A stellar-mass black hole population in the globular cluster NGC 6101? (1609.01720v1)

Abstract: Dalessandro et al. observed a similar distribution for blue straggler stars and main-sequence turn-off stars in the Galactic globular cluster NGC 6101, and interpreted this feature as an indication that this cluster is not mass-segregated. Using direct N-body simulations, we find that a significant amount of mass segregation is expected for a cluster with the mass, radius and age of NGC 6101. Therefore, the absence of mass segregation cannot be explained by the argument that the cluster is not yet dynamically evolved. By varying the retention fraction of stellar-mass black holes, we show that segregation is not observable in clusters with a high black hole retention fraction (>50% after supernova kicks and >50% after dynamical evolution). Yet all model clusters have the same amount of mass segregation in terms of the decline of the mean mass of stars and remnants with distance to the centre. We also discuss how kinematics can be used to further constrain the presence of a stellar-mass black hole population and distinguish it from the effect of an intermediate-mass black hole. Our results imply that the kick velocities of black holes are lower than those of neutron stars. The large retention fraction during its dynamical evolution can be explained if NGC 6101 formed with a large initial radius in a Milky Way satellite.

• The paper demonstrates that a high retention of stellar-mass black holes can mask expected mass segregation in NGC 6101.
• It employs NBODY6 simulations to compare models with varied black hole retention, quantifying impacts on stellar distributions.
• The findings imply that detailed kinematic analyses are essential to distinguish between stellar-mass BH populations and potential intermediate-mass black holes.

A Stellar-Mass Black Hole Population in the Globular Cluster NGC 6101?

The paper examines the globular cluster NGC 6101, challenging previous observations that suggested the cluster lacked mass segregation. By employing $N$-body simulations, the authors propose the presence of a significant stellar-mass black hole (BH) population in NGC 6101, offering an alternative explanation for the absence of observable mass segregation.

Key Findings

The paper explores the unusual features of NGC 6101, specifically the equal distribution of blue straggler stars (BSSs) and main-sequence turn-off (MSTO) stars, which typically indicates non-segregation. The authors refute the interpretation that this lack of segregation results from NGC 6101 being dynamically unevolved. Their simulations reveal:

• Mass Segregation Expectation: Considerable mass segregation is expected given the cluster's mass, radius, and age.
• High Black Hole Retention: Clusters with a high retention fraction of stellar-mass BHs (>50% after supernova kicks and post-dynamical evolution) conceal observable mass segregation similar to NGC 6101's characteristics. This suggests BHs play a significant role in the dynamics of the cluster.
• Kinematic Proxies: They examine how kinematics can distinguish between the effects of a stellar-mass BH population and an intermediate-mass black hole (IMBH).

Numerical and Theoretical Contributions

The simulations conducted illustrate comparable levels of mass segregation across clusters irrespective of BH retention. The authors employed various models with the NBODY6 integrator to substantiate their hypothesis, varying BH retention to demonstrate its influence on perceived mass dynamics.

Table Highlights:

• The initial conditions of NGC 6101 estimated via emacss indicate a survival possibility of a large BH population during its dynamic history. The substantial relaxation time contrasts with observable mass demographics.
• Three distinct $N$-body models (N0, N0.5, N1) show evolutionary retention of BHs, correlating to variations in core size and star distribution.

Implications

The retention of significant BH populations can explain the sizable core radii seen in clusters like NGC 6101 and potentially others in dwarf galaxies or extragalactic origins. This observation contrasts with conventional thinking that clusters should be devoid of significant BH populations due to BH ejection through dynamic interactions.

Future Directions

• Observational Verification: High-resolution kinematic studies are essential to ascertain or refute the presence of pervasive BH populations, specifically assessing central velocity dispersions in NGC 6101.
• Broader Application: Similar methodologies can apply to other globular clusters displaying atypical core radii or similar mass segregation profiles, potentially redefining the understanding of BH influence in stellar systems.

Conclusion

This research provides a compelling case for reassessing the role of stellar-mass BHs in globular clusters. By demonstrating the potential for significant BH retention to obscure classical markers of mass segregation, it challenges existing paradigms around cluster dynamics and evolution. The implications extend to our broader understanding of BH formation, retention, and influence across varied celestial environments.