150 light-years away cluster could host multiple black holes

The Hyades cluster is a young open star cluster that is likely around 650 million years old. This 20-light-year-wide cluster contains hundreds of stars. Due to its close proximity to Earth, it is one of the prominent objects in the night sky, easily seen in the direction of the constellation Taurus. 

In this study, the team created computer simulations to search for black holes nestled in the Hyades cluster.

The researchers began by simulating the present movement and evolution of all the stars in the cluster. 

The results were then compared to the precise locations and trajectories of the stars acquired from data provided by the European Space Agency's Gaia spacecraft. 

In recent years, the Gaia space telescope's advancements have enabled extensive analyses of stars' precise locations and velocities inside open clusters, including Hyades. 

"Our simulations can only simultaneously match the mass and size of the Hyades if some black holes are present at the center of the cluster today (or until recently)", said Stefano Torniamenti, a postdoctoral researcher at the University of Padua and first author of the paper, in an official release.

Observations important to understanding stars' evolution

The model comparison suggests that black holes originating from Hyades may still exist or be relatively close to the cluster. The simulations indicate that there are likely two to three black holes within the Hyades star cluster. However, it's also possible that black holes were ejected from the cluster within the last 150 million years. 

If confirmed, this discovery positions them as the black holes nearest to the Sun, significantly closer than the previous contender, Gaia BH1, located 480 parsecs away from the Sun.

"This observation helps us understand how the presence of black holes affects the evolution of star clusters and how star clusters, in turn, contribute to gravitational wave sources. These results also give us insight into how these mysterious objects are distributed across the galaxy,” said Mark Gieles, a member of the UB Department of Quantum Physics and Astrophysics and host of the first author in Barcelona. 

The results have been reported in the journal Monthly Notices of the Royal Astronomical Society.

Study abstract:

Astrophysical models of binary-black hole mergers in the universe require a significant fraction of stellar-mass black holes (BHs) to receive negligible natal kicks to explain the gravitational wave detections. This implies that BHs should be retained even in open clusters with low escape velocities (≲1 km s−1). We search for signatures of the presence of BHs in the nearest open cluster to the Sun – the Hyades – by comparing density profiles of direct N-body models to data from Gaia. The observations are best reproduced by models with 2–3 BHs at present. Models that never possessed BHs have an half-mass radius ∼30  per cent smaller than the observed value, while those where the last BHs were ejected recently (≲150 Myr ago) can still reproduce the density profile. In 50  per cent of the models hosting BHs, we find BHs with stellar companion(s). Their period distribution peaks at ∼103 yr, making them unlikely to be found through velocity variations.