Population Of Intermediate-mass Black Holes Research Articles

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.

A Chandra survey of milky way globular clusters – III. Searching for X-ray signature of intermediate-mass black holes

ABSTRACT Globular clusters (GCs) are thought to harbor the long-sought population of intermediate-mass black holes (IMBHs). We present a systematic search for a putative IMBH in 81 Milky Way GCs, based on archival Chandra X-ray observations. We find in only six GCs a significant X-ray source positionally coincident with the cluster centre, which have 0.5–8 keV luminosities between ∼1 × 1030 erg s−1 and ∼4 × 1033 erg s−1. However, the spectral and temporal properties of these six sources can also be explained in terms of binary stars. The remaining 75 GCs do not have a detectable central source, most with 3σ upper limits ranging between 1029–32 erg s−1 over 0.5–8 keV, which are significantly lower than predicted for canonical Bondi accretion. To help understand the feeble X-ray signature, we perform hydrodynamic simulations of stellar wind accretion on to a 1000 M⊙ IMBH from the most-bound orbiting star, for stellar wind properties consistent with either a main-sequence (MS) star or an asymptotic giant branch (AGB) star. We find that the synthetic X-ray luminosity for the MS case ($\sim 10^{19}\rm ~erg \, s^{-1}$) is far below the current X-ray limits. The predicted X-ray luminosity for the AGB case ($\sim 10^{34}\rm ~erg \, s^{-1}$), on the other hand, is compatible with the detected central X-ray sources, in particular the ones in Terzan 5 and NGC 6652. However, the probability of having an AGB star as the most-bound star around the putative IMBH is very low. Our study strongly suggests that it is very challenging to detect the accretion-induced X-ray emission from IMBHs, even if they were prevalent in present-day GCs.

A parsec-scale radio jet launched by the central intermediate-mass black hole in the dwarf galaxy SDSS J090613.77+561015.2

ABSTRACT The population of intermediate-mass black holes (IMBHs) in nearby dwarf galaxies plays an important ‘ground truth’ role in exploring black hole formation and growth in the early Universe. In the dwarf elliptical galaxy SDSS J090613.77+561015.2 (z = 0.0465), an accreting IMBH has been revealed by optical and X-ray observations. Aiming to search for possible radio core and jet associated with the IMBH, we carried out very long baseline interferometry (VLBI) observations with the European VLBI Network at 1.66 GHz. Our imaging results show that there are two 1-mJy components with a separation of about 52 mas (projected distance 47 pc) and the more compact component is located within the 1σ error circle of the optical centroid from available Gaia astrometry. Based on their positions, elongated structures and relatively high brightness temperatures, as well as the absence of star-forming activity in the host galaxy, we argue that the radio morphology originates from the jet activity powered by the central IMBH. The existence of the large-scale jet implies that violent jet activity might occur in the early epochs of black hole growth and thus help to regulate the co-evolution of black holes and galaxies.

Discovering intermediate-mass black hole lenses through gravitational wave lensing

Intermediate-mass black holes are the missing link that connects stellar-mass to supermassive black holes and are key to understanding galaxy evolution. Gravitational waves, like photons, can be lensed, leading to discernable effects such as diffraction or repeated signals. We investigate the detectability of intermediate-mass black hole deflectors in the LIGO-Virgo detector network. In particular, we simulate gravitational waves with variable source distributions lensed by an astrophysical population of intermediate-mass black holes, and use standard LIGO tools to infer the properties of these lenses. We find detections of intermediate-mass black holes at $98\%$ confidence level over a wide range of binary and lens parameters. Therefore, we conclude that intermediate-mass black holes could be detected through lensing of gravitational waves in the LIGO-Virgo detector network.

Constraining the population of intermediate-mass black holes by gamma-ray data of the Fornax cluster

Recent studies of merging black holes suggest that the cosmological mass fraction of primordial black holes (PBHs) is not completely negligible. The mass of a PBH can be as massive as an intermediate mass black hole (IMBH). In this article, we assume that dark matter particles are thermal relic particles and they can self-annihilate. The dark matter around an IMBH may have formed a density spike so that the rate of dark matter annihilation would be greatly enhanced. Using the gamma-ray data of the Fornax cluster and assuming merging events in the cluster are not important, we show that the amount of IMBHs (including PBHs) is very sub-dominant compared with the amount of baryons and dark matter. The upper limit on the IMBH-to-baryon ratio is $\sim 7\times 10^{-4}$ for $m_{\rm DM} \le 10$ TeV.

Intermediate-Mass Black Hole Feedback in Dwarf Galaxies: a View from Cosmological Simulations

AbstractBlack holes are usually observed to be of stellar-mass or supermassive. By natural extension, there should be a population of Intermediate-Mass Black Holes (IMBHs: with mass between 100 to 106M⊙) in the Universe; which has started to been observed. An exciting claim has been made recently by Silk (2017): that early feedback by IMBHs in gas-rich dwarf galaxies at z = 5–8, can potentially solve multiple dwarf galaxy problems within the Λ-cold-dark-matter cosmology. We are performing Cosmological Hydrodynamical Simulations of (2Mpc)3 volumes, starting from z = 100, to test the case for IMBHs in Dwarf Galaxies. Black holes of mass 1000M⊙ are seeded inside halos when they reach a mass of 107M⊙. The black holes grow by accretion of gas from their surroundings and by merger with other black holes, and consequently eject feedback energy. We analyze the simulation output in post-processing to study the growth of the first IMBHs, and their impact on star-formation. Our conclusions, based on numerical simulation results, support the phenomenological ideas made by Silk (2017). IMBHs at the centers of dwarf galaxies can be a strong source of feedback to quench star-formation and generate outflows. At the same time, these IMBHs form the missing link between stellar-mass and supermassive BHs.

Intermediate-mass Black Holes and Dark Matter at the Galactic Center

Abstract Could there be a large population of intermediate-mass black holes (IMBHs) formed in the early universe? Whether primordial or formed in Population III, these are likely to be very subdominant compared to the dark matter density, but could seed early dwarf galaxy/globular cluster and supermassive black hole formation. Via survival of dark matter density spikes, we show here that a centrally concentrated relic population of IMBHs, along with ambient dark matter, could account for the Fermi gamma-ray “excess” in the Galactic center because of dark matter particle annihilations.

Erratum: Detecting floating black holes as they traverse the gas disc of the Milky Way

A population of intermediate-mass black holes (BHs) is predicted to be freely floating in the Milky Way (MW) halo, due to gravitational wave recoil, ejection from triple BH systems, or tidal stripping in the dwarf galaxies that merged to make the MW. As these BHs traverse the gaseous MW disk, a bow shock forms, producing detectable radio and mm/sub-mm synchrotron emission from accelerated electrons. We calculate the synchrotron flux to be $\sim \rm 0.01-10\, mJy$ at GHz frequency, detectable by Jansky Very Large Array, and $\sim 10-100\,\mu\rm Jy$ at $\sim10^{10}-10^{12} \,\rm Hz$ frequencies, detectable by Atacama Large Millimeter/sub-millimter Array. The discovery of the floating BH population will provide insights on the formation and merger history of the MW as well as on the evolution of massive BHs in the early Universe.

Detecting floating black holes as they traverse the gas disc of the Milky Way

A population of intermediate-mass black holes (BHs) is predicted to be freely floating in the Milky Way (MW) halo, due to gravitational wave recoil, ejection from triple BH systems, or tidal stripping in the dwarf galaxies that merged to make the MW. As these BHs traverse the gaseous MW disk, a bow shock forms, producing detectable radio and mm/sub-mm synchrotron emission from accelerated electrons. We calculate the synchrotron flux to be $\sim \rm 0.01-10\, mJy$ at GHz frequency, detectable by Jansky Very Large Array, and $\sim 10-100\,\mu\rm Jy$ at $\sim10^{10}-10^{12} \,\rm Hz$ frequencies, detectable by Atacama Large Millimeter/sub-millimter Array. The discovery of the floating BH population will provide insights on the formation and merger history of the MW as well as on the evolution of massive BHs in the early Universe.

Constraining the black hole mass spectrum with gravitational wave observations - I. The error kernel

Many scenarios have been proposed for the origin of the supermassive black holes (SMBHs) that are found in the centres of most galaxies. Many of these formation scenarios predict a high-redshift population of intermediate-mass black holes (IMBHs), with masses in the range 100 to 100000 times that of the Sun. A powerful way to observe these IMBHs is via gravitational waves the black holes emit as they merge. The statistics of the observed black hole population should, in principle, allow us to discriminate between competing astrophysical scenarios for the origin and formation of SMBHs. However, gravitational wave detectors such as LISA will not be able to detect all such mergers nor assign precise black hole parameters to the merger, due to weak gravitational wave signal strengths. In order to use LISA observations to infer the statistics of the underlying population, these errors must be taken into account. We describe here a method for folding the LISA gravitational wave parameter error estimates into an `error kernel' designed for use at the population model level. The effects of this error function are demonstrated by applying it to several recent models of black hole mergers, and some tentative conclusions are made about LISA's ability to test scenarios of the origin and formation of supermassive black holes.

Cosmic backgrounds from miniquasars

Abstract A large population of intermediate-mass black holes (IMBHs) might be produced at early cosmic times as a leftover of the evolution of the very massive first stars. Accretion on to IMBHs provides a source of (re)ionizing radiation. We show that the baryon mass fraction locked into IMBHs and their growth is strongly constrained by the observed residual soft X-ray background (SXRB) intensity. Thus, unless they are extremely X-ray quiet, miniquasars must be quite rare and/or have a short shining phase. As a byproduct, we find that miniquasars cannot be the only source of reionization and that their alleged contribution to the near-infrared bands is completely negligible.

Dynamical evolution of intermediate-mass black holes and their observable signatures in the nearby Universe

We investigate the consequences of a model of the assembly and growth of massive black holes (MBHs) from primordial seeds, remnants of the first generation of stars in a hierarchical structure formation scenario. Our model traces the build-up of MBHs from an early epoch, and follows the merger history of dark matter haloes and their associated holes via Monte Carlo realizations of the merger hierarchy, from early times to the present time, in a Λ cold dark matter cosmology. The sequence of minor and major mergers experienced by galactic haloes in their hierarchical growth affects the merger history of MBHs embedded in their nuclei. So, if the formation route for the assembly of supermassive black holes dates back to the early universe, a large number of black hole (BH) interactions is inevitable. The coalescence time-scales of binary black holes can be long enough for a third BH to fall in and interact with the central binary. These BH triple interactions lead typically to the final expulsion of one of the three bodies and to the recoil of the binary. Also, asymmetric emission of gravitational waves in the last stages of the BH merging can give a recoil velocity to the centre of mass of the coalescing binary. This scenario leads to the prediction of a population of intermediate-mass black holes (IMBHs) wandering in galaxy haloes at the present epoch. We compute the luminosity distribution produced by these IMBHs accreting from their circumstellar medium. We find that in a Milky Way-sized galaxy they are unable to account for sources with luminosities ≳ 1039 erg s−1, unless they carry a baryonic remnant from which they are able to accrete for a long time. We also find that, for typical spiral galaxies, the bright end of the point-source distribution correlates with the mass of the galaxy and the most luminous sources are expected to be found in the disc.

Population X: Are the super-Eddington X-ray sources beamed jets in microblazars or intermediate mass black holes?

Recent x-ray observations reveal an increasing number of x-ray sources in nearby galaxies exceeding luminosities of Lx >~ 2 x 10^39 erg s-1. Assuming isotropic emission, the Eddington limit suggests a population of intermediate-mass black holes of M >> 10 Msun. However, Markoff et al. proposed that jets may be contributing to the x-ray emission from x-ray binaries (XRBs), implying that some x-ray sources may be relativistically beamed. This could reduce the required black hole masses to standard values. To test this hypothesis, we investigate a simple x-ray population synthesis model for x-ray point sources in galaxies with relativistic beaming and compare it with an isotropic emission model. The model is used to explain a combined data set of x-ray point sources in nearby galaxies. We show that the current distributions are consistent with black hole masses M < ~10 Msun and bulk Lorentz factors for jets in microquasars of gammaj ~ 5. Alternatively, intermediate mass black holes up to 1000 Msun are required which are distributed in a powerlaw with roughly (dN)/(dM) ~ M^-2.