Arquivo de maio 28, 2009

Super Massive Black Hole of the Center of Our Galaxy Via Lactea - European Spacial Agency
Supermassive black hole
 

XMM-Newton takes astronomers to a black hole’s edge

 

27 May 2009
Using new data from ESA’s XMM-Newton spaceborne observatory, astronomers have probed closer than ever to a supermassive black hole lying deep at the core of a distant active galaxy.
 
The galaxy – known as 1H0707-495 – was observed during four 48-hr-long orbits of XMM-Newton around Earth, starting in January 2008. The black hole at its centre was thought to be partially obscured from view by intervening clouds of gas and dust, but these current observations have revealed the innermost depths of the galaxy. 

“We can now start to map out the region immediately around the black hole,” says Andrew Fabian, at the University of Cambridge, who headed the observations and analysis. 

X-rays are produced as matter swirls into a supermassive black hole. The X-rays illuminate and are reflected from the matter before its eventual accretion. Iron atoms in the flow imprint characteristic iron lines on the reflected light. The iron lines are distorted in a number of characteristic ways: they are affected by the speed of the orbiting iron atoms, the energy required for the X-rays to escape the black hole’s gravitational field, and the spin of the black hole. All these features show that the astronomers are tracking matter to within twice the radius of the black hole itself.  
 

“The black hole is swallowing the equivalent of two Earths per hour.”

XMM-Newton detected two bright features of iron emission in the reflected X-rays that had never been seen together in an active galaxy. These bright features are known as the iron L and K lines, and they can be so bright only if there is a high abundance of iron. Seeing both in this galaxy suggests that the core is much richer in iron than the rest of the galaxy. 

The direct X-ray emission varies in brightness with time. During the observation, the iron L line was bright enough for its variations to be followed.
 

A painstaking statistical analysis of the data revealed a time lag of 30 seconds between changes in the X-ray light observed directly, and those seen in its reflection from the disc. This delay in the echo enabled the size of the reflecting region to be measured, which leads to an estimate of the mass of the black hole at about 3 to 5 million solar masses. 

The observations of the iron lines also reveal that the black hole is spinning very rapidly and eating matter so quickly that it verges on the theoretical limit of its eating ability, swallowing the equivalent of two Earths per hour. 

The team are continuing to track the galaxy using their new technique. There is a lot for them to study. Far from being a steady process, like water slipping down a plughole, a feeding black hole is a messy eater. “Accretion is a very messy process because of the magnetic fields that are involved,” says Fabian. 

Their new technique will enable the astronomers to map out the process in all its glorious complexity, taking them to previously unseen regions at the very edges of this and other supermassive black holes.
  
Notes for editors:
 
‘The detection of Broad Iron K and L line emission in the Narrow-Line Seyfert 1 Galaxy 1H0707-495 using XMM-Newton’, by A. Fabian et al. will be published in Nature tomorrow.

From Wikipedia, the free encyclopedia

For the Muse song, see Supermassive Black Hole (song).

Top: artist’s conception of a supermassive black hole tearing apart a star. Bottom: images believed to show a supermassive black hole devouring a star in galaxy RXJ 1242-11. Left: X-ray image, Right: optical image.[1]

A supermassive black hole is a black hole with a mass of the order of between 105 and 1010 solar masses. Most, if not all, galaxies, including the Milky Way,[2] are believed to contain supermassive black holes at their centers.[3][4]

Supermassive black holes have properties which distinguish them from their relatively low-mass cousins:

  • The average density of a supermassive black hole (measured as the mass of the black hole divided by its Schwarzschild volume) 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 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.

Formation

An artist’s conception of a supermassive black hole & accretion disk. Credit: NASA/JPL-Caltech

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 [5] 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 [6] 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. 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 [7] suggest that ultraluminous X-ray sources (ULXs) may be black holes from this missing group.

Doppler measurements

Direct Doppler measures of water masers surrounding the nucleus of nearby galaxies have revealed a very fast keplerian motion, only possible with a high concentration of matter in the center. Currently, the only known objects that can pack enough matter in such a small space are black holes, or things that will evolve into black holes within astrophysically short timescales. For active galaxies farther away, the width of broad spectral lines can be used to probe the gas orbiting near the event horizon. The technique of reverberation mapping uses variability of these lines to measure the mass and perhaps the spin of the black hole that powers the active galaxy’s “engine”.

Such supermassive black holes in the center of many galaxies are thought to be the “engine” of active objects such as Seyfert galaxies and quasars.

Milky Way galactic center black hole

Astronomers are confident that our own Milky Way galaxy has a supermassive black hole at its center, in a region called Sagittarius A*[8] because:

  • The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter (closest distance) of 17 light hours from the central object.[9]
  • 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[10] 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[11] have provided the strongest evidence to date that Sagittarius A* is the site of a supermassive black hole,[8] based on data from the ESO[12] and the Keck telescope.[13] Our galactic central black hole is calculated to have a mass of approximately 4.1 million solar masses,[14] or about 8.2 × 1036 kg.

Supermassive black holes outside the Milky Way

It is now widely accepted that the center of nearly every galaxy contains a supermassive black hole.[15][16] 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.[15]

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.[17][18]

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 [19]. As of November 2008[update], another binary pair, in OJ 287, contains the most massive black hole known, with a mass estimated at 18 billion solar masses.[20]

Currently, there is no compelling evidence for massive black holes at the centers of globular clusters, or smaller stellar systems.[citation needed]

See also