Hypercompact stellar system
Encyclopedia
Star cluster
Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...
around a supermassive black hole
Supermassive black hole
A supermassive black hole is the largest type of black hole in a galaxy, in the order of hundreds of thousands to billions of solar masses. Most, and possibly all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.Supermassive black holes have...
that has been ejected from the centre of its host galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
. Stars that are close to the black hole at the time of the ejection will remain bound to the black hole after it leaves the galaxy, forming the HCSS.
The term "hypercompact" refers to the fact HCSSs are small in size compared with ordinary star clusters of similar luminosity. This is because the gravitational force from the supermassive black hole keeps the stars moving in very tight orbits about the centre of the cluster.
No HCSS has yet been found, although they are believed to be fairly common. Finding an HCSS would confirm the theory of gravitational wave
Gravitational wave
In physics, gravitational waves are theoretical ripples in the curvature of spacetime which propagates as a wave, traveling outward from the source. Predicted to exist by Albert Einstein in 1916 on the basis of his theory of general relativity, gravitational waves theoretically transport energy as...
recoil, and would prove that supermassive black holes can exist outside of galaxies.
Properties
Astronomers believe that supermassive black holes (SMBHs) can be ejected from the centres of galaxies by gravitational wave recoil. This happens when two SMBHs in a binary system coalesce, after losing energy in the form of gravitational waveGravitational wave
In physics, gravitational waves are theoretical ripples in the curvature of spacetime which propagates as a wave, traveling outward from the source. Predicted to exist by Albert Einstein in 1916 on the basis of his theory of general relativity, gravitational waves theoretically transport energy as...
s. Because the gravitational waves are not emitted isotropically, some momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
is imparted to the coalescing black holes, and they feel a recoil, or "kick," at the moment of coalescence. Computer simulations
Numerical relativity
Numerical relativity is one of the branches of general relativity that uses numerical methods and algorithms to solve and analyze problems. To this end, supercomputers are often employed to study black holes, gravitational waves, neutron stars and many other phenomena governed by Einstein's Theory...
suggest that the kick can be as large as 10000 kilometres per second,
which exceeds escape velocity
Escape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...
from the centres of even the most massive galaxies.
Stars that are orbiting around the SMBH at the moment of the kick will be dragged along with the SMBH, providing their orbital velocity exceeds the kick velocity Vk. This is what determines the size of the HCSS: its radius is roughly the radius of the orbit that has the same velocity around the SMBH as the kick velocity, or
where
is the mass of the SMBH. The size 
works out to be roughly one-half parsecParsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
(pc) (two light years) for a kick of 1000 km/s and a SMBH mass of 100 million solar masses. The largest HCSSs would have sizes of about 20 pc, roughly the same as a large globular cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
, and the smallest would be about a thousandth of a parsec across, smaller than any known star cluster.
The number of stars that remain bound to the SMBH after the kick depends both on Vk, and on how densely the stars were clustered about the SMBH before the kick. A number of arguments suggest that the total stellar mass would be roughly 0.1% of the mass of the SMBH or less. The biggest HCSSs would carry perhaps a few million stars, making them comparable in luminosity to a globular cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
or ultra-compact dwarf galaxy.
Aside from being very compact, the main difference between an HCSS and an ordinary star cluster is the much greater mass of the HCSS, due to the SMBH at its centre. The SMBH itself is dark and undetectable, but its gravity causes the stars to move at much higher velocities than in an ordinary star cluster. Normal star clusters have internal velocities of a few kilometres per second, while in an HCSS, essentially all the stars are moving faster than
, i.e. hundreds or thousands of kilometres per second.If the kick velocity is less than the escape velocity from the galaxy, the SMBH will fall back toward the galaxy nucleus, oscillating many times through the galaxy before finally coming to rest. In this case, the HCSS would only exist as a distinct object for a relatively short time, of order hundreds of millions of years, before disappearing back into the galaxy nucleus. During this time the HCSS would be difficult to detect since it would be superposed on or behind the galaxy.
Even if an HCSS escapes from its host galaxy, it will remain bound to the group or cluster
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...
that contains the galaxy, since the escape velocity from a cluster of galaxies is much larger than that from a single galaxy. When observed, the HCSS will be moving more slowly than
, since it will have climbed out through the gravitational potential wellPotential well
A potential well is the region surrounding a local minimum of potential energy. Energy captured in a potential well is unable to convert to another type of energy because it is captured in the local minimum of a potential well...
of the galaxy and/or cluster.
The stars in an HCSS would be similar to the types of stars that are observed in galactic nuclei. This would make the stars in an HCSS more metal-rich and younger than the stars in a typical globular cluster.
Finding a HCSS
Since the black hole at the centre of the HCSS is essentially invisible, an HCSS would look very similar to a faint cluster of stars (see figure). Determining that an observed star cluster is a HCSS requires measuring the orbital velocities of the stars in the cluster via their Doppler shifts and verifying that they are moving much faster than expected for stars in an ordinary star cluster. This is a challenging observation to make because an HCSS would be relatively faint, requiring many hours of exposure time even on a 10m class telescope.The most promising places to look for HCSSs are cluster of galaxies
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...
, for two reasons: (1) Most of the galaxies in a galaxy cluster are elliptical galaxies
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
which are believed to have formed through mergers
Galaxy merger
Galaxy mergers can occur when two galaxies collide. They are the most violent type of galaxy interaction. Although galaxy mergers do not involve stars or star systems actually colliding, due to the vast distances between stars in most circumstances, the gravitational interactions between galaxies...
. A galaxy merger is a prerequisite for forming a binary SMBH, which is a prerequisite for a kick. (2) The escape velocity from a galaxy cluster is large enough that a HCSS would be retained even if it escaped from its host galaxy.
It has been estimated that the nearby Fornax
Fornax Cluster
At a distance of approximately 62.0 Mly , the Fornax Cluster is the second richest cluster of galaxies within 100 million light-years, although it is much smaller than the Virgo Cluster. It lies primarily in the constellation Fornax, and may be associated with the nearby Eridanus Group...
and Virgo
Virgo Cluster
The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly away in the constellation Virgo. Comprising approximately 1300 member galaxies, the cluster forms the heart of the larger Local Supercluster, of which the Local Group is an outlying member...
galaxy clusters may contain hundreds or thousands of HCSSs. These galaxy clusters have been surveyed for compact galaxies and star clusters. It is possible that some of the objects picked up in these surveys were HCSSs that were misidentified as ordinary star clusters. A few of the compact objects in the surveys are known to have rather high internal velocities, but none appear to be massive enough to qualify as HCSSs.
Another likely place to find a HCSS would be near the site of a recent galaxy merger
Galaxy merger
Galaxy mergers can occur when two galaxies collide. They are the most violent type of galaxy interaction. Although galaxy mergers do not involve stars or star systems actually colliding, due to the vast distances between stars in most circumstances, the gravitational interactions between galaxies...
.
From time to time, the black hole at the centre of an HCSS will disrupt a star that passes too close, producing a very luminous flare. A few such flares have been observed at the centres of galaxies, presumably caused by a star coming too close to the SMBH in the galaxy nucleus. It has been estimated that a recoiling SMBH will disrupt about a dozen stars during the time it takes to escape from its galaxy. Since the lifetime of a flare is a few months, the chances of seeing such an event are small unless a large volume of space is surveyed. A star in a HCSS could also explode as a Type I (white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
) supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
.
Importance
Discovery of an HCSS would be important for several reasons.- It would constitute proof that supermassive black holes can exist outside of galaxies.
- It would verify the computer simulations that predict gravitational wave recoils of thousands of kilometres per second.
- Existence of HCSSs would imply that some galaxies do not have supermassive black holes at their centres. This would have important consequences for theories that link the growth of galaxies to the growth of supermassive black holes, and for empirical correlationsM-sigma relationThe M-sigma relation is an empirical correlation between the stellar velocity dispersion \sigma of a galaxy bulge and the mass M of the supermassive black hole atthe galaxy's center.The relation can be expressed mathematically as...
between SMBH mass and galaxy properties.
- If many HCSSs could be discovered, it would be possible to reconstruct the distributionProbability distributionIn probability theory, a probability mass, probability density, or probability distribution is a function that describes the probability of a random variable taking certain values....
of kick velocities, which contains information about the merger history of galaxies, the masses and spinSpin (physics)In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
s of binary black holes, etc.
External links
- New Type of Cosmic Object to Be Revealed? National Geographic News article on hypercompact stellar systems.
- Mangled Stars Could Reveal Ejected Black Holes New Scientist article on tidal disruption flares from recoiling black holes.