km s−1 around the central engine, leading to a black hole mass estimation of ∼3.87−1.1+1.1 ×105 M⊙.

Ultradeep ATCA Imaging of 47 Tucanae Reveals a Central Compact Radio Source

We present the results of an ultradeep radio continuum survey, containing ∼480 hr of observations, of the Galactic globular cluster 47 Tucanae with the Australia Telescope Compact Array. This comprehensive coverage of the cluster allows us to reach rms noise levels of 1.19 μJy beam−1 at 5.5 GHz, 940 nJy beam−1 at 9 GHz, and 790 nJy beam−1 in a stacked 7.25 GHz image. This is the deepest radio image of a globular cluster and the deepest image ever made with the Australia Telescope Compact Array. We identify ATCA J002405.702-720452.361, a faint (6.3 ± 1.2 μJy at 5.5 GHz, 5.4 ± 0.9 μJy at 9 GHz), flat-spectrum (α = −0.31 ± 0.54) radio source that is positionally coincident with the cluster center and potentially associated with a faint X-ray source. No convincing optical counterpart was identified. We use radio, X-ray, optical, and UV data to show that explanations involving a background active galactic nucleus, a chromospherically active binary, or a binary involving a white dwarf are unlikely. The most plausible explanations are that the source is an undiscovered millisecond pulsar or a weakly accreting black hole. If the X-ray source is associated with the radio source, the fundamental plane of black-hole activity suggests a black hole mass of ∼54–6000 M ⊙, indicating an intermediate-mass black hole or a heavy stellar-mass black hole.

Extreme N-emitters at high redshift: Possible signatures of supermassive stars and globular cluster or black hole formation in action

Context. Recent James Webb Space Telescope (JWST) spectroscopic observations of the z = 10.6 galaxy GN-z11 have revealed a very peculiar UV spectrum exhibiting intense emission lines of nitrogen, which are not typically detected in galaxy spectra. This observation indicates a super-solar N/O abundance ratio at low metallicity, which only resembles the abundances seen in globular cluster (GC) stars. This discovery suggests that we might be seeing proto-GCs in formation or possibly even signatures of supermassive stars. Aims. To examine whether other objects with strong N IV and/or N III emission lines (N-emitters, hereafter) exist and to better understand their origin and nature, we have examined the available JWST spectra and data from the literature. Methods. Using the NIRSpec/JWST observations from CEERS, we found an extreme N-emitter, CEERS-1019 at z = 8.6782, showing intense N IV] λ1486 and N III] λ1750 emission. From the observed rest-UV and optical lines, we conclude that it is compatible with photoionization from stars and we have determined accurate abundances for C, N, O, and Ne, relative to H. We also (re-)analyzed other N-emitters from the literature, including three lensed objects at z = 2.3 − 3.5 (Sunburst cluster, SMACS2031, and Lynx arc) and a low-redshift compact galaxy, Mrk 996. We carried out a comparison among the observed abundance ratios to observations from normal star-forming galaxies, predicted wind yields from massive stars, and predictions from supermassive stars (SMS with ∼104 − 105M⊙). Results. For CEERS-1019, we find a highly supersolar ratio log(N/O)= − 0.18 ± 0.11, and abundances of log(C/O)= − 0.75 ± 0.11 and log(Ne/O)= − 0.63 ± 0.07, which are normal compared to other galaxies at the low metallicity (12 + log(O/H) = 7.70 ± 0.18) of this galaxy. The three lensed N-emitters also show strongly enhanced N/O ratios and two of them normal C/O. The high N/O abundances can be reproduced by massive star winds assuming a special timing and essentially no dilution with the ambient interstellar medium (ISM). Alternatively, these N/O ratios can be explained by mixing the ejecta of SMS with comparable amounts of unenriched ISM. Massive star ejecta (from WR stars) are needed to explain the galaxies with enhanced C/O (Lynx arc, Mrk 996). On the other hand, a SMS in the “conveyer-belt model” (put forward to explain globular clusters) would predict a high N/O and small changes in C/O, compatible with CEERS-1019, the Sunburst cluster, SMACS2031, and GN-z11. Based on the chemical abundances, possible enrichment scenarios, and other properties (e.g., their compactness and high ISM density), we discuss which objects could contain proto-GCs. We suggest that this is the case for CEERS-1019, SMACS2031, and the Sunburst cluster. Enrichment in the Lynx arc and Mrk 996 is likely due to normal massive stars (WR), which implies that the star-forming regions in these objects cannot become GCs. Finally, we propose that some N-emitters enriched by SMS could also have formed intermediate mass black holes and we suggest that this might be the case for GN-z11. Conclusions. Our observations and analysis reinforce the suggested link between some N-emitters and proto-GC formation, which is supported both by empirical evidence and quantitative models. Furthermore, the observations provide possible evidence for the presence of supermassive stars in the early Universe (z > 8) and at z ∼ 2 − 3. Our analysis also suggests that the origin and nature of the N-emitters is diverse, including objects such as GN-z11, which may possibly host an active galactic nucleus (AGN).

XMM-Newton-discovered Fast X-ray Transients: host galaxies and limits on contemporaneous detections of optical counterparts

ABSTRACT Extragalactic fast X-ray transients (FXTs) are a class of soft (0.3–10 keV) X-ray transients lasting a few hundred seconds to several hours. Several progenitor mechanisms have been suggested to produce FXTs, including supernova shock breakouts, binary neutron star mergers, or tidal disruptions involving an intermediate-mass black hole and a white dwarf. We present detailed host studies, including spectroscopic observations of the host galaxies of seven XMM-Newton-discovered FXTs. The candidate hosts lie at redshifts 0.0928 <z < 0.645 implying peak X-ray luminosities of 1043 erg s−1<LX < 1045 erg s−1 and physical offsets of 1 kpc < rproj < 22 kpc. These observations increase the number of FXTs with a spectroscopic redshift measurement by a factor of 2, although we note that one event is re-identified as a Galactic flare star. We infer host star formation rates and stellar masses by fitting the combined spectroscopic and archival photometric data. We also report on a contemporaneous optical counterpart search to the FXTs in Pan-STARRS and ATLAS by performing forced photometry at the position of the FXTs. We do not find any counterpart in our search. Given our constraints, including peak X-ray luminosities, optical limits, and host properties, we find that XRT 110 621 is consistent with an supernova shock breakout (SN SBO) event. Spectroscopic redshifts of likely host galaxies for four events imply peak X-ray luminosities that are too high to be consistent with SN SBOs, but we are unable to discard either the binary neutron star or white dwarf–intermediate-mass black hole tidal disruption event scenarios for these FXTs.

Modeling the High-energy Ionizing Output from Simple Stellar and X-Ray Binary Populations

We present a methodology for modeling the joint ionizing impact due to a “simple X-ray population” (SXP) and its corresponding simple stellar population (SSP), where “simple” refers to a single age and metallicity population. We construct composite spectral energy distributions (SEDs) including contributions from ultraluminous X-ray sources and stars, with physically meaningful and consistent consideration of the relative contributions of each component as a function of instantaneous burst age and stellar metallicity. These composite SEDs are used as input for photoionization modeling with Cloudy, from which we produce a grid for the time- and metallicity-dependent nebular emission from these composite populations. We make the results from the photoionization simulations publicly available. We find that the addition of the SXP prolongs the high-energy ionizing output from the population—and correspondingly increases the intensity of nebular lines such as He ii λ1640,4686, [Ne v] λ3426,14.3 μm, and [O iv] 25.9 μm by factors of at least two relative to models without an SXP spectral component. This effect is most pronounced for instantaneous bursts of star formation on timescales >10 Myr and at low metallicities (∼0.1 Z ⊙), due to the imposed time- and metallicity-dependent behavior of the SXP relative to the SSP. We propose nebular emission line diagnostics accessible with JWST suitable for inferring the presence of a composite SXP + SSP, and we discuss how the ionization signatures compare to models for sources such as intermediate-mass black holes.

Constraining a Companion of the Galactic Center Black Hole Sgr A*

We use 23 yr of astrometric and radial velocity data on the orbit of the star S0-2 to constrain a hypothetical intermediate-mass black hole orbiting the massive black hole Sgr A* at the Galactic center. The data place upper limits on variations of the orientation of the stellar orbit at levels between 0.°02 and 0.°07 per year. We use a combination of analytic estimates and full numerical integrations of the orbit of S0-2 in the presence of a black hole binary. For a companion intermediate-mass black hole outside the orbit of S0-2 (1020 au), we find that a companion black hole with mass m c between 103 and 105 M ⊙ is excluded, with a boundary behaving as . For a companion with a c < 1020 au, a black hole with mass between 103 and 105 M ⊙ is excluded, with . These bounds arise from quadrupolar perturbations of the orbit of S0-2. Significantly stronger bounds on an inner companion arise from the fact that the location of S0-2 is measured relative to the bright emission of Sgr A* and that separation is perturbed by the “wobble” of Sgr A* about the center of mass between it and the companion. The result is a set of bounds as small as 400 M ⊙ at 200 au; the numerical simulations suggest a bound from these effects varying as . We compare and contrast our results with those from a recent analysis by the GRAVITY collaboration.

A LOFAR sample of luminous compact sources coincident with nearby dwarf galaxies

The vast majority of extragalactic compact continuum radio sources are associated with star formation or jets from (super)massive black holes and, as such, are more likely to be found in association with starburst galaxies or early-type galaxies. Two new populations of radio sources were recently identified: (a) compact and persistent sources (PRSs) associated with fast radio bursts (FRBs) in dwarf galaxies and (b) compact sources in dwarf galaxies that could belong to the long-sought population of intermediate-mass black holes. Despite the interesting aspects of these newly found sources, the current sample size is small, limiting scrutiny of the underlying population. Here, we present a search for compact radio sources coincident with dwarf galaxies. We search the LOFAR Two-meter Sky Survey (LoTSS), the most sensitive low-frequency (144 MHz central frequency) large-area survey for optically thin synchrotron emission to date. Exploiting the high spatial resolution (6″) and low astrometric uncertainty (∼0.″2) of the LoTSS, we match its compact sources to the compiled sample of dwarf galaxies in the Census of the Local Universe, an Hα survey with the Palomar Observatory 48 inch Samuel Oschin Telescope. We identify 29 over-luminous compact radio sources, evaluate the probability of chance alignment within the sample, investigate the potential nature of these sources, and evaluate their volumetric density and volumetric rate. While optical line-ratio diagnostics on the nebular lines from the host galaxies support a star-formation origin rather than an AGN origin, future high-angular-resolution radio data are necessary to ascertain the origin of the radio sources. We discuss planned strategies to differentiate between candidate FRB hosts and intermediate-mass black holes.

Intermediate mass black hole feedback in dwarf galaxy simulations with a resolved ISM and accurate nuclear stellar dynamics

AbstractRecent observations have established that dwarf galaxies can host black holes of intermediate mass (IMBH, 100Mȯ &lt; MIMBH ≲ 105 Mȯ). With modern numerical models, we can test the growth of IMBHs as well as their evolutionary impact on the host galaxy. Our novel subsolar-mass (0.8 solar mass) resolution simulations of dwarf galaxies (M* = 2 × 107 Mȯ) have a resolved three-phase interstellar medium and account for non-equilibrium heating, cooling, and chemistry processes. The stellar initial mass function is fully sampled between 0.08–150 Mȯ while massive stars can form HII regions and explode as resolved supernovae. The stellar dynamics around the IMBH is integrated accurately with a regularization scheme. We present a viscous accretion disk model for the IMBH with momentum, energy, and mass conserving wind feedback. We demonstrate how the IMBH can grow from accretion of the cold and warm gas phase and how the presence of the IMBH and its feedback impacts the gas phase structure.

Evidence of dark contents in the centre of NGC 6517

ABSTRACT Millisecond pulsars can serve as effective probes to investigate the presence of intermediate-mass black holes (IMBHs) within Galactic globular clusters (GCs). Based on the standard structure models for GCs, we conduct simulations to analyse the distributions of pulsar accelerations within the central region of NGC 6517. By comparing the measured accelerations of pulsars obtained from their period derivatives $\dot{P}$ to the simulated distribution profiles, we demonstrate that a central excess of dark mass is required to account for the measured accelerations. Our analysis, which relies on existing pulsar timing observations, is currently unable to differentiate between two possible scenarios: an IMBH precisely situated at the core of the cluster with mass ${\gtrsim} 9000^{+4000}_{-3000} \, {\rm M}_{\odot }$ or a central concentration of stellar-mass dark remnants with a comparable total mass. However, with additional acceleration measurements from a few more pulsars in the cluster, it will be possible to differentiate the source of the non-luminous matter.

Short-lived repeating fast radio bursts from tidal disruption of white dwarfs by intermediate-mass black holes

ABSTRACT The origin of repeating fast radio bursts (RFRBs) is still a mystery. We propose that short-lived RFRBs might be triggered from the tidal disruption of white dwarfs (WDs) by intermediate-mass black holes (BHs). In this model, we show that the remnant WD clusters after tidal collapse cuts the magnetic lines on the BH accretion discs, and during each fall of the clump, so that electrons are torn from the surface of the mass and instantly accelerated to the relativistic energy. The subsequent movement of these electrons along magnetic field lines will result in coherent curvature radiation. This short-lived radio transients might accompany with the accretion process. The luminosity and the time-scale can be estimated to be Ltot ∼ 1.96 × 1040 erg s−1 and Δt ∼ 1.14 ms, respectively, which are consistent with the typical properties of RFRBs. Moreover, the total event rate of our model for generating RFRBs might be as high as $\sim 10~\rm {yr^{-1}~Gpc^{-3}}$.

Detecting intermediate-mass black hole binaries with atom interferometer observatories: Using the resonant mode for the merger phase

Atom interferometry detectors like AION, ZAIGA, and AEDGE will be able to detect gravitational waves (GWs) at dHz covering the band between large space-based laser interferometers LISA/TianQin/Taiji and ground-based facilities LIGO/Virgo/KAGRA. They will detect the late inspiral and merger of GW sources containing intermediate-mass black holes (IMBHs) in the mass range 102−105 M⊙. We study how accurately the parameters of an IMBH binary can be measured using AION's power spectral density. Furthermore, we propose a detection scheme where the early inspiral of the binary is detected using the regular broadband mode while the merger is detected using the resonant mode. We find that using such a detection scheme, the signal-to-noise ratio of the detection and the detection accuracy of the parameters can be enhanced compared to the full detection of the signal using the broadband mode. We, further, assess the impact of the necessary detection gap while switching from broadband to resonant mode studying the case of a short (30 s) and a long (600 s) gap. We find that the improvement in the detection accuracy for both gaps is around 40% for the total mass and the spin of the heavier black hole. For the short gap, the accuracy always improves ranging between 2% and 31% for the other parameters. For the long gap, there is a decrease in the detection accuracy for the luminosity distance, the inclination, and the initial phase but only by 1%–6% while for the remaining parameters, we have improved accuracies of around 2%–20%.

Tidal disruption of white dwarfs in a modified gravity theory with SPH

Abstract Low energy imprints of modifications to general relativity are often found in pressure balance equations inside stars. These modifications are then amenable to tests via astrophysical phenomena, using observational effects in stellar astrophysics that crucially depend on such equations. One such effect is tidal disruption of stars in the vicinity of black holes. In this paper, using a numerical scheme modelled with smoothed particle hydrodynamics, we study real time tidal disruption of a class of white dwarfs by intermediate-mass black holes, in the low energy limit of a theory of modified gravity that alters the internal physics of white dwarfs, namely the Eddington inspired Born-Infeld theory. In this single parameter extension of general relativity, the mass-radius relation of white dwarfs as well as their tidal disruption radius depend on the modified gravity parameter, and these capture the effect of modifications to general relativity. Our numerical simulations incorporating these show that departure from general relativity in these scenarios might be observationally significant, and should therefore be contrasted with data. In particular, we study observationally relevant physical quantities, i.e., tidal kick velocity and trajectory deviation of the remnant core and fallback rates of the tidal debris in this theory and compare them to the Newtonian limit of general relativity. We also comment on the qualitative differences between the modified gravity theory and one with stellar rotation.

Probing intermediate-mass black hole binaries

Atom interferometers can observe the intermediate band of gravitational waves needed for the long-sought-after detection of merging intermediate-mass black hole binaries.

Ram-pressure stripped radio tail and two ULXs in the spiral galaxy HCG 97b

ABSTRACT We report LOFAR and Very Large Array (VLA) detections of extended radio emission in the spiral galaxy HCG 97b, hosted by an X-ray bright galaxy group. The extended radio emission detected at 144 MHz, 1.4 GHz, and 4.86 GHz is elongated along the optical disc and has a tail that extends 27 kpc in projection towards the centre of the group at GHz frequencies or 60 kpc at 144 MHz. Chandra X-ray data show two off-nuclear ultra-luminous X-ray sources (ULXs), with the farther one being a plausible candidate for an accreting intermediate-mass black hole (IMBH). The asymmetry observed in both CO emission morphology and kinematics indicates that HCG 97b is undergoing ram-pressure stripping, with the leading side at the south-eastern edge of the disc. Moreover, the VLA 4.86 GHz image reveals two bright radio blobs near one ULX, aligning with the disc and tail, respectively. The spectral indices in the disc and tail are comparable and flat (α &amp;gt; −1), suggesting the presence of recent outflows potentially linked to ULX feedback. This hypothesis gains support from estimates showing that the bulk velocity of the relativistic electrons needed for transport from the disc to the tail is approximately ${\sim}1300\, \rm km~s^{-1}$. This velocity is much higher than those observed in ram-pressure stripped galaxies ($100{-}600\, \rm km~s^{-1}$), implying an alternative mechanism aiding the stripping process. Therefore, we conclude that HCG 97b is subject to ram pressure, with the formation of its stripped radio tail likely influenced by the putative IMBH activities.

Double black hole mergers in nuclear star clusters: eccentricities, spins, masses, and the growth of massive seeds

ABSTRACT We investigate the formation of intermediate mass black holes (IMBHs) through hierarchical mergers of stellar origin black holes (BHs), as well as BH mergers formed dynamically in nuclear star clusters. Using a semi-analytical approach that incorporates probabilistic mass-function-dependent double BH (DBH) pairing, binary-single encounters, and a mass-ratio-dependent prescription for energy dissipation in hardening binaries, we find that IMBHs with masses of $\mathcal {O}(10^2)$–$\mathcal {O}(10^4)\, \rm M_\odot$ can be formed solely through hierarchical mergers in time-scales of a few 100 Myrs to a few Gyrs. Clusters with escape velocities ≳400 km s−1 inevitably form high-mass IMBHs. The spin distribution of IMBHs with masses ≳ 103 M⊙ is strongly clustered at χ ∼ 0.15; while for lower masses, it peaks at χ ∼ 0.7. Eccentric mergers are more frequent for equal-mass binaries containing first- and/or second-generation BHs. Metal-rich, young, dense clusters can produce up to 20 per cent of their DBH mergers with eccentricity ≥0.1 at $10\, \rm Hz$, and ∼2–9 per cent of all in-cluster mergers can form at &amp;gt;10 Hz. Nuclear star clusters are therefore promising environments for the formation of highly eccentric DBH mergers, detectable with current gravitational-wave detectors. Clusters of extreme mass (∼108 M⊙) and density (∼108 M⊙ pc−3) can have about half of all of their DBH mergers with primary masses ≥100 M⊙. The fraction of in-cluster mergers increases rapidly with increasing cluster escape velocity, being nearly unity for vesc ≳ 200 km s−1. Cosmological merger rate of DBHs from nuclear clusters varies ⪅0.01–1 Gpc−3 yr−1, where the large error bars come from uncertainties in the cluster initial conditions, number density distribution, and redshift evolution of nucleated galaxies.

Chirping compact stars: gravitational radiation and detection degeneracy with binaries

Compressible, Riemann S-type ellipsoids can emit gravitational waves (GWs) with a chirp-like behavior (hereafter chirping ellipsoids, CELs). We show that the GW frequency-amplitude evolution of CELs (mass ∼ 1 M⊙, radius ∼ 103 km, polytropic equation of state with index n ≈ 3) is indistinguishable from that emitted by double white dwarfs and by extreme mass-ratio inspirals (EMRIs) composed of an intermediate-mass (e.g. 103 M⊙) black hole and a planet-like (e.g. 10-4 M⊙) companion, in the frequency interval within the detector sensitivity band in which the GW emission of these systems is quasi-monochromatic.For reasonable astrophysical assumptions, the local universe density rate of CELs, double white dwarfs, and EMRIs in the mass range here considered are very similar, posing a detection-degeneracy challenge for space-based GW detectors. We outline the astrophysical implications of this CEL-binary detection degeneracy by space-based GW-detection facilities.