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Register a new userBlack Holes merging with Low Mass Gap Objects inside Globular Clusters
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Recently, the LIGO-Virgo collaborations have reported the coalescence of a binary involving a black hole and a low-mass gap object (LMGO) with mass in the range $sim2.5-5M_odot$. Such detections, challenge our understanding of the black hole and neutron star mass spectrum, as well as how such binaries evolve especially if isolated. In this work we study the dynamical formation of compact object pairs, via multiple binary-single exchanges that occur at the cores of globular clusters. We start with a population of binary star systems, which interact with single compact objects as first generation black holes and LMGOs. We evaluate the rate of exchange interactions leading to the formation of compact object binaries. Our calculations include all possible types of binary-single exchange interactions and also the interactions of individual stars with compact object binaries that can evolve their orbital properties, leading to their eventual merger. We perform our calculations for the full range of the observed Milky Way globular cluster environments. We find that the exchanges are efficient in forming hard compact object binaries at the cores of dense astrophysical stellar environments. Furthermore, if the population size of the LMGOs is related to that of neutron stars, the inferred merger rate density of black hole-LMGO binaries inside globular clusters in the local Universe is estimated to be about $0.1 , text{Gpc}^{-3}text{yr}^{-1}$.
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Intermediate-mass black holes (IMBHs) have masses of about 100 to 100,000 solar masses. They remain elusive. Observing IMBHs in present-day globular clusters (GCs) would validate a formation channel for seed black holes in the early universe and inform event predictions for gravitational wave facilities. Reaching a large number of GCs per galaxy is key, as models predict that only a few percent will have retained their gravitational-wave fostering IMBHs. Related, many galaxies will need to be examined to establish a robust sample of IMBHs in GCs. These needs can be meet by using a next-generation Very Large Array (ngVLA) to search for IMBHs in the GCs of hundreds of galaxies out to a distance of 25 Mpc. These galaxies hold tens of thousands of GCs in total. We describe how to convert an ngVLA signal from a GC to an IMBH mass according to a semi-empirical accretion model. Simulations of gas flows in GCs would help to improve the robustness of the conversion. Also, self-consistent dynamical models of GCs, with stellar and binary evolution in the presence of IMBHs, would help to improve IMBH retention predictions for present-day GCs.
Current theoretical models predict a mass gap with a dearth of stellar black holes (BHs) between roughly $50,M_odot$ and $100,M_odot$, while, above the range accessible through massive star evolution, intermediate-mass BHs (IMBHs) still remain elusive. Repeated mergers of binary BHs, detectable via gravitational wave emission with the current LIGO/Virgo/Kagra interferometers and future detectors such as LISA or the Einstein Telescope, can form both mass-gap BHs and IMBHs. Here we explore the possibility that mass-gap BHs and IMBHs are born as a result of successive BH mergers in dense star clusters. In particular, nuclear star clusters at the centers of galaxies have deep enough potential wells to retain most of the BH merger products after they receive significant recoil kicks due to anisotropic emission of gravitational radiation. We show that a massive stellar BH seed can easily grow to $sim 10^3 - 10^4,M_odot$ as a result of repeated mergers with other smaller BHs. We find that lowering the cluster metallicity leads to larger final BH masses. We also show that the growing BH spin tends to decrease in magnitude with the number of mergers, so that a negative correlation exists between final mass and spin of the resulting IMBHs. Assumptions about the birth spins of stellar BHs affect our results significantly, with low birth spins leading to the production of a larger population of massive BHs.
The NSFs Karl G. Jansky Very Large Array (VLA) was used at 3~cm to search for accretion signatures from intermediate-mass black holes (IMBHs) in 19 globular star clusters (GCs) in NGC,3115, an early-type galaxy at a distance of 9.4 Mpc. The 19 have stellar masses $M_{star} sim (1.1 - 2.7) times 10^6~M_odot$, with a mean $overline{M_{star}} sim 1.8 times 10^6~M_odot$. None were detected. An IMBH accretion model was applied to the individual GCs and their radio stack. The radio-stacked GCs have an IMBH mass $overline{M_{rm IMBH}} < 1.7 times 10^5~M_odot$ and mass fraction $overline{M_{rm IMBH}} / overline{M_{star}} < 9.5%$, with each limit being uncertain by a factor of about 2.5. The latter limit contrasts with the extremes of some stripped nuclei, suggesting that the set of stacked GCs in NGC,3115 is not a set of such nuclei. The radio luminosities of the individual GCs correspond to X-ray luminosities $L_{rm X} < (3.3 - 10) times 10^{38}$ erg~s$^{-1}$, with a factor of about 2.5 uncertainty. These limits predicted for putative IMBHs in the GCs are consistent with extant {em Chandra} observations. Finally, a simulated observation with a next-generation VLA (ngVLA) demonstrates that accretion signatures from IMBHs in GCs can be detected in a radio-only search, yet elude detection in an X-ray-only search due to confusion from X-ray binaries in the GCs.
For a sample of nine Galactic globular clusters we measured the inner kinematic profiles with integral-field spectroscopy that we combined with existing outer kinematic measurements and HST luminosity profiles. With this information we are able to detect the crucial rise in the velocity-dispersion profile which indicates the presence of a central black hole. In addition, N-body simulations compared to our data will give us a deeper insight in the properties of clusters with black holes and stronger selection criteria for further studies. For the first time, we obtain a homogeneous sample of globular cluster integral- field spectroscopy which allows a direct comparison between clusters with and without an intermediate-mass black hole.
Intermediate-mass black holes (IMBHs) by definition have masses of $M_{rm IMBH} sim 10^{2-5}~M_odot$, a range with few observational constraints. Finding IMBHs in globular star clusters (GCs) would validate a formation channel for massive black-hole seeds in the early universe. Here, we simulate a 60-hour observation with the next-generation Very Large Array (ngVLA) of 728 GC candidates in the Virgo Cluster galaxy NGC,4472. Interpreting the radio detection thresholds as signatures of accretion onto IMBHs, we benchmark IMBH mass thresholds in three scenarios and find the following: (1) Radio analogs of ESO,243-49 HLX-1, a strong IMBH candidate with $M_{rm IMBH}^{rm HLX} sim 10^{4-5}~M_odot$ in a star cluster, are easy to access in all 728 GC candidates. (2) For the 30 GC candidates with extant X-ray detections, the empirical fundamental-plane relation involving black hole mass plus X-ray and radio luminosities suggests access to $M_{rm IMBH}^{rm FP} sim 10^{1.7-3.6}~M_odot$, with an uncertainty of 0.44 dex. (3) A fiducial Bondi accretion model was applied to all 728 GC candidates and to radio stacks of GC candidates. This model suggests access to IMBH masses, with a statistical uncertainty of 0.39 dex, of $M_{rm IMBH}^{rm B} sim 10^{4.9-5.1}~M_odot$ for individual GC candidates and $M_{rm IMBH}^{rm B,stack} sim 10^{4.5}~M_odot$ for radio stacks of about 100-200 GC candidates. The fiducial Bondi model offers initial guidance, but is subject to additional systematic uncertainties and should be superseded by hydrodynamical simulations of gas flows in GCs.
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