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Black holes

Black holes (A Mystry)

This 1998 Hubble telescope image shows the relativistic jet shooting out from a supermassive black hole at the center of the M87 galaxy. New research has yielded insights into the jet's size and how it is formed.
Black holes are fascinating and mysterious, but they’re hard to study. Not only are they black—nothing can escape them, including light—they’re unfathomably distant. But by linking a network of radio telescopes around the country, a team of astronomers has glimpsed for the first time the environment immediately surrounding a supermassive black hole at the center of the M87 galaxy, more than 50 million light years away.
Specifically, they were able to estimate the size of the base of a galaxy-long plasma jet that shoots out from the hole on either side. It was smaller than anticipated, which suggests the black hole is spinning with incredible velocity—and that the giant ring of superheated debris whirling around it, called an accretion disc, is spinning in the same direction.
The findings were published online Thursday in the journal Science. They lend empirical support to a theoretical model of how black holes generate those plasma beams, called relativistic jets.
Researchers in the past have studied the black hole at the center of our own galaxy, the Milky Way, explains Shep Doeleman, assistant director of MIT’s Haystack observatory and lead author of the study. But that one is apparently a bit of a weakling as far as black holes go, with a mass just 4 million times that of the sun, and it doesn’t appear to produce these polar jets. The black hole at the center of M87 is 6.5 billion times the mass of the sun, and its plasma beams are marvelous.
This computer-generated image shows how the extreme gravity of the black hole distorts the jet's appearance near the event horizon.
This is the first time we’ve gotten this close of a look at any jet issuing from a black hole," Doeleman tells me. "So how these jets are formed has been a mystery. The best work has been simulations on supercomputers, which produce beautiful pictures of what jets should look like very close to a black hole." But by combining telescopes in California, Arizona, and Hawaii into essentially one huge telescope, called the Event Horizon Telescope, Doeleman’s team was able to begin piecing together a picture of what they actually look like.
In the future, the plan is to further refine that picture by expanding the Event Horizon Telescope to include other telescopes around the world. The findings reported in Science this week are "just the tip of the iceberg," Doeleman says.
　

　
　
Supermassive Black Holes (Gravity gone mad)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—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. First, 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 less than the density of water in the case of some supermassive black holes. 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, the density of a black hole is inversely proportional to the square of the mass, and thus higher mass black holes have lower average density. Also, 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.
Formation

The origin of supermassive black holes (SMBH) remains an open field of research. Astrophysicists agree that once a black hole is in place in the center of a galaxy, it can grow by accretion of matter and by merging with other black holes. There are, however, several hypotheses for the formation mechanisms and initial masses of the progenitors, or "seeds", of supermassive black holes. The most obvious hypothesis is that the seeds are black holes of tens or perhaps hundreds of solar masses that are left behind by the explosions of massive stars and grow by accretion of matter. Another model involves a large gas cloud in the period before the first stars formed collapsing into a "quasi-star" and then a black hole of initially only around ~20 solar masses, and then rapidly accreting to become relatively quickly an intermediate-mass black hole, and possibly a SMBH if the accretion-rate is not quenched at higher masses. The initial "quasi-star" would become unstable to radial perturbations because of 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 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 moments after the Big Bang. Formation of black holes from the deaths of the first stars has been extensively studied and corroborated by observations. The other models for black hole formation listed above are theoretical.

Artist’s impression of the huge outflow ejected from the quasar SDSS J1106+1939.
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. 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. This is a major component of the theory of accretion disks. Gas accretion is the most efficient, and also the most conspicuous, way in which black holes grow. The majority of the mass growth of supermassive black holes is thought to occur through episodes of rapid gas accretion, which are observable as active galactic nuclei or quasars. Observations reveal that quasars were much more frequent when the Universe was younger, indicating that supermassive black holes formed and grew early. A major constraining factor for theories of supermassive black hole formation is the observation of distant luminous quasars, which indicate that supermassive black holes of billions of solar masses had already formed when the Universe was less than one billion years old. This suggests that supermassive black holes arose very early in the Universe, inside the first massive galaxies.
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.
Some Others Belive on Formation os Supermassive Black Hole One theory is that an individual star-like black hole forms and swallows up enormous amounts of matter over the course of millions of years to produce a supermassive black hole. Another possibility is that a cluster of star-like black holes forms and eventually merges into a single, supermassive black hole. Or, a single large gas cloud could collapse to form a supermassive black hole.
Recent research suggests that galaxies and their central black holes do not grow steadily, but in fits and starts. In the beginning of a growth cycle, the galaxy and its central black hole accumulate matter. The energy generated by the jets that accompany the growth of the supermassive black hole eventually brings the in-fall of matter and the growth of the galaxy to a halt. The activity around the central black hole then ceases because of the lack of a steady supply of matter, and the jets disappear. Millions of years later the hot gas around the galaxy cools and resumes falling into the galaxy, initiating a new season of growth.This artist's concept depicts a supermassive black hole at the center of a galaxy. The blue color here represents radiation pouring out from material very close to the black hole. The grayish structure surrounding the black hole, called a torus, is made up of gas and dust. Image credit: NASA/JPL-CaltechSupermassive black hole hypothesis

Inferred orbits of 6 stars around supermassive black hole candidate Sagittarius A* at the Milky Way galactic centre.
Astronomers are confident that our own Milky Way galaxy has a supermassive black hole at its center, 26,000 light-years from the Solar System, in a region called Sagittarius A* because:
The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter (closest distance) of 17 light hours (1.8×1013 m or 120 AU) from the center of the central object.
From the motion of star S2, the object's mass can be estimated as 4.1 million solar masses.
The radius of the central object must be significantly less than 17 light hours, because otherwise, S2 would either collide with it or be ripped apart by tidal forces. In fact, recent observations indicate that the radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit.
Only a black hole is dense enough to contain 4.1 million solar masses in this volume of space.

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, or about 8.2 × 1036 kg.