A luminous aggregation of stars, seemingly scattered haphazardly across the celestial expanse, might conceal a remarkable secret within its core: a dense congregation of over a hundred stellar-mass black holes.
This celestial grouping, identified as Palomar 5, is a stellar stream extending approximately 30,000 light-years and situated roughly 80,000 light-years distant.
Such compact stellar collections are frequently regarded as relics from the nascent Universe. They are characterized by their extreme density and spherical form, typically housing between 100,000 and 1 million ancient stars; some, such as NGC 6397, approach the age of the Universe itself.
Within any given globular cluster, all constituent stars originated simultaneously from a singular gaseous nebula. The Milky Way hosts over 150 recognized globular clusters; these entities serve as invaluable instruments for examining, for instance, cosmic history or the distribution of dark matter within their host galaxies.
However, another category of stellar assembly is currently commanding considerable scientific interest – tidal streams, which manifest as elongated celestial currents of stars traversing the sky.
Previously, the identification of these streams posed significant challenges. Yet, with the Gaia space observatory’s precise three-dimensional mapping of the Milky Way, an increased number of these streams have been brought into clearer view.
The ensuing video provides a concise overview of this revelation:
“The formation mechanisms of these streams remain enigmatic, but one prevailing hypothesis posits that they originate from the disintegration of star clusters,” articulated astrophysicist Mark Gieles of the University of Barcelona in Spain in 2021, coinciding with the initial announcement of the detection by researchers.
“Nevertheless, none of the recently detected streams exhibit an associated star cluster, which introduces an element of uncertainty.
“Consequently, to elucidate the genesis of these streams, it is imperative to scrutinize one that possesses a linked stellar system. Palomar 5 represents a singular instance, rendering it a ‘Rosetta Stone’ for comprehending stream formation and thus the focal point of our detailed investigation.”
Palomar 5 distinguishes itself through its exceptionally broad and diffuse stellar distribution, coupled with an extensive tidal stream that spans over a remarkable 20 degrees of the sky, prompting Gieles and his research cohort to concentrate their efforts on this object.
The research team employed sophisticated N-body simulations to meticulously reconstruct the orbital trajectories and evolutionary pathways of each star within the cluster, aiming to ascertain their present-day positions.
Given recent evidence suggesting the potential presence of black hole populations within the central regions of globular clusters, and acknowledging the known propensity of gravitational interactions with black holes to eject stars at high velocities, the scientists incorporated black holes into a subset of their simulations.
Their findings indicated that a substantial population of stellar-mass black holes residing within Palomar 5 could account for its present configuration. Gravitational dynamics would have been responsible for flinging stars out of the cluster and into the tidal stream, but this scenario necessitates a considerably higher number of black holes than initially hypothesized.
The more readily and efficiently escaping stars, relative to black holes, would have consequently skewed the proportional prevalence of black holes, elevating it significantly.
“The estimated number of black holes is approximately three times greater than what would be anticipated based on the stellar population within the cluster, implying that black holes constitute more than 20 percent of the cluster’s total mass,” stated Gieles.
“Each of these objects possesses a mass roughly 20 times that of our Sun and originated from supernova explosions marking the final stages of massive stars’ lifecycles during the cluster’s early existence.”
Simulations conducted by the team project that, within approximately a billion years, the cluster will undergo complete dissolution. Just prior to this event, the residual cluster will consist exclusively of black holes in orbit around the galactic center. This projection suggests that Palomar 5 may not be an anomaly after all – it is destined to disperse entirely into a stellar stream, mirroring the fate of other streams that have already been identified.
Furthermore, this implies that other globular clusters are likely to follow a similar trajectory over time. It also reinforces the notion that globular clusters represent prime locations for the detection of colliding black holes, as well as the enigmatic class of intermediate-mass black holes, situated between the lighter stellar-mass category and the colossal supermassive black holes.
“It is theorized that a significant proportion of binary black hole mergers originate within star clusters,” commented astrophysicist Fabio Antonini of Cardiff University in the United Kingdom .
“A substantial unknown in this context is the precise quantity of black holes present in these clusters, a figure that is challenging to ascertain observationally due to our inability to directly detect black holes. Our methodology offers a means to estimate the number of black holes in a star cluster by examining the stars they eject.”
The findings of this research have been disseminated in the journal Nature Astronomy.
