From University of Barcelona [Universitat de Barcelona](ES) via phys.org : “Astronomers discover an oversized black hole population in the star cluster Palomar 5”

From University of Barcelona [Universitat de Barcelona](ES)

via

phys.org

July 5, 2021

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Credit: CC0 Public Domain.

Palomar 5 is a unique star cluster. In a paper published today in Nature Astronomy, an international team of astrophysicists led by the University of Barcelona show that distinguishing features of Palomar 5 are likely the result of an oversized black hole population of more than 100 of them in the center of the cluster.

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“The number of black holes is roughly three times larger than expected from the number of stars in the cluster, and it means that more than 20% of the total cluster mass is made up of black holes. They each have a mass of about 20 times the mass of the Sun, and they formed in supernova explosions at the end of the lives of massive stars, when the cluster was still very young,” says Prof Mark Gieles, from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and lead author of the paper.

Tidal streams are streams of stars that were ejected from disrupting star clusters or dwarf galaxies. In the last few years, nearly thirty thin streams have been discovered in the Milky Way halo. “We do not know how these streams form, but one idea is that they are disrupted star clusters. However, none of the recently discovered streams have a star cluster associated with them, hence we can not be sure. So, to understand how these streams formed, we need to study one with a stellar system associated with it. Palomar 5 is the only case, making it a Rosetta Stone for understanding stream formation and that is why we studied it in detail,” explains Gieles.

The authors simulate the orbits and the evolution of each star from the formation of the cluster until the final dissolution. They varied the initial properties of the cluster until a good match with observations of the stream and the cluster was found. The team finds that Palomar 5 formed with a lower black hole fraction, but stars escaped more efficiently than black holes, such that the black hole fraction gradually increased. The black holes dynamically puffed up the cluster in gravitational slingshot interactions with stars, which led to even more escaping stars and the formation of the stream. Just before it completely dissolves—roughly a billion years from now—the cluster will consist entirely of black holes. “This work has helped us understand that even though the fluffy Palomar 5 cluster has the brightest and longest tails of any cluster in the Milky Way, it is not unique. Instead, we believe that many similarly puffed up, black hole-dominated clusters have already disintegrated in the Milky Way tides to form the recently discovered thin stellar streams,” says co-author Dr. Denis Erkal at the University of Surrey.

Gieles says, “We have shown that the presence of a large black hole population may have been common in all the clusters that formed the streams.” This is important for our understanding of globular cluster formation, the initial masses of stars and the evolution of massive stars. This work also has important implications for gravitational waves. “It is believed that a large fraction of binary black hole mergers form in star clusters. A big unknown in this scenario is how many black holes there are in clusters, which is hard to constrain observationally because we can not see black holes. Our method gives us a way to learn how many BHs there are in a star cluster by looking at the stars they eject.'”, says Dr. Fabio Antonini from Cardiff University, a co-author of the paper.

Palomar 5 is a globular cluster discovered in 1950 by Walter Baade. It is in the Serpens constellation at a distance of about 80,000 light-years, and it is one of the roughly 150 globular clusters that orbit around the Milky Way. It is older than 10 billion years, like most other globular clusters, meaning that it formed in the earliest phases of galaxy formation. It is about 10 times less massive and five times more extended than a typical globular cluster and in the final stages of dissolution.

June 3, 2002

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Sky Survey Unveils Star Cluster Shredded By The Milky Way

Albuquerque, N.M. — A team of astronomers from the Sloan Digital Sky Survey (SDSS) collaboration has discovered a spectacular stream of stellar debris emanating from a star cluster that is being torn apart by the Milky Way.

Sloan Digital Sky Survey telescope (US) at Apache Point near Sunspot, New Mexico Altitude 2,788 meters (9,147 ft.)

SDSS Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude 2,788 meters (9,147 ft).

Dr. Michael Odenkirchen and Dr. Eva Grebel from the MPG Institute for Astronomy [MPG Institut für Astronomie](DE) in Heidelberg, Germany, are presenting these findings today at the American Astronomical Society (US) meeting in Albuquerque, New Mexico. The detection of this stream, the first of its kind, supports theorists’ view that star clusters get destroyed by the tidal forces of the Milky Way. Researchers say such extended streams of tidal debris provide a new way to determine the mass distribution of the dark matter halo of our Galaxy.

The stars in the newly discovered stream are being torn from an ancient globular cluster named Palomar 5, which is located in the outer part of our Galaxy 75,000 light years away from the Sun. While typical globular clusters are massive, luminous concentrations of some hundred thousand stars, Palomar 5 by comparison looks faint and diffuse and contains only about ten thousand stars. This led astronomers to suspect that Palomar 5 might be a likely victim of the disruptive tides of the Milky Way. These “tides” arise because the Milky Way’s gravitational pull is stronger on the cluster’s near side than on the far side, thus tearing the cluster apart. However, the telltale debris from the disruption was difficult to find since it is hidden in a sea of foreground and background objects.

Using data from the SDSS and a special filtering technique, Odenkirchen and his collaborators have succeeded in making the stream of debris from Palomar 5 directly visible. “The excellent homogeneity, resolution, depth, and multi-color information of the SDSS observations have allowed us to separate faint former members of Palomar 5 from contaminating field stars and background galaxies,” says Odenkirchen, who is a postdoctoral researcher at the MPIA.

The SDSS is an international project that is creating a deep map of one quarter of the sky in five colors. The SDSS records objects up to 10 million times fainter than the faintest stars visible with the naked eye. The observations are carried out with a special wide-field camera on a dedicated 2.5-meter telescope at Apache Point Observatory, New Mexico. Team member Dr. Connie Rockosi of the University of Washington (US) was one of the builders of the camera.

First direct evidence for the tidal disruption of Palomar 5 emerged two years ago from SDSS commissioning data that happened to include Palomar 5. Odenkirchen and collaborators were amazed to recover the characteristic S-shape signature of tidal debris from these data. “This was the first time that tidal tails of a star cluster were seen with convincing clarity”, says Grebel, an astronomer who leads the Galactic structure group at MPIA.

Meanwhile the SDSS has scanned a much larger region on the sky. Analyzing the new data the researchers found that the two tails emanating from Palomar 5 extend over an arc of ten degrees on the sky. This vast area corresponds to 20 times the diameter of the full moon on the sky or to a length of 13,000 light years in space. “Remarkably, we now find more mass in the tails than in the remaining cluster. We expect to detect the stream over an even larger area as the survey progresses,” Odenkirchen said.

The tails of Palomar~5 delineate the orbital path of this cluster and thus provide a unique opportunity to determine its motion around the Milky Way. “The motions of objects orbiting the Galactic halo are still poorly known. It normally takes decades to measure even only the instantaneous displacement of a globular cluster on the sky,” Grebel points out. “Finding additional coherent streams that extend over large portions of the sky we would be able to reconstruct Galactic orbits independent of a specific Galactic model,” says the MPIA’s Dr. Walter Dehnen, who carried out extensive numerical simulations on the disruption of Palomar~5. The researchers expect that the geometry and the velocities of those tidal streams will become important tools for determining the mass of the dark matter halo of the Milky Way.

Together with the so-called Sagittarius stream, which emerges from a dwarf galaxy that is currently being accreted by the Milky Way, there are now two different examples of extended stream-like structures in the Galactic halo. Computer simulations suggest that globular clusters were much more numerous in the early days of the Milky Way, and that many of them have already been shredded by Galactic tides. As the survey proceeds the SDSS researchers will be able to test this prediction by searching for signs of tidal mass loss around other globular clusters. “The SDSS data base will ultimately allow us to estimate the total number of such streams,” says Professor Hans-Walter Rix, director of the MPIA. “This will clarify the role of tidal disruption in the build-up of the Galactic halo and provide a crucial test for galaxy formation models.

The researchers participating in this work are Michael Odenkirchen, Eva Grebel, Walter Dehnen, and Hans-Walter Rix from the Max Planck Institute for Astronomy, Connie Rockosi of the University of Washington, Brian Yanny from DOE’s Fermi National Accelerator Laboratory (US), and Heidi Newberg from the Rensselaer Polytechnic Institute (US).
The SDSS is a joint project of The University of Chicago (US), Fermilab, the Institute for Advanced Study (US), the Japan Participation Group, The Johns Hopkins University (US), DOE’s Los Alamos National Laboratory (US), the Max-Planck Institute for Astronomy (MPIA), the MPG Institute for Astrophysics [MPG Institut für Astrophysik](DE), New Mexico State University (US), Princeton University (US), the United States Naval Observatory, the University of Pittsburgh (US), and the University of Washington.
Funding for the SDSS has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society.

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Fig. 1: Color-coded map of the distribution of stars emerging from the star cluster Palomar 5 (white blob). The two long tidal tails (orange) contain 1.3 times the mass of the cluster and delineate its orbit around the Milky Way (yellow line).

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