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| Artist's impression of black holes in a cluster. (ESA/Hubble, N. Bartmann) |
Palomar 5 is a unique star cluster. This is firstly because it is one of the
“fluffiest” clusters in the halo of our Galaxy, with the average distance
between the stars being a few light-years, comparable to the distance from
the Sun to the nearest star. Secondly, it has a specular stellar stream
associated with it that spans more than 20 degrees across the sky. In a
paper published on July 5, 2021, in Nature Astronomy, an international team
of astronomers and astrophysicists led by the University of Barcelona show
that both distinguishing features of Palomar 5 are likely the result of an
oversized black hole population of more than 100 black holes in the center
of the cluster.
“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.
Gieles points out that in this paper “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.
Palomar 5 is a globular cluster discovered in 1950 by Walter Baade. It is in
the Serpens constellation at a distance of about 65,000 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 5 times more extended than a typical
globular cluster and in the final stages of dissolution.
Reference:
“A supra-massive population of stellar-mass black holes in the globular
cluster Palomar 5” by Mark Gieles, Denis Erkal, Fabio Antonini, Eduardo
Balbinot and Jorge Peñarrubia, 5 July 2021, Nature Astronomy. DOI:
10.1038/s41550-021-01392-2
Tags:
Space & Astrophysics