Supermassive Black Hole in the Milky Way Galaxy
Largest Type of Black Hole in a Galaxy

Supermassive Black Hole in the Milky Way Galaxy
Largest Type of Black Hole in a Galaxy

From a distance, our galaxy would look like a flat spiral, some 100,000 light years across, with pockets of gas, clouds of dust, and about 400 billion stars rotating around the galaxys center. Thick dust and blinding starlight have long obscured our vision into the mysterious inner regions of the galactic center.

And yet, the clues have been piling up, that something important, something strange is going on in there. Astronomers tracking stars in the center of the galaxy have found the best proof to date that black holes exist. Now, they are shooting for the first direct image of a black hole.

A supermassive black hole is the largest type of black hole in a galaxy, on the order of hundreds of thousands to billions of solar masses.

Most, if not all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.

Supermassive black holes have properties which distinguish them from lower-mass classifications:

The average density of a supermassive black hole (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) can be very low, and may actually be lower than the density of air.

This is because the Schwarzschild radius is directly proportional to mass, while density is inversely proportional to the volume.

Since the volume of a spherical object (such as the event horizon of a non-rotating black hole) is directly proportional to the cube of the radius, and mass merely increases linearly, the volume increases by a much greater factor than the mass as a black hole grows. Thus, average density decreases for increasingly larger radii of black holes (due to volume increasing much faster than mass).

The tidal forces in the vicinity of the event horizon are significantly weaker. Since the central singularity is so far away from the horizon, a hypothetical astronaut traveling towards the black hole center would not experience significant tidal force until very deep into the black hole.

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There are many models for the formation of black holes of this size. The most obvious is by slow accretion of matter starting from a black hole of stellar size.

Another model of supermassive black hole formation involves a large gas cloud collapsing into a relativistic star of perhaps a hundred thousand solar masses or larger.

The star would then become unstable to radial perturbations due to electron-positron pair production in its core, and may collapse directly into a black hole without a supernova explosion, which would eject most of its mass and prevent it from leaving a supermassive black hole as a remnant.

Yet another model involves a dense stellar cluster undergoing core-collapse as the negative heat capacity of the system drives the velocity dispersion in the core to relativistic speeds.

Finally, primordial black holes may have been produced directly from external pressure in the first instants after the Big Bang. The difficulty in forming a supermassive black hole resides in the need for enough matter to be in a small enough volume. This matter needs to have very little angular momentum in order for this to happen.

Black Hole Information Paradox

The black hole information paradox results from the combination of quantum mechanics and general relativity. It suggests that physical information could disappear in a black hole, allowing many physical states to evolve into the same state.

This is a contentious subject since it violates a commonly assumed tenet of science—that in principle complete information about a physical system at one point in time should determine its state at any other time.

A postulate of quantum mechanics is that complete information about a system is encoded in its wave function, an abstract concept not present in classical physics. The evolution of the wave function is determined by a unitary operator, and unitarity implies that information is conserved in the quantum sense.

There are two things to keep in mind here: quantum determinism, and reversibility. Quantum determinism means that given a present wave function, its future changes are uniquely determined by the evolution operator. Reversibility refers to the fact that the evolution operator has an inverse, meaning that the past wave functions are similarly unique.

With quantum determinism, reversibility, and a conserved Liouville measure, the von Neumann entropy ought to be conserved, if coarse graining is ignored.

Stephen Hawking presented rigorous theoretical arguments based on general relativity and thermodynamics which threatened to undermine these ideas about information conservation in the quantum realm. Several proposals have been put forth to resolve this paradox.

Supermassive Black Holes

A supermassive black hole is a black hole with a mass in the range of hundreds of thousands to tens of billions of solar masses.

It is currently thought that most, if not all galaxies, including the Milky Way, contain a supermassive black hole at their galactic center.

Normally the process of accretion involves transporting a large initial endowment of angular momentum outwards, and this appears to be the limiting factor in black hole growth, and explains the formation of accretion disks.

Currently, there appears to be a gap in the observed mass distribution of black holes.

There are stellar-mass black holes, generated from collapsing stars, which range up to perhaps 33 solar masses. The minimal supermassive black hole is in the range of a hundred thousand solar masses.

Between these regimes there appears to be a dearth of intermediate-mass black holes. Such a gap would suggest qualitatively different formation processes. However, some models suggest that ultraluminous X-ray sources (ULXs) may be black holes from this missing group.

It is now widely accepted that the center of nearly every galaxy contains a supermassive black hole. The close observational correlation between the mass of this hole and the velocity dispersion of the host galaxy's bulge, known as the M-sigma relation, strongly suggests a connection between the formation of the black hole and the galaxy itself.

The explanation for this correlation remains an unsolved problem in astrophysics. It is believed that black holes and their host galaxies coevolved between 300-800 million years after the Big Bang, passing through a quasar phase and developing correlated characteristics, but models differ on the causality of whether black holes triggered galaxy formation or vice versa, and sequential formation cannot be excluded.

The unknown nature of dark matter is a crucial variable in these models. At least one galaxy, Galaxy 0402+379, appears to have two supermassive black holes at its center, forming a binary system.

If they collide, the event would create strong gravitational waves. Binary supermassive black holes are believed to be a common consequence of galactic mergers.

As of November 2008, another binary pair, in OJ 287, contains the most massive black hole known, with a mass estimated at 18 billion solar masses. Currently, there is no compelling evidence for massive black holes at the centers of globular clusters, or smaller stellar systems.