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Register a new userIntermediate-mass black holes in Globular Clusters
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Nora L\\\"utzgendorf
Publication date
2013
fields
Physics
and research's language is
English
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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.
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Decades after the first predictions of intermediate-mass black holes (IMBHs) in globular clusters (GCs) there is still no unambiguous observational evidence for their existence. The most promising signatures for IMBHs are found in the cores of GCs, where the evidence now comes from the stellar velocity distribution, the surface density profile, and, for very deep observations, the mass-segregation profile near the cluster center. However, interpretation of the data, and, in particular, constraints on central IMBH masses, require the use of detailed cluster dynamical models. Here we present results from Monte Carlo cluster simulations of GCs that harbor IMBHs. As an example of application, we compare velocity dispersion, surface brightness and mass-segregation profiles with observations of the GC M10, and constrain the mass of a possible central IMBH in this cluster. We find that, although M10 does not seem to possess a cuspy surface density profile, the presence of an IMBH with a mass up to 0.75% of the total cluster mass, corresponding to about 600 Msun, cannot be excluded. This is also in agreement with the surface brightness profile, although we find it to be less constraining, as it is dominated by the light of giants, causing it to fluctuate significantly. We also find that the mass-segregation profile cannot be used to discriminate between models with and without IMBH. The reason is that M10 is not yet dynamically evolved enough for the quenching of mass segregation to take effect. Finally, detecting a velocity dispersion cusp in clusters with central densities as low as in M10 is extremely challenging, and has to rely on only 20-40 bright stars. It is only when stars with masses down to 0.3 Msun are included that the velocity cusp is sampled close enough to the IMBH for a significant increase above the core velocity dispersion to become detectable.
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.
The study of intermediate-mass black holes (IMBHs) is a young and promising field of research. Formed by runaway collisions of massive stars in young and dense stellar clusters, intermediate-mass black holes could still be present in the centers of globular clusters, today. Our group investigated the presence of intermediate-mass black holes for a sample of 10 Galactic globular clusters. We measured the inner kinematic profiles with integral-field spectroscopy and determined masses or upper limits of central black holes in each cluster. In combination with literature data we further studied the positions of our results on known black-hole scaling relations (such as M_bh - sigma) and found a similar but flatter correlation for IMBHs. Applying cluster evolution codes, the change in the slope could be explained with the stellar mass loss occurring in clusters in a tidal field over its life time. Furthermore, we present results from several numerical simulations on the topic of IMBHs and integral field units (IFUs). We ran N-body simulations of globular clusters containing IMBHs in a tidal field and studied their effects on mass-loss rates and remnant fractions and showed that an IMBH in the center prevents core collapse and ejects massive objects more rapidly. These simulations were further used to simulate IFU data cubes. For the specific case of NGC 6388 we simulated two different IFU techniques and found that velocity dispersion measurements from individual velocities are strongly biased towards lower values due to blends of neighbouring stars and background light. In addition, we use the Astrophysical Multipurpose Software Environment (AMUSE) to combine gravitational physics, stellar evolution and hydrodynamics to simulate the accretion of stellar winds onto a black hole.
For galaxies hosting supermassive black holes (SMBHs), it has been observed that the mass of the central black hole (M_BH) tightly correlates with the effective or central velocity dispersion (sigma) of the host galaxy. The origin of this M_BH - sigma scaling relation is assumed to lie in the merging history of the galaxies but many open questions about its origin and the behavior in different mass ranges still need to be addressed. The goal of this work is to study the black-hole scaling relations for low black-hole masses, where the regime of intermediate-mass black holes (IMBHs) in globular clusters (GCs) is entered. We collect all existing reports of dynamical black-hole measurements in globular clusters, providing black-hole masses or upper limits for 14 candidates. We plot the black-hole masses versus different cluster parameters including total mass, velocity dispersion, concentration and half-mass radius. We search for trends and test the correlations in order to quantify their significance using a set of different statistical approaches. For correlations showing a large significance we perform a linear fit, accounting for uncertainties and upper limits. We find a clear correlation between the mass of the IMBH and the velocity dispersion of the globular cluster. As expected, the total mass of the globular cluster then also correlates with the mass of the IMBH. While the slope of the M_BH - sigma correlation differs strongly from the one observed for SMBHs, the other scaling relations M_BH - M_TOT, and M_BH - L are similar to the correlations in galaxies. Significant correlations of black-hole mass with other cluster properties were not found in the present sample.
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.
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