Getting to the heart of stellar systems
GALNUC Astrophysical Dynamics and Statistical Physics of Galactic Nuclei
Recent observations have shed new light on astrophysical dynamics and the behaviour of stellar systems, now researchers aim to build on these foundations. The GalNUC project is developing sophisticated dynamical models with the wider aim of investigating the properties of dense stellar systems, as Professor Bence Kocsis explains.
Project Objectives
GALNUC strives to develop a comprehensive model to describe the long term behavior of galactic nuclei using revolutionary multidisciplanary methods. GALNUC explains the astrophysical origin of electromagnetic and gravitational waves from these systems, which host a central supermassive black hole and the densest population of stars and compact objects in the Universe.
A region at the centre of a galaxy, galactic nuclei host supermassive black holes and are densely populated with stars and other compact objects. Based at Eötvös Loránd University in Budapest, Professor Bence Kocsis and his colleagues in the GalNUC project aim to probe deeper into the nature of these stellar systems. “We are trying to understand the physical properties of these dense stellar systems,” he explains. The number of stars and other objects in these systems is much higher than in other regions of the universe. “For example, the next star beyond our own Sun is more than a light year away from the Sun. It’s 1.3 parsec away, which is approximately 4.2 light years,” outlines Professor Kocsis. “Whereas in these regions that we’re interested in, you have millions of stars within just a few light years. So that’s a very high number of stars in a very small volume.”
Dense stellar systems These stars in a dense stellar system are distributed in both spherical and counterrotating disk-like structures, a major topic of interest to Professor Kocsis. In one of these structures, stars have been observed to rotate in a clockwise direction, while in the other, another set of stars rotate in a counter-clockwise direction. “This observation was made a couple of years ago. Since then there have been new observations, and neutron stars and black holes have been discovered in the Galactic nucleus,” says Professor Kocsis. Researchers now aim to develop a model from which more can be learnt about galactic nuclei. “We’re trying to develop some simple models, and to understand which types of models lead to which type of activity,” continues Professor Kocsis. “From there, we can then look to see whether the model matches some of the features that have been observed.” This research brings together statistical physics and astrophysics. Typically statistical physics is used to understand quite smallscale objects, but Professor Kocsis says it can also be applied on astrophysical observations. “The way stars interact in the galactic centre is very similar, in a mathematical sense, to the interaction among liquid crystal molecules,” he explains. These molecules have an axisymmetric shape, and the statistical behaviour of the
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Project Funding
Overall budget: € 1 511 436
Contact Details
Black holes (shown with blue) settle in a disk in a galactic nucleus simulation.
system as a whole can be described by deriving its Hamiltonian, which helps to determine the energy of the system. “The stars in the Galactic center move around in a manner unlike bees in a beehive, their respective orbits cover disks with a geometry that resembles that of liquid crystal molecules. Liquid crystals exhibit a phenomena called phase transition, which is very interesting in physics,” says Professor Kocsis. “If the liquid crystals are cooled down to very low temperatures, then an ordered state forms.” A phase transition can be thought of as the point at which a system assumes a completely different form, such as when ice changes into water for example, while it will eventually enter into a disordered state if heated above a certain temperature. All that is needed to describe the phase transition of a system is its Hamiltonian; researchers have found that the Hamiltonian of stars orbiting in a galactic nuclei is actually very similar to that of liquid crystals. “This discovery led us to hypothesise that maybe this model would have similar phases. So maybe at low temperatures a coherent, ordered phase can exist, where the stellar orbits align in a disk, while at high temperatures the distribution will be disordered resembling a sphere,” explains Professor Kocsis. “We’re very interested in finding out what the order/disorder transition depends on, and whether this can be applied in the galactic nuclei.” Researchers have found that the transition between order and disorder depends on the mass of the object, or the population. Professor Kocsis points to the example of a stellar system with a large population of stars – some with a high mass, some with a lower mass – in which there is also a distribution of black holes. “These black holes are typically more massive than regular stars – they
The GALNUC team at the Eotvos Lorand University observatory in Budapest.
are usually somewhere between 5 and 50 solar masses,” he outlines. The more massive objects tend to settle in a more ordered state than lower mass objects. “So higher mass objects may represent ordered states and low mass objects will represent the disordered states in many cases,” continues Professor Kocsis. “This also ties in with observations, which show that highmass stars are in a disk and lower-mass stars are distributed spherically.”
Gravitational waves This work could also lead to interesting new insights into the distribution of black holes. Globular clusters, a type of star cluster which does not have a supermassive black hole at its centre, are of great interest in this respect. “Calculations show that these clusters are expanding. They lose mass, because stars are ejected beyond the escape velocity. The cluster expands, while the inner region gets denser and denser,” outlines Professor Kocsis. These stellar systems hold a lot of interest in the wider field, as it has been hypothesised that they may be the sites of stellar mergers and the mergers of black holes. “It is now possible to detect the merger of black holes using gravitational wave observations. We’re trying to make predictions about the origin of pulses of gravitational waves,” says Professor Kocsis. Researchers from the LIGO-VIRGO collaboration recently managed to detect the merger of two black holes at a distance of at least one billion light years from the Earth, now Professor Kocsis aims to build further on these findings. The aim is to make specific theoretical predictions about the types of gravitational wave sources that can be expected and at what rate, which can then be
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compared with observed data. “If the theory does not match the observations, then we will know that some important pieces are missing,” explains Professor Kocsis. Models may predict the presence of gravitational waves which have not yet been observed, yet these waves could be detected in future with more sophisticated instruments, so Professor Kocsis says theoretical models have an important role to play in informing how observatories operate. “One of the predictions we have made is that intermediate mass black holes should be observed in the future,” he outlines.
millihertz is required. This type of instrument would need different technologies than those which are currently used.”
LISA gravitational wave detector A lot of energy is currently being devoted to this work, while research is also ongoing into the development of space-based instruments to detect gravitational waves. LISA, a spacebased gravitational wave detector, is currently being developed by the European Space Agency. “This instrument is currently planned to be launched in 2034. A lot of predictions are being
The next star beyond our own Sun is more than a light year away from the Sun, it’s 1.3 parsec away. Whereas in these regions that we’re interested in, you have millions of stars within just a few light years. These black holes have a mass of above 100 solar masses, however they are not supermassive black holes, which have a much larger mass. It has also been predicted that these black holes will probably merge with other back holes. “We can even predict the rate at which they will merge as a function of redshift,” says Professor Kocsis. Current facilities are not capable of observing these objects, so the question then arises on how they could be observed in future. “The frequency of the gravitational waves that they emit are too low to be detected with the noise in current instruments,” explains Professor Kocsis. “As the noise at frequencies between 10 and 30 hertz is reduced by ongoing upgrades, these instruments may detect intermediate mass black holes. To detect supermassive black holes, an instrument capable of detecting frequencies between around 0.01 and 100
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made about its possible impact,” says Professor Kocsis. This will form an important part of Professor Kocsis’ future research agenda. “We are planning to make specific predictions for gravitational wave observatories,” he continues. “Current gravitational wave detectors – namely LIGO-VIRGO – are observing much higher numbers of mergers than originally expected. The big question is; what is the astrophysical origin of these types of mergers? What can these observatories see?” Researchers will also look to further improve the theoretical models. It has been established that there is a connection between statistical physics and astrophysics, now Professor Kocsis and his colleagues aim to build further on these findings. “We’re trying to make use of this connection, to make better and better models,” he outlines.
Project Coordinator, Bence Kocsis Assistant Professor, Department of Atomic Physics Institute of Physics Eötvös Loránd University Pázmány Péter sétány 1/A Budapest H-1117 Hungary T: +36 1-372-2500/6342 E: bkocsis@caesar.elte.hu W: http://galnuc.elte.hu/
Bence Kocsis
Bence Kocsis obtained a PhD from Eotvos University (Hungary). He has held prestigious independent postdoctoral positions at the Harvard Center for Astrophysics as a NASA Einstein Fellow and the Institute of Advanced Study Princeton. He is currently an assistant professor at Eotvos University where he leads the GALNUC project since 2015.
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