Supermassive Black Holes

A supermassive black hole is a black hole with the mass on the order of hundreds of thousands to billions of solar masses. Most galaxies are believed to contain supermassive black holes at their centers.


This artist's concept depicts a supermassive black hole and its
accretion disk at the center of a galaxy (Credit: NASA)

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

• The average density of a supermassive black hole 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 is directly proportional to the cube of the radius, and mass merely increases linearly, the volume increases at a greater rate than mass. Thus, average density decreases for increasingly larger radii of black holes.
• 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 travelling towards the black hole center would not experience significant tidal force until very deep into the black hole.

There are several 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.

Astronomers are confident that our own Milky Way galaxy has a supermassive black hole at its center, in a region called Sagittarius A* because:

• The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter of 17 light hours from the central object.
• Early estimates indicated that the central object contains 2.6 million solar masses and has a radius of less than 17 light hours. Only a black hole can contain such a vast mass in such a small volume.
• Further observations strengthened the case for a black hole, by showing that the central object's mass is about 3.7 million solar masses and its radius no more than 6.25 light-hours.

The Max Planck Institute for Extraterrestrial Physics and UCLA Galactic Center Group have provided the strongest evidence to date that Sagittarius A* is the site of a supermassive black hole, based on data from the ESO and the Keck telescope. Our galactic central black hole is calculated to have a mass of approximately 4.1 million solar masses.


Sagittarius A* (centre) and two light echoes
from a recent explosion (Credit: NASA)

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. Should these collide, the event would create strong gravitational waves. Binary supermassive black holes are believed to be a common consequence of galaxy 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.

The Key to the Cosmos

Prof. Jim Al-Khalili investigates the atom by looking at radioactivity, the Atom Bomb and the Big Bang, and even why we are here and how we were made. He shows that, in the quest to understand the atom, the mystery of how the entire universe was created became revealed.

The Clash of the Titans - 60min Video

Nuclear physicist Jim Al-Khalili looks at what led to the discovery that everything is made of atoms. The program looks at how the discovery affected the scientific world including the atomic energy theories of Albert Einstein and Werner Heisenberg and quantum mechanics.

Exceptional Thermal Characteristics of Water

(Frozen lake on Mather Island, Antarctica; Author: Hannes Grobe, Alfred Wegener Institute)

The expansion of water with temperature is approximately linear. However, on a fine scale it is not exactly linear. In the temperature range from 0°C to 4°C water decreases in volume with increasing temperature, its coefficient of volume expansion is negative. Water also expands when it freezes, while most materials contract when they freeze. This has important consequences for plant and animal life in lakes. A lake cools at the surface, the cooler and denser water from the surface flows to the bottom. But when the air temperature drops below 4°C, the surface water is less dense than the water below, the downward flow ceases. The ice is less dense than water, and it floats. The water at the bottom always remains at 4°C unless the entire lake is frozen.

Related link: Water Structure and Science, an excellent web site on physics and chemistry of water.

The Hawking Paradox: Video

The strange and exciting mix of general relativity and quantum mechanics results in the so called black hole information paradox. The Discovery Channel show "The Hawking Paradox” explains that information lost in black holes could be saved in parallel universes where no black holes exist. See the complete 49 min video here: