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Register a new userHSTs hunt for intermediate-mass black holes in star clusters
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Authors
Julio Chaname
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Establishing or ruling out, either through solid mass measurements or upper limits, the presence of intermediate-mass black holes (IMBHs) at the centers of star clusters would profoundly impact our understanding of problems ranging from the formation and long-term dynamical evolution of stellar systems, to the nature of the seeds and the growth mechanisms of supermassive black holes. While there are sound theoretical arguments both for and against their presence in todays clusters, observational studies have so far not yielded truly conclusive IMBH detections nor upper limits. We argue that the most promising approach to solving this issue is provided by the combination of measurements of the proper motions of stars at the centers of Galactic globular clusters and dynamical models able to take full advantage of this type of data set. We present a program based on HST observations and recently developed tools for dynamical analysis designed to do just that.
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Intermediate mass black holes (IMBHs), with masses between 100 to 10^5 M_odot, represent the link between stellar mass black holes and the supermassive black holes that reside in galaxy centers. While IMBHs are crucial to our understanding of black hole seed formation, black holes of less than approx 10^4 M_odot eluded detection by traditional searches. Observations of the infrared coronal lines (CLs) offer us one of the most promising tools to discover IMBHs in galaxies. We have modeled the infrared emission line spectrum that is produced by gas photoionized by an AGN radiation field and explored for the first time the dependence of the infrared CL spectrum on black hole mass over the range of 10^2 M_odot to 10^8 M_odot. We show that infrared CLs are expected to be prominent in the spectra of accreting IMBHs and can potentially be a powerful probe of the black hole mass in AGNs. We identify key emission line ratios in the 1-30 mu m range that are most sensitive to black hole mass. While variations in accretion rate and the physical parameters of the gas can also affect the CL spectrum, we demonstrate that the effect of black hole mass is likely to be the most dramatic over the mass range explored in our models. With the unprecedented sensitivity of JWST, a large number of CLs will be detectable for the first time, providing important insight into the existence and properties of IMBHs in the local universe, potentially revolutionizing our understanding of this class of object.
Intermediate-mass black holes (IMBHs) could form via runaway merging of massive stars in a young massive star cluster (YMC). We combine a suite of numerical simulations of YMC formation with a semi-analytic model for dynamical friction and merging of massive stars and evolution of a central quasi-star, to predict how final quasi-star and relic IMBH masses scale with cluster properties (and compare with observations). The simulations argue that inner YMC density profiles at formation are steep (approaching isothermal), producing some efficient merging even in clusters with relatively low effective densities, unlike models which assume flat central profiles resembling those of globular clusters (GCs) {em after} central relaxation. Our results can be approximated by simple analytic scalings, with $M_{rm IMBH} propto v_{rm cl}^{3/2}$ where $v_{rm cl}^{2} = G,M_{rm cl}/r_{rm h}$ is the circular velocity in terms of initial cluster mass $M_{rm cl}$ and half-mass radius $r_{rm h}$. While this suggests IMBH formation is {em possible} even in typical clusters, we show that predicted IMBH masses for these systems are small, $sim 100-1000,M_{odot}$ or $sim 0.0003,M_{rm cl}$, below even the most conservative observational upper limits in all known cases. The IMBH mass could reach $gtrsim 10^{4},M_{odot}$ in the centers nuclear star clusters, ultra-compact dwarfs, or compact ellipticals, but in all these cases the prediction remains far below the present observed supermassive BH masses in these systems.
Collisions were suggested to potentially play a role in the formation of massive stars in present day clusters, and have likely been relevant during the formation of massive stars and intermediate mass black holes within the first star clusters. In the early Universe, the first stellar clusters were particularly dense, as fragmentation typically only occurred at densities above $10^9$cm$^{-3}$, and the radii of the protostars were enhanced due to the larger accretion rates, suggesting a potentially more relevant role of stellar collisions. We present here a detailed parameter study to assess how the number of collisions as well as the mass growth of the most massive object depends on the properties of the cluster, and we characterize the time evolution with three effective parameters, the time when most collisions occur, the duration of the collisions period, as well as the normalization required to obtain the total number of collisions. We apply our results to typical Population III (Pop.III) clusters of about $1000$M$_odot$, finding that a moderate enhancement of the mass of the most massive star by a factor of a few can be expected. For more massive Pop.III clusters as expected in the first atomic cooling halos, we expect a more significant enhancement by a factor of $15-32$. We therefore conclude that collisions in massive Pop.III clusters were likely relevant to form the first intermediate mass black holes.
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.
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.
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