Gaia Discovers a Swarm of Black Holes Roaming the Milky Way

Palomar 5, a seemingly calm star cluster, may hide a swarm of over 100 stellar-mass black holes. Spanning 30,000 light-years and located 80,000 light-years away, it provides clues about star cluster evolution and tidal stream formation. Unique in its loose structure and 20-degree tidal stream, Palomar 5 contrasts with typical dense globular clusters of ancient stars. While globular clusters help study the Universe’s history and dark matter, tidal streams, mapped by the Gaia observatory, are becoming a key focus.

A Rosetta Stone for Understanding Tidal Streams

Astrophysicist Mark Gieles from the University of Barcelona described Palomar 5 as a crucial case for studying tidal streams. Unlike most streams, which lack associated star clusters, Palomar 5 combines both a loose star distribution and a prominent tidal stream, making it a “Rosetta Stone” for unraveling how these streams form.

To investigate, Gieles and his team conducted detailed N-body simulations to trace the cluster’s evolution and explain its current structure. Their models included black holes, as these objects are known to influence the dynamics of star clusters by ejecting stars through gravitational interactions.

Black Holes Driving Palomar 5’s Evolution

The team’s simulations revealed that Palomar 5 likely hosts a surprisingly large population of stellar-mass black holes. These black holes, each roughly 20 times the Sun’s mass, likely formed from supernova explosions during the cluster’s youth. Over time, gravitational interactions with these black holes propelled stars out of the cluster, forming its tidal stream.

Interestingly, the simulations suggest that black holes now make up over 20% of the cluster’s total mass, with their numbers tripling compared to initial predictions. As stars escape more readily than black holes, the proportion of black holes has steadily increased.

A Glimpse into the Future of Star Clusters

The simulations predict that Palomar 5 will completely dissolve in about a billion years, leaving behind only black holes orbiting the galactic center. This suggests that Palomar 5’s fate may not be unique; other globular clusters could also dissolve into tidal streams over time. Additionally, globular clusters may be prime locations for studying black hole populations, including binary black holes that merge, and the elusive middleweight black holes between stellar-mass and supermassive categories.

Fabio Antonini of Cardiff University highlighted how this research sheds light on black hole populations in star clusters. By studying the stars ejected by black holes, scientists can estimate their numbers. Palomar 5’s hidden black holes redefine cluster dynamics and offer new insights into black holes’ cosmic role.

Read Original Article: Science Alert

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