There Are No Black Holes in NGC 6397 (Astronomy)

Globular clusters (GC’s) are old and dense star systems in the Galaxy halo and bulge. Their average age is almost equal to the age of our Universe. They are hosting a wide range of stellar phenomena, particularly involving compact objects. Recently, analyzing a heightened central velocity dispersion in NGC 6397, Vitral & Mamon detected a central dark population, which they suggested may be a subcluster of stellar-mass black holes (BHs) of total mass 10³ M within the central 6 arcsec (0.07 pc). Now, Nicholas Rui and colleagues showed that there are no stellar or intermediate black holes in NGC 6397. Instead, it contains heavy white dwarfs with a mean mass ≈ 1 M.

No stellar-mass and intermediate mass BH population in NGC 6397

Massive star evolution produces hundreds to thousands of BHs in typical GCs, most of which (unlike neutron stars) are initially retained. These BHs quickly mass-segregate (inclined to sink) to the cluster core, assembling a dense central BH subsystem. Three-body encounters within this BH-dominated core produce many dynamically-hard BH binaries, which then provide energy to passing stars through scattering interactions. This further hardens the binaries while heating the rest of the cluster. BHs undergoing these binary-mediated encounters receive significant recoil kicks that displace them away from the core (and eventually may eject them from the cluster altogether); they further heat the cluster through dynamical friction while sinking back to the core. Overall, the dynamics of BHs acts as a strong energy source in a process we call “BH burning”. This process is well-understood and well-supported by a wide scientific consensus.

The enclosed mass of white dwarfs and neutron stars versus projected radius for the best-fitting NGC 6397 model found in the CMC Cluster Catalog. Solid curves bounded by shaded regions indicate the expectation value and spread from all possible projections. In contrast to black holes, white dwarfs are a natural and obvious “dark” population contributing 10³ M (black dashed line) within 6 arcsec (green dotted line) of the cluster center, in concordance with a recent analysis of NGC 6397 data by Vitral & Mamon. Not shown here are black holes, as this model for NGC 6397 contains only a single remaining black hole, of mass 17.4 M. © Nicholas et al

The energy generated by BH burning inflates the cluster and supports it against gravothermal contraction. Hence, clusters that retain sizable BH populations today exhibit large core radii with flat central surface brightness profiles (well fit by King models) and reduced mass segregation in the luminous stellar populations. BH burning is so powerful that star clusters exceeding a critical mass fraction in BHs may even evolve towards 100% BH clusters, after ejecting their entire luminous stellar populations. Importantly, a star cluster will only evolve towards a traditional “corecollapsed” surface brightness profile after almost all BHs have been ejected. Since NGC 6397 is core-collapsed, Nicholas Rui and colleagues therefore expect that it may have very few, if any, BHs at present (the model illustrated in Fig. 1 contains just one BH of mass 17.4 M in NGC 6397). Thus, this eliminates BHs as a plausible explanation for the central dark population in this cluster.

For similar reasons, they showed that the claims of intermediate-mass BHs (IMBHs) at the centers of core-collapsed GCs have not withstood follow-up studies and are almost certainly incorrect. Like stellar-mass BHs, an IMBH would provide a central dynamical heat source which would act to inflate the cluster core significantly. Clusters with IMBHs are expected to resemble standard King models except within the small radius of influence of the IMBH. Searches for IMBHs in core-collapsed clusters in pursuit of the theorized cusp in the surface density are therefore misguided. On the contrary, such searches should focus on clusters which have not undergone core collapse, where a stronger case for an IMBH might be made.

Thus, their model pointed towards the presence of the white dwarf population, which is dominated by heavy white dwarfs with a mean mass ≈ 1 M; most of these are carbon-oxygen white dwarfs (86%), with a sizable minority of oxygen-neon white dwarfs (14%). They concluded that they will further explore the implications of these white dwarf subsystems in core-collapsed clusters in a forthcoming work.

Featured image: The globular cluster NGC 6397, located at a distance of approx. 7,200 light-years in the southern constellation Ara. It has undergone a “core collapse” and the central area is very dense. It contains about 400,000 stars and its age (based on evolutionary models) is 13,400 ± 800 million years. The photo is a composite of exposures in the B-, V- and I-bands obtained in the frame of the Pilot Stellar Survey with the Wide-Field-Imager (WFI) camera at the 2.2-m ESO/MPI telescope at the ESO La Silla Observatory. It was prepared and provided by the ESO Imaging Survey team. The spikes seen at some of the brighter stars are caused by the effect of overexposure (CCD “bleeding”). Credit: ESO

Reference: (1) Nicholas Z. Rui, Newlin Weatherford, Kyle Kremer, Sourav Chatterjee, Giacomo Fragione, Frederic A. Rasio, Carl L. Rodriguez, Claire S. Ye, “No Black Holes in NGC 6397”, ArXiv, pp. 1-3, 2021. (2) Eduardo Vitral and Gary A. Mamon, “Does NGC 6397 contain an intermediate-mass black hole or a more diffuse inner subcluster?”, A&A 646, A63 (2021).

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