Intermediate-mass Black Hole Candidate Research Articles

Identification of Intermediate-mass Black Hole Candidates among a Sample of Sd Galaxies

We analyzed images of every northern hemisphere Sd galaxy listed in the Third Reference Catalogue of Bright Galaxies with a relatively face-on inclination (θ ≤ 30°). Specifically, we measured the spiral arms’ winding angle, ϕ, in 85 galaxies. We applied a novel black hole mass planar scaling relation involving the rotational velocities (from the literature) and pitch angles of each galaxy to predict central black hole masses. This yielded 23 galaxies, each having at least a 50% chance of hosting a central intermediate-mass black hole (IMBH), 102 < M • ≤ 105 M ☉. These 23 nearby (≲50 Mpc) targets may be suitable for an array of follow-up observations to check for active nuclei. Based on our full sample of 85 Sd galaxies, we estimate that the typical Sd galaxy (which tends to be bulgeless) harbors a black hole with log(M•/M☉)=6.00±0.14 , but with a 27.7% chance of hosting an IMBH, making this morphological type of galaxy fertile ground for hunting elusive IMBHs. Thus, we find that a ∼106 M ☉ black hole corresponds roughly to the onset of bulge development and serves as a conspicuous waypoint along the galaxy–supermassive black hole coevolution journey. Our survey suggests that >1.22% of bright galaxies (B T ≲ 15.5 mag) in the local Universe host an IMBH (i.e., the “occupation fraction”), which implies a number density >4.96 × 10−6 Mpc−3 for central IMBHs. Finally, we observe that Sd galaxies exhibit an unexpected diversity of properties that resemble the general population of spiral galaxies, albeit with an enhanced signature of the eponymous prototypical traits (i.e., low masses, loosely wound spiral arms, and smaller rotational velocities).

DAVOS: Dwarf Active Galactic Nuclei from Variability for the Origins of Seeds: Properties of Variability-selected Active Galactic Nuclei in the COSMOS Field and Expectations for the Rubin Observatory

We study the black hole mass–host galaxy stellar mass relation, M BH–M *, of a sample of z < 4 optically variable active galactic nuclei (AGNs) in the COSMOS field. The parent sample of 491 COSMOS AGNs were identified by optical variability from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) program. Using publicly available catalogs and spectra, we consolidate their spectroscopic redshifts and estimate virial black hole masses using broad-line widths and luminosities. We show that variability searches with deep, high-precision photometry like the HSC-SSP can identity AGNs in low-mass galaxies up to z ∼ 1. However, their black holes are more massive given their host galaxy stellar masses than predicted by the local relation for active galaxies. We report that z ∼ 0.5–4 variability-selected AGNs are meanwhile more consistent with the M BH–M * relation for local inactive early-type galaxies. This result is in agreement with most previous studies of the M BH–M * relation at similar redshifts and indicates that AGNs selected from variability are not intrinsically different from the broad-line Type 1 AGN population at similar luminosities. Our results demonstrate the need for robust black hole and stellar mass estimates for intermediate-mass black hole candidates in low-mass galaxies at similar redshifts to anchor this scaling relation. Assuming that these results do not reflect a selection bias, they appear to be consistent with self-regulated feedback models wherein the central black hole and stars in galaxies grow in tandem.

Possible Supercritical Accretion on the Ultraluminous X-Ray Source in the Metal-poor Galaxy I Zw 18

We present an analysis of X-ray observations of the ultraluminous X-ray source (ULX) in I Zw 18 based on archival data taken with Chandra, XMM-Newton, and Suzaku. This ULX is considered to be an intermediate-mass black hole candidate simply because it is in the lowest-metallicity environment among ULXs, where the formation of heavy black holes is facilitated. However, actual study of the ULX based on observations spanning for a long period has been too limited to determine its nature. In this study, we investigate the spectral evolution of the ULX up to 2014, combining the previously unpublished Suzaku data with those from the other two satellites. We derive a positive correlation of L∝Tin2.1±0.4 between the bolometric luminosity L and inner-disk temperature T in on the basis of the multicolor disk blackbody model, where we exclude the Chandra data, which have the lowest luminosity and systematic residuals in the fitting. The nominal relation L∝Tin4 for the standard disk is rejected at a significance level of 1.5%. These results suggest that the ULX was in the slim-disk state during these observations except at the time of the Chandra observation, in which the ULX was likely to be in a different state. The apparent inner-disk radius appears negatively correlated with the inner-disk temperature. Moreover, we find a radial dependence of the disk temperature of T(r) ∝ r −p with p < 0.75, which also supports the hypothesis that the ULX has a slim disk. Therefore, the I Zw 18 ULX is most likely to be powered by a stellar-mass compact object in supercritical accretion.

Detecting Wandering Intermediate-Mass Black Holes with AXIS in the Milky Way and Local Massive Galaxies

This white paper explores the detectability of intermediate-mass black holes (IMBHs) wandering in the Milky Way (MW) and massive local galaxies, with a particular emphasis on the role of AXIS. IMBHs, ranging within 103−6M⊙, are commonly found at the centers of dwarf galaxies and may exist, yet undiscovered, in the MW. By using model spectra for advection-dominated accretion flows (ADAFs), we calculated the expected fluxes emitted by a population of wandering IMBHs with masses of 105M⊙ in various MW environments and extrapolated our results to massive local galaxies. Around 40% of the potential population of wandering IMBHs in the MW can be detected in an AXIS deep field. We proposed criteria to aid with selecting IMBH candidates using already available optical surveys. We also showed that IMBHs wandering in &gt;200 galaxies within 10 Mpc can be easily detected with AXIS when passing within dense galactic environments (e.g., molecular clouds and cold neutral medium). In summary, we highlighted the potential X-ray detectability of wandering IMBHs in local galaxies and provided insights for guiding future surveys. Detecting wandering IMBHs is crucial for understanding their demographics and evolution and the merging history of galaxies. This white paper is part of a series commissioned for the AXIS Probe Concept Mission; additional AXIS white papers can be found at the AXIS website.

Primordial intermediate and supermassive black hole formation during the electron–positron annihilation epoch

ABSTRACT Some of the Intermediate Mass Black Hole candidates observed at the centre of galaxies or in globular clusters and some of the Supermassive Black Holes seen at the centre of many galaxies might be of primordial origin. Indeed, Primordial Black Holes (PBHs) of such mass could have formed when the Universe was ∼1–103 s old, due to the collapse of density fluctuations. In particular, when the Universe was ∼1 s in age, Electron–Positron Annihilation (EPA) took place. We explore the formation of intermediate mass and supermassive PBHs, taking into account the effect of the EPA when the fluctuations have a running tilt power-law spectrum: when these cross the 10−0.5–103.0 s Universe horizon they could produce 5 × 103–5 × 108 M⊙ PBHs with a density as high as ∼1010/Gpc3. On average, this implies a population of about one thousand PBHs in the Local Group of Galaxies, with the nearest one at about 250 kpc, just under half the distance to the Andromeda galaxy (M31).

Exploring X-Ray Properties of Low-metallicity Dwarf Galaxies

One of the primary outstanding questions in extragalactic astronomy is the formation and early evolution of the supermassive black holes that are seen in nearly every massive galaxy. Low-metallicity dwarf galaxies may offer the most representative local analogs to pristine early galaxies, making them a vital tool in probing black hole seed models through the study of the intermediate-mass black holes (IMBHs) possibly hosted therein; though these dwarf galaxies, and the IMBHs they may host, are typically not as well-studied in this context as their higher-metallicity and higher-mass counterparts. In this paper, we explore the X-ray properties of a sample of 37 low-metallicity dwarf galaxies using archival XMM observations, and we compare the properties of this population against a representative sample of higher-metallicity counterparts. We report the detection of 10 sources with 0.3–10 keV luminosity in excess of 1040 erg s−1 within the low-metallicity sample, which we highlight for followup as potential IMBH candidates. Finally, we discuss the differing multiwavelength scaling relations (e.g., L X–L W2, L X–star formation rate) between the two galaxy populations, as well as the sample’s L X as a function of metallicity.

Intermediate-Mass Black Holes: The Essential Population to Explore the Unified Model for Accretion and Ejection Processes

We study radio and X-ray emissions from intermediate-mass black holes (IMBHs) and explore the unified model for accretion and ejection processes. The radio band survey of IMBH (candidate) hosted galaxies indicates that only a small fraction (∼0.6%) of them are radio-band active. In addition, very long baseline interferometry observations reveal parsec-scale radio emission of IMBHs, further resulting in a lower fraction of actively ejecting objects (radio emission is produced by IMBHs other than hosts), which is consistent with a long quiescent state in the evolution cycle of IMBHs. Most (75%, i.e., 3 out of 4 samples according to a recent mini-survey) of the radio-emitting IMBHs are associated with radio relics and there is also evidence of dual radio blobs from episodic ejecting phases. Taking the radio emission and the corresponding core X-ray emission of IMBH, we confirm a universal fundamental plane relation (FMP) of black hole activity. Furthermore, state transitions can be inferred by comparing a few cases in XRBs and IMBHs in FMP, i.e., both radio luminosity and emission regions evolve along these state transitions. These signatures and evidence suggest an analogy among all kinds of accretion systems which span from stellar mass to supermassive black holes, hinting at unified accretion and ejection physics. To validate the unified model, we explore the correlation between the scale of outflows (corresponding to ejection powers) and the masses of central engines; it shows that the largest scale of outflows LS^out follows a power-law correlation with the masses of accretors Mcore, i.e., logLS^out=(0.73±0.01)logMcore−(3.34±0.10). In conclusion, this work provides evidence to support the claim that the ejection (and accretion) process behaves as scale-invariant and their power is regulated by the masses of accretors.

The hyperluminous X‐ray source population

AbstractWe have recently published a catalog of 1843 candidate ultraluminous X‐ray sources (ULXs). This is the largest catalog of ULXs to date and was built by cross‐correlating recent serendipitous source catalogs from the XMM‐Newton, Swift, and Chandra observatories with a large sample of galaxies, primarily from HyperLEDA. The catalog contains 71 hyperluminous X‐ray source (HLX) candidates, the most extreme members of the ULX population with luminosities above 1041 erg s−1. These sources are often considered the best candidates for intermediate‐mass black hole (IMBH) accretors and include the archetypal IMBH candidate ESO 243–49 HLX‐1. However, the most luminous of the known pulsating ULXs, NGC 5907 ULX1, is also an HLX at its brightest. We demonstrate that these two objects occupy distinct areas of the hardness‐intensity parameter space, and use this to contextualize the results from a pilot study of three data‐rich examples of the 42 HLXs we select as the best candidates based on their multi‐wavelength counterparts and X‐ray data quality. We briefly discuss the implications of this work.

Radio Observations of Four Active Galactic Nuclei Hosting Intermediate-mass Black Hole Candidates: Studying the Outflow Activity and Evolution

Observational searches for intermediate-mass black holes (IMBHs; 102–106 M ⊙) include relatively isolated dwarf galaxies. For those that host active galactic nuclei (AGNs), the IMBH nature may be discerned through the accretion–jet activity. We present radio observations of four AGN-hosting dwarf galaxies, which potentially harbor IMBHs. Very large array (VLA) observations indicate steep spectra (indices of −0.63 to −1.05) between 1.4 and 9 GHz. However, a comparison with the 9 GHz in-band spectral index shows a steepening for GH047 and GH158 (implying older/relic emission) and flattening for GH106 and GH163 (implying recent activity). Overlapping emission regions in the VLA 1.4 GHz and our very long baseline array (VLBA) 1.5 GHz observations, and possibly symmetric pc-scale extensions, are consistent with recent activity in the latter two. Using the compact VLBA radio luminosity, X-ray luminosity (probing the accretion activity), and the black hole masses, all AGNs are found to lie on the empirical fundamental plane relation. The four AGNs are radio-quiet with relatively higher Eddington ratios (0.04–0.32) and resemble X-ray binaries during spectral state transitions that entail an outflow ejection. Furthermore, the radio to X-ray luminosity ratio of −3.9 to −5.6 in these four sources support the scenarios that include corona mass ejection from the accretion disk and wind activity. The growth to kpc-scales likely proceeds along a similar trajectory to young AGNs and peaked spectrum sources. These complex clues can thus aid in the detection and monitoring of IMBHs in the nearby universe.

A fast-rising tidal disruption event from a candidate intermediate-mass black hole

Massive black holes (BHs) at the centres of massive galaxies are ubiquitous. The population of BHs within dwarf galaxies, on the other hand, is not yet known. Dwarf galaxies are thought to harbour BHs with proportionally small masses, including intermediate-mass BHs, with masses 102 < MBH < 106 solar masses (M⊙). Identification of these systems has historically relied on the detection of light emitted from accreting gaseous disks close to the BHs. Without this light, they are difficult to detect. Tidal disruption events, the luminous flares produced when a star strays close to a BH and is shredded, are a direct way to probe massive BHs. The rise times of these flares theoretically correlate with the BH mass. Here we present AT 2020neh, a fast-rising tidal disruption event candidate, hosted by a dwarf galaxy. AT 2020neh can be described by the tidal disruption of a main sequence star by a 104.7–105.9 M⊙ BH. We find the observable rate of fast-rising nuclear transients like AT 2020neh to be low, at ≲2 × 10−8 events Mpc−3 yr−1. Finding non-accreting BHs in dwarf galaxies is important to determine how prevalent BHs are within these galaxies, and to constrain models of BH formation. AT 2020neh-like events may provide a galaxy-independent method of measuring the masses of intermediate-mass BHs. The rapid rise in brightness of a tidal disruption event is attributed to the destruction of a main sequence star by a black hole of intermediate mass in a dwarf galaxy. Such events are rare, and non-accreting intermediate-mass black holes are challenging to find.

Variability-selected Intermediate-mass Black Hole Candidates in Dwarf Galaxies from ZTF and WISE

While it is difficult to observe the first black hole seeds in the early universe, we can study intermediate-mass black holes (IMBHs) in local dwarf galaxies for clues about their origins. In this paper we present a sample of variability-selected active galactic nuclei (AGN) in dwarf galaxies using optical photometry from the Zwicky Transient Facility (ZTF) and forward-modeled mid-IR photometry of time-resolved Wide-field Infrared Survey Explorer (WISE) co-added images. We found that 44 out of 25,714 dwarf galaxies had optically variable AGN candidates and 148 out of 79,879 dwarf galaxies had mid-IR variable AGN candidates, corresponding to active fractions of 0.17% ± 0.03% and 0.19% ± 0.02%, respectively. We found that spectroscopic approaches to AGN identification would have missed 81% of our ZTF IMBH candidates and 69% of our WISE IMBH candidates. Only nine candidates have been detected previously in radio, X-ray, and variability searches for dwarf galaxy AGN. The ZTF and WISE dwarf galaxy AGN with broad Balmer lines have virial masses of 105 M ⊙ < M BH < 107 M ⊙, but for the rest of the sample, BH masses predicted from host galaxy mass range between 105.2 M ⊙ < M BH < 107.25 M ⊙. We found that only 5 of 152 previously reported variability-selected AGN candidates from the Palomar Transient Factory in common with our parent sample were variable in ZTF. We also determined a nuclear supernova fraction of 0.05% ± 0.01% yr−1 for dwarf galaxies in ZTF. Our ZTF and WISE IMBH candidates show the promise of variability searches for the discovery of otherwise hidden low-mass AGN.

A follow-up on intermediate-mass black hole candidates in the second LIGO–Virgo observing run with the Bayes Coherence Ratio

ABSTRACT The detection of an intermediate-mass black hole population (102–106 M⊙) will provide clues to their formation environments (e.g. discs of active galactic nuclei, globular clusters) and illuminate a potential pathway to produce supermassive black holes. Ground-based gravitational-wave detectors are sensitive to mergers that can form intermediate-mass black holes weighing up to ∼450 M⊙. However, ground-based detector data contain numerous incoherent short duration noise transients that can mimic the gravitational-wave signals from merging intermediate-mass black holes, limiting the sensitivity of searches. Here, we follow-up on binary black hole merger candidates using a ranking statistic that measures the coherence or incoherence of triggers in multiple-detector data. We use this statistic to rank candidate events, initially identified by all-sky search pipelines, with lab-frame total masses ≳ 55 M⊙ using data from LIGO’s second observing run. Our analysis does not yield evidence for new intermediate-mass black holes. However, we find support for eight stellar-mass binary black holes not reported in the first LIGO–Virgo gravitational wave transient catalogue GWTC-1, seven of which have been previously reported by other catalogues.

An Ultraluminous Supersoft Source in a Dwarf Galaxy of A85: An Intermediate-mass Black Hole Candidate

Abstract We study a large sample of dwarf galaxies using archival Chandra X-ray observations, with the aim of detecting accreting intermediate-mass black holes (IMBHs). IMBHs are expected to inhabit dwarf galaxies and to produce specific signatures in terms of luminosity and X-ray spectra. We report the discovery of an X-ray source associated with an A85 dwarf galaxy that fits the IMBH description. The stellar mass of the host galaxy is estimated to be 2 × 108 M ⊙, which makes it one of the least massive galaxies to potentially host an accreting black hole. The source is detected in the soft band, under 1 keV, and is undetected at higher energies. The X-ray luminosity is ≈1041 erg s−1, making it almost three orders of magnitude more luminous than the most luminous stellar-mass supersoft emitters. From the galaxy stellar mass versus black hole mass relation, we estimate the mass to be within the intermediate regime. Another method that resulted in an intermediate mass relies on the fact that supersoft emission is expected to be associated with high accretion rates, approaching the Eddington limit. We suggest that the observed offset of the X-ray source from the galactic center (≈1.8 kpc) is due to galaxy interactions, and we present evidence from the literature that supports the relation between black hole activity and galaxy interactions.

Search for Intermediate-mass Black Holes at Low Redshift with Intra-night Variability

Abstract We present a sample of intermediate-mass black hole (IMBH) candidates based on the detection of a broad Hα emission line and variability, which are selected from the Sloan Digital Sky Survey Data Release 7. By performing spectral decomposition of emission lines as well as a visual inspection, we initially identified 131 targets with a broad Hα line among a large sample of emission line galaxies. We further selected 25 IMBH candidates whose estimated black hole mass (M BH) is less than 106 M ⊙. To constrain the nature of these candidates, we analyzed X-ray properties and performed an intra-night variability monitoring campaign with optical telescopes. Based on the optical variability analysis, we report a sample of 11 targets with detected intra-night variability as the best IMBH candidates, which are suitable for follow-up observations for accurate M BH determination such as reverberation mapping campaigns.

Central X-Ray Point Sources Found to Be Abundant in Low-mass, Late-type Galaxies Predicted to Contain an Intermediate-mass Black Hole

Abstract Building upon three late-type galaxies in the Virgo cluster with both a predicted black hole mass of less than ∼105 M ⊙ and a centrally located X-ray point source, we reveal 11 more such galaxies, more than tripling the number of active intermediate-mass black hole candidates among this population. Moreover, this amounts to a ∼36 ± 8% X-ray detection rate (despite the sometimes high, X-ray-absorbing, H i column densities), compared to just 10 ± 5% for (the largely H i-free) dwarf early-type galaxies in the Virgo cluster. The expected contribution of X-ray binaries from the galaxies’ inner field stars is negligible. Moreover, given that both the spiral and dwarf galaxies contain nuclear star clusters, the above inequality appears to disfavor X-ray binaries in nuclear star clusters. The higher occupation, or rather detection, fraction among the spiral galaxies may instead reflect an enhanced cool gas/fuel supply and Eddington ratio. Indeed, four of the 11 new X-ray detections are associated with known LINERs or LINER/H ii composites. For all (four) of the new detections for which the X-ray flux was strong enough to establish the spectral energy distribution in the Chandra band, it is consistent with power-law spectra. Furthermore, the X-ray emission from the source with the highest flux (NGC 4197: L X ≈ 1040 erg s−1) suggests a non-stellar-mass black hole if the X-ray spectrum corresponds to the “low/hard state”. Follow-up observations to further probe the black hole masses, and prospects for spatially resolving the gravitational spheres of influence around intermediate-mass black holes, are reviewed in some detail.

Resonant Dynamical Friction in Nuclear Star Clusters: Rapid Alignment of an Intermediate-mass Black Hole with a Stellar Disk

Abstract We investigate the dynamical evolution of an intermediate-mass black hole (IMBH) in a nuclear star cluster hosting a supermassive black hole (SMBH) and both a spherical and a flattened disk-like distribution of stellar-mass objects. We use a direct N-body (φ GPU) and an orbit-averaged (N-ring) numerical integrator to simulate the orbital evolution of stars and the IMBH. We find that the IMBH’s orbit gradually aligns with the stellar disk if their mutual initial inclination is less than 90°. If it is larger than 90°, i.e., counter-rotating, the IMBH does not align. Initially, the rate of orbital reorientation increases linearly with the ratio of the mass of the IMBH over the SMBH mass, and it is orders of magnitude faster than ordinary (i.e., Chandrasekhar) dynamical friction, particularly for high SMBH masses. The semimajor axes of the IMBH and the stars are approximately conserved. This suggests that the alignment is predominantly driven by orbit-averaged gravitational torques of the stars, a process that may be called resonant dynamical friction. The stellar disk is warped by the IMBH, and ultimately increases its thickness. This process may offer a test for the viability of IMBH candidates in the Galactic Center. Resonant dynamical friction is not limited to IMBHs; any object much more massive than disk particles may ultimately align with the disk. This may have implications for the formation and evolution of black hole disks in dense stellar systems and gravitational wave source populations for LIGO, VIRGO, KAGRA, and LISA.

Dynamical modelling of globular clusters: challenges for the robust determination of IMBH candidates

ABSTRACTThe presence or absence of intermediate-mass black holes (IMBHs) at the centre of Milky Way globular clusters (GCs) is still an open question. This is due to either observational restrictions or limitations in the dynamical modelling method; in this work, we explore the latter. Using a sample of high-end Monte Carlo simulations of GCs, with and without a central IMBH, we study the limitations of spherically symmetric Jeans models assuming constant velocity anisotropy and mass-to-light ratio. This dynamical method is one of the most widely used modelling approaches to identify a central IMBH in observations.With these models, we are able to robustly identify and recover the mass of the central IMBH in our simulation with a high-mass IMBH ($M_{\rm IMBH}/M_{\rm GC}\sim 4{{\ \rm per\ cent}}$). Simultaneously, we show that it is challenging to confirm the existence of a low-mass IMBH ($M_{\rm IMBH}/M_{\rm GC}\sim 0.3{{\ \rm per\ cent}}$), as both solutions with and without an IMBH are possible within our adopted error bars. For simulations without an IMBH, we do not find any certain false detection of an IMBH. However, we obtain upper limits that still allow for the presence of a central IMBH. We conclude that while our modelling approach is reliable for the high-mass IMBH and does not seem to lead towards a false detection of a central IMBH, it lacks the sensitivity to robustly identify a low-mass IMBH and to definitely rule out the presence of an IMBH when it is not there.

Multiwavelength Follow-up of the Hyperluminous Intermediate-mass Black Hole Candidate 3XMM J215022.4−055108

Abstract We recently discovered the X-ray/optical outbursting source 3XMM J215022.4−055108. It was best explained as the tidal disruption of a star by an intermediate-mass black hole of mass of a few tens of thousand solar masses in a massive star cluster at the outskirts of a large barred lenticular galaxy at D L = 247 Mpc. However, we could not completely rule out a Galactic cooling neutron star as an alternative explanation for the source. In order to further pin down the nature of the source, we have obtained new multiwavelength observations by XMM-Newton and the Hubble Space Telescope (HST). The optical counterpart to the source in the new HST image is marginally resolved, which rules out the Galactic cooling neutron star explanation for the source and suggests a star cluster of half-light radius ∼27 pc. The new XMM-Newton observation indicates that the luminosity was decaying as expected for a tidal disruption event and that the disk was still in the thermal state with a supersoft X-ray spectrum. Therefore, the new observations confirm the source as one of the best intermediate-mass black hole candidates.

The Fifth Candidate for an Intermediate-mass Black Hole in the Galactic Center

Abstract We report the results of high-resolution molecular line observations of the high-velocity compact cloud HCN–0.085–0.094 with the Atacama Large Millimeter/submillimeter Array. The HCN J = 4–3, HCO+ J = 4–3, and CS J = 7–6 line images reveal that HCN–0.085–0.094 consists mainly of three small clumps with extremely broad velocity widths. Each of the three clumps has a 5.5 GHz radio continuum counterpart in its periphery toward Sgr A*. The positional relationship indicates that their surfaces have been ionized by ultraviolet photons from young stars in the central cluster, suggesting the clumps are in close proximity to the Galactic nucleus. One of the three clumps has a ring-like structure with a very steep velocity gradient. This kinematical structure suggests an orbit around a point-like object with a mass of ∼104 M ⊙. The absence of stellar counterparts indicates that the point-like object may be a quiescent black hole. This discovery adds another intermediate-mass black hole candidate in the central region of our Galaxy.

Stellar properties of the host galaxy of an ultraluminous X-ray source in NGC 5252

ABSTRACT An ultraluminous X-ray source (ULX) in NGC 5252 has been known as a strong candidate for an off-nuclear intermediate-mass black hole (IMBH). We present near-infrared imaging data of the ULX obtained with the William Herschel Telescope. Using this data we estimate a stellar mass associated with the ULX of ≈107.9 ± 0.1$\, \mathrm{M}_\odot$, suggesting that it could be (the remnant of) a dwarf galaxy that is in the process of merging with NGC 5252. Based on a correlation between the mass of the central black hole (BH) and host galaxy, the ULX is powered by a 105 $\, \mathrm{M}_\odot$ BH. Alternatively, if the BH mass is ≈106$\, \mathrm{M}_\odot$ or larger, the host galaxy of the ULX must have been heavily stripped during the merger. The ULX Ks-band luminosity is two orders of magnitude smaller than that expected from an ordinary active galactic nucleus with the observed [O iii] luminosity, which also suggests the ULX lacks a dusty torus. We discuss how these findings provide suggestive evidence that the ULX is hosting an IMBH.