Observations of the Messier 15 Globular Cluster using Multi-Unit Spectroscopic Explorer (MUSE) revealed complex central kinematics distinct from the ones observed in the clusters’ outer parts.
A Globular cluster (GC) is a collection of stars tightly bound by gravity, giving them a spherical shape and higher densities towards their center. Astronomers consider GCs as natural laboratories, enabling them to study the evolution of stars and galaxies.
Messier 15 (M15 or NGC 7078) is a Globular cluster located some 33,000 light-years from the Earth. It was discovered by Jean-Dominique Maraldi, an Italian astronomer in 1746 and is estimated to be around 13 billion light-years old. It is one of the oldest discovered global clusters. M15 has a diameter of about 175 light-years, and its core has undergone core collapse.
A team of astronomers led by Christopher Usher of the Stockholm University in Sweden used the MUSE instrument on the Very Large Telescope (VLT) to shed light on some of the earlier observations of M15. Previous studies have revealed a fast-spinning, kinematically decoupled core in the inner region and the core to be having a higher rotation and a different rotation axis. The study results were published in arXiv.org.
Using MUSE, the team measured radial velocities of 864 stars within 8 arcseconds of the M15 center, making it the largest radial velocity sample of M15 to date. The team found that M15 showcases complex central kinematics different from those observed in the outer parts. Also, the core’s rotation axis is offset from the rotation axis of the bulk of the cluster.
According to the astronomers, kinematically distinct cores (KDCs) observed in M15 are found in the centers of early galaxies in the Universe and are thought to be the result of mergers. But GC mergers are rare; therefore, the astronomers are looking for a hypothesis that could explain KDC in M15. They suspect phenomena such as post-core-collapse oscillations for the KDC in M15.
Future studies using dynamical models will shed more light on more possible hypotheses.
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