# Where intermediate-mass black holes could hide in the Galactic Centre: A full parameter study with the S2 orbit

**Authors:**  The GRAVITY Collaboration, O. Straub, M. Bauböck, R. Abuter, N. Aimar, P. Amaro Seoane, A. Amorim, J. P. Berger, H. Bonnet, G. Bourdarot, W. Brandner, V. Cardoso, Y. Clénet, Y. Dallilar, R. Davies, P. T. de Zeeuw, J. Dexter, A. Drescher, F. Eisenhauer, N. M. Förster Schreiber  
**arXiv:** [2303.04067](https://arxiv.org/abs/2303.04067)  
**Date:** 2023-03-07  
**Summary page:** https://www.summarizepaper.com/en/paper/2303.04067v1/

## Key points

- The Galactic Center (GC) of the Milky Way contains a massive black hole, SgrA*, and a compact nuclear star cluster with fast-moving young stars called S-stars
- Current star formation models cannot easily explain the presence and location of these stars, leading to the suggestion that an intermediate-mass black hole (IMBH) could be present in the GC
- Researchers used GRAVITY astrometric and SINFONI, KECK, and GNIRS spectroscopic data of S2 to investigate whether a second massive object could exist deep in the GC in the form of an IMBH binary companion to SgrA*
- They explored two types of settings: a hierarchical set-up where the star S2 orbits the SgrA*-IMBH binary and a non-hierarchical set-up where the IMBH trajectory lies outside the S2 orbit
- For the hierarchical case, IMBH masses greater than 2000 Msun on orbits with smaller semi-major axes than S2 were largely excluded. In contrast, for the non-hierarchical case, there were several pockets of valid IMBH solutions within the parameter space.
- However, further analysis revealed that IMBHs on semi-major axes larger than S2 tended to disrupt the resident star cluster in less than one million years.
- The current data did not formally require an IMBH's presence; if one existed in GC, it would have to be either a low mass IMBH inside the S2 orbit that moves on a short and significantly inclined trajectory or an IMBH with a semi major axis greater than 1".
- The researchers provided parameter maps of valid IMBH solutions in GC and discussed their general structure.
- They concluded that while an IMBH could potentially exist in GC, it would have to be on a specific trajectory to avoid disrupting its surrounding star cluster.
- The study highlights how important it is to understand dynamics when dealing with moving masses in GC as well as provides insights into potential future research directions.

## Summary

The Galactic Center (GC) of the Milky Way is home to a massive black hole, SgrA*, and a compact nuclear star cluster that contains a sub-parsec concentration of fast-moving young stars called S-stars. The presence and location of these stars cannot be easily explained by current star formation models, leading to the suggestion that an intermediate-mass black hole (IMBH) could be present in the GC. To investigate this possibility, researchers used GRAVITY astrometric and SINFONI, KECK, and GNIRS spectroscopic data of S2 to study whether a second massive object could exist deep in the GC in the form of an IMBH binary companion to SgrA*. They employed a post-Newtonian framework and considered two types of settings: (i) a hierarchical set-up where the star S2 orbits the SgrA*-IMBH binary and (ii) a non-hierarchical set-up where the IMBH trajectory lies outside the S2 orbit. The team explored the full 20-dimensional parameter space using Bayesian dynamic nested sampling methods for both cases. For the hierarchical case, they found that IMBH masses greater than 2000 Msun on orbits with smaller semi-major axes than S2 were largely excluded. In contrast, for the non-hierarchical case, there were several pockets of valid IMBH solutions within the parameter space. However, further analysis revealed that IMBHs on semi-major axes larger than S2 tended to disrupt the resident star cluster in less than one million years. This made it highly unlikely for an IMBH to exist among the S-stars. The current data did not formally require an IMBH's presence; if one existed in GC, it would have to be either a low mass IMBH inside the S2 orbit that moves on a short and significantly inclined trajectory or an IMBH with a semi major axis greater than 1". The researchers provided parameter maps of valid IMBH solutions in the GC and discussed their general structure. They concluded that while an IMBH could potentially exist in GC, it would have to be on a specific trajectory to avoid disrupting its surrounding star cluster. The study highlights how important it is to understand dynamics when dealing with moving masses in GC as well as provides insights into potential future research directions.