11/09/2011

Supermassive Black Holes were first theorized to exist in the 1790’s. John Michell of England and Pierre LaPlace of France independently suggested the existence of an "invisible star." Michell and LaPlace calculated the mass and size — which is now called the
"event horizon" — that an object needs in order to have an escape velocity greater than the speed of light. In 1967 John Wheeler, an American theoretical physicist, applied the term "black hole" to these collapsed objects.
It wasn’t until the early 1970’s that an intense an intense X-ray source was found in the constellation Cygnus called Cygnus X-1. As the years passed, in the spring of 1972, Cygnus X-1 was identified with a star known by its classification number HDE226868 (which is a radio source). Astronomers have since found convincing evidence for a supermassive black hole in the center of the giant elliptical galaxy M87, as well as in several other galaxies.
The discovery is based on velocity measurements of a whirlpool of hot gas orbiting the black hole. In 1994, Hubble Space Telescope data produced an unprecedented measurement of the mass of an unseen object at the center of M87. Based on the kinetic energy of the material whirling about the center (as in Wheeler's dance, see
Question 4 above), the object is about 3 billion times the mass of our Sun and appears to be concentrated into a space smaller than our solar system.

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 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 much less than the density of water (the densities are similar for 108 solar mass black hole). 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, average density decreases for larger black holes, being inversely proportional to the square of the 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. (http://en.wikipedia.org, 2011) Formation A black hole is formed when a star with 100’s of times the mass of our sun runs out of Hydrogen, which is what keeps a star burning, collapses in upon itself. The force of gravity is so great, due to the mass of the star, that nothing can stop the collapse of the core. Once this happens, the surface of the star instantly collapses to the core and the core material ejects into space, causing a massive release of energy called a Gama Ray bust. To put this in perspective, imagine the Sun shrunk down to the size of the Earth, while still retaining all of its mass. The remaining object is not a hole in space, rather an infinitely dense sphere. These as known as stellar mass black holes, and generally don’t grow much bigger than roughly 20 miles in size. Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars this usually occurs either because a star has too little "fuel" left to maintain its temperature through stellar nucleosynthesis, or because a star that would have been stable receives extra matter in a way that does not raise its core temperature. In either case the star's temperature is no longer high enough to prevent it from collapsing under its own weight (the ideal gas law explains the connection between pressure, temperature, and volume). (wikipedia.org, 2011)

Supermassive blackholes lurking at the heart of galaxies, are billions, even trillions miles in size. Many theories abound as to how supermassive black holes formed. One theory suggests that the Universe cooled enough for the first stars to form out of the original hydrogen and

helium. This was pure material, unpolluted by previous generations of stars. Astronomers

have calculated that these first stars, called Population III, would have a maximum rate that

they could gather material together to form a star. But what if there was much more gas around?

With a regular star, material comes in relatively slowly, creating a central mass. With enough

mass, the star ignites, and this creates and outward pressure that stops further material from

compacting too tightly. The problem with that theory is that you would never have a star, you

would just go from a cloud of hydrogen to a tightly bound central mass, and then a black hole.

Once there are a few solar masses of accumulated gas, the core begins to shrink under the pull of its increasing mass. The object goes through a brief period of nuclear fusion when it reaches 100 solar masses, but it passes through this phase so rapidly that it doesn’t get a chance to expand again. Eventually the object reaches several thousand solar masses, and its temperature has climbed to several hundred million degrees. At this point, gravity finally takes over, collapsing the core, and turning the object into a 10-20 solar mass black hole which then starts consuming all the mass around it. (CAIN, 2007)

Recent information suggests that these massive monsters form from galactic mergers.

Galaxies collide to form bigger galaxies which in turn cause the black holes to grow in size.

When the galaxy and black hole are still young there is so much gas and dust feeding the

black hole, that it vents jets of superheated gas into space, called “gamma ray bursts”.

These jets can be billions of light years in length. These jests are so bright that they can outshine the entire galaxy.

A new model of the evolution of galaxies and black holes show collisions show that colliding galaxies likely spawned black holes that formed about 13 billion years ago. The discovery fills in a missing chapter of our universe’s early history, and could help write the next chapter — in which scientists better understand how gravity and dark matter formed the universe as we know it. (ATKINSO, 2010)

The first supermassive black holes were formed shortly after the "Big Bang". That is the conclusion reached by an international research group led by Prof. Lucio Mayer from the University of Zurich. As the researchers write in "Nature", the supermassive black holes were formed through the collision of galaxies 13 billion years ago. The new findings are important in order to understand the origin of gravitation and cosmological structures. (L. Mayer, 2011)

Supermassive Black Holes and the Surrounding Galaxy

The supermassive black hole at the center of our galaxy, dubbed Sgr*A or Sagittarius A

because it lies in the Sagittarius near the border of the constellations Sagittarius and Scorpius,

was discovered from measuring the stars obverting the vicinity of a super bright and super

compact radio source. The star (dubbed S2) orbiting closest to the massive radio source, has

been measured at an orbit of 15 years at just 17 light hours from the source at a speed of

roughly 11,000,000 mph. The mass of the supermassive black hole is thought to be roughly 3

million times the mass of the sun and the size of Neptune’s orbit around the sun in size. As

massive as this sounds, it is rather mundane by galactic standards. Our closest neighboring

galaxy Andromeda has a mass of 4 million suns in size.

The nearby Andromeda Galaxy, 2.5 million light-years away, contains a (1.1–2.3) × 108 solar mass central black hole, significantly larger than the Milky Way's. As of November 2008, the binary pair in OJ 287, 3.5 billion light years away, contains the most massive black hole known, with a mass estimated at 18 billion solar masses. (http://en.wikipedia.org, 08)

The recent finding, that the mass of the supermassive black holes is closely related to that of the bulge, shows that the formation of supermassive black holes is also closely linked to that of the host galaxy. That is, they (the galaxies and the supermassive black holes) probably grow together. (www.nasa.gov, 2011)
Obeservational Evidence

A black hole can not been seen directly due to the fact that light can not escape the gravity well

that is a black hole. Even the massive black hole at the center of our galaxy is shrouded by

massive amounts of gas, dust and insterstellar radiation. Black holes can be obeserved indirectly

through gravational lensing. This is the distribution of matter (such as a cluster of galaxies)

between a distant source (a background galaxy) and an observer, that is capable of bending

(lensing) the light from the source, as it travels towards the observer. Another method used is the

measurement of X-ray and radio waves. A black hole emits massive amounts of energy by the

gas that is consumed as it falls onto the black hole and converted into X-rays.

Event Horizon
Contrary to myth, a black hole, will not instantly “suck” anything in that comes near it. There is
A defining threshold in spacetime called the “event horizon” that once crossed nothing, not even light can escape the gravitational pull of the black hole. Far away from the event horizon the speed of an object of particle is limited by the speed of light, and can move freely in any direction. As an object or particle of light approaches the event horizon, spacetime begins to distort slowing the object black hole.

As an object gets closer to the event horizon spacetime starts to distort slowing the speed of the object. Once crossed the gravitational pull is so great that nothing can escape. One of the most well-known examples of an event horizon derives from general relativity's description of a black hole, a celestial object so dense that no nearby matter or radiation can escape its gravitational field. Often, this is described as the boundary within which the black hole's escape velocity is greater than the speed of light. (wikipedia.org, 2011)

Singularity

The heart of a black hole is called the singularity. This is the point where physics as we know it

breaks down. The singularity is defined as a point of infinite mass and infinite density.

At the center of a black hole as described by general relativity lies a gravitational singularity, a region where the spacetime curvature becomes infinite. For a non-rotating black hole this region takes the shape of a single point and for a rotating black hole it is smeared out to form a ring singularity lying in the plane of rotation. In both cases the singular region has zero volume. It can also be shown that the singular region contains all the mass of the black hole solution. The singular region can thus be thought of as having infinite density. (http://en.wikipedia.org, 2011)

Measuring the spin of black holes

A black hole rotates at massive speeds due to the force of gravity which not only pulls on

objects, but space itself. It is thought that black holes that are consuming matter will have very

little spin. However as these objects merge, they tend to rotate faster and faster over time.

To understand these processes, astronomers must attempt to measure the rotation of black holes in different types of galaxies. Since black holes are perfectly featureless `smooth' objects, measuring their rotation is a very difficult task. They must search for the subtle effect that a rotating black hole has on its environment and, in particular, the accretion disk. So far, there are hints and suggestions that some massive black holes are indeed rapidly spinning. Unfortunately, X-ray telescopes are not yet sensitive enough to provide definitive proof that some massive black holes are rapidly spinning.

.Growth

Once a black hole is formed it continues to absorb gas and dust in the interstellar regions

surrounding it, as is the case with supermassive black holes, even growing in size by merging

with other black holes that it may come in contact with. One byproduct of this process is the

creating of an “interstellar wind” which is generated by all the matter being consumed, which in

turn pushes or blows surrounding matter out of reach of the event horizon, thereby causing the

supermassive black hole to halt its feeding process, that is unless a star or other object strays too

close to the black hole in which case it is consumed.

Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb gas and interstellar dust from its direct surroundings and omnipresent cosmic background radiation. This is the primary process through which supermassive black holes seem to have grown. A similar process has been suggested for the formation of intermediate-mass black holes in globular clusters.

Another possibility is for a black hole to merge with other objects such as stars or even other black holes. This is thought to have been important especially for the early development of supermassive black holes, which could have formed from the coagulation of many smaller objects. The process has also been proposed as the origin of some intermediate-mass black holes. (http://en.wikipedia.org, 2011)

[pic]

The graph above demonstrates how stars at the heart of our galaxy orbit the supermassive black hole. It was through observing these massive stars, the black hole was discovered. (wikipedia.org, 2011)