Omega Centauri is a large globular cluster, containing almost ten million stars, in the direction of the constellation of Centaurus, which has been studied to understand its stellar kinematics, the motions of its stars under the action of the gravitational forces which act on them.
A research team at the Instituto de Astrofísica de Canarias (IAC) has just published a study which shows that a group of black holes dominates the movements of its stellar kinematics. This result can be extended to certain other structures in the universe and goes against some previous claims about the role of low-mass black holes in the motions of the stars in globular clusters. The study has recently been published in the journal Astronomy & Astrophysics, with the first author Andrés Bañares Hernández, who works in the team led by Jorge Martín Camalich.
The article is the result of an international collaboration between the IAC and the University of Surrey (Guildford, UK) and the Annecy-le-Vieux Laboratoire de Physique Théorique LAPTh in Annecy (France).
The team has carried out extensive kinematic studies to determine the structure of galaxies and star clusters in the Local Group, the galaxies closest to the Milky Way. This specific study has been of the globular cluster Omega Centauri, the largest known globular cluster in the Milky Way. A much discussed question in current astrophysical circles is whether there is an intermediate-mass black hole within this cluster (that is a black hole with a mass between a few hundred and a few hundred thousand times the mass of the Sun) and, if so, its overall effects on the cluster.
The study by the IAC, with Bañares as first author appears to have clarified this question, by discovering that what is affecting the internal motions of the stars in the cluster is not an intermediate-mass black hole, but a collection of a number of stellar-mass black holes, which form after the collapse of massive stars at the ends of their lives, and are much smaller, each one with a mass less than a few tens of solar masses.
This discovery opens up a new viewpoint in the observation of the different types of back holes and their role in stellar evolution. Until now there is good agreement that there are supermassive black holes, with masses greater than a million solar masses, found at the centres of galaxies; there is one at the centre of the Milky Way. It is also well known that there are black holes with much lower masses, stellar-mass black holes, which have been well observed within our Galaxy. Two interesting questions are “How were they produced, and what effects do they have?” Andres Bañares answers “We know that large galaxies have black holes in their centres, but at the present time we do not know for sure whether this is also true for dwarf galaxies. It is thought that Omega Centauri is a small galaxy which has been split when it merged with the Milky Way. This has made astronomers look for a central black hole in this cluster, which could perhaps explain some of its more complicated properties, which would constitute a significant advance in our understanding of its formation and evolution”.
In fact the existence of intermediate-mass black holes is not certain, because up to now observations have confirmed only the existence of stellar-mass black holes, of up to a few tens of solar masses. The existence or non-existence of the intermediate mass black holes is important because they are a missing link predicted by models of the supermassive black hole formation.
The question of the presence of an intermediate mass black hole in Omega Centauri has been debated for almost two decades, with a number of studies suggesting its presence, based on the kinematics of its stars. Whether it contains an intermediate mass black hole, or a population of stellar mass black holes and other stellar remnants has intensively researched, mainly because of the possibility that Omega Centauri is the result of a dwarf galaxy’s merging with the Milky Way.
“Our analysis is an important step in clearing up this debate, because it has allowed us to distinguish between these two possible solutions using more complete and rigorous methodology than in previous analyses, as well as more recent and new data” explains the first author.
Among the novelties in this study is use of pulsar accelerations as an additional constraint to the kinematics of the cluster. “Pulsars are neutron stars which spin at a regular frequency, emitting a signal with a very short period which we can measure very accurately. When the pulsars are part of a galaxy, or in this case of a globular star cluster, they undergo acceleration which we can measure using the variations in this periodic signal. This is a manifestation if what is known as the Doppler effect.”
Andres Bañares explains that “The formation of pulsars is also a field of active study. Due to the fact that a large number of them have recently been detected, and due to its dynamical state, Omega Centauri is an ideal environment to study models of their formation, which we have been able to do for the first time in our analysis.”
This result brings out the effectiveness of this novel methodology which, using stellar kinematics and observations of pulsars, with extensive modelling, can be used to explore the structure of star clusters, setting a promising precedent in the context of a field undergoing rapid growth of observations and discoveries.
Contacts
Jorge Martin Camalich
jcamalich [at] iac.es (jcamalich[at]iac[dot]es)
Andrés Bañares Hernández
andres.banares.hdez [at] iac.es (andres[dot]banares[dot]hdez[at]iac[dot]es)
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