Most monster black holes lurk at the heart of massive galaxies, slurping up matter from the galactic center with a pull so strong that nothing, not even light, can escape.
But a team of European astronomers reported in the journal Nature that a particular black hole some 5 billion light-years away has no evidence of a host galaxy. A light-year is about 6 trillion miles (10 trillion km), the distance light travels in a year.
The black hole was detected when the scientists went hunting for quasars -- extremely bright, small, distant objects that are strongly associated with black holes. Astronomers believe a quasar is produced by cosmic gas as it is drawn toward the edge of a supermassive black hole.
Most quasars and black holes are in the middle of supermassive galaxies and in their survey of 20 relatively nearby quasars, the scientists found 19 followed this expected pattern. But one showed no signs of having a galactic home.
The astronomers, using the Hubble telescope and the Very Large Telescope in Chile, reported that this rogue black hole may be the result of a rare collision between a seemingly normal spiral galaxy and an exotic object harboring a very massive black hole.
One problem in quasar-hunting is that they are so bright, they outshine most galaxies that surround them, just as the headlights from an oncoming vehicle can make the vehicle hard to see. So even if a surrounding galaxy is present, it can be difficult to detect.
The European astronomers used the two telescopes to overcome this problem by comparing the quasars they were watching with a reference star. This let them differentiate the light from the quasar from the light from any possible underlying galaxy.
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On Detecting Black Holes
To be sure that an object is a black hole, we should have some means of measuring its mass. In our own galaxy, several black hole candidates are all members of binary systems, where the other object is a usual star emitting visible radiation! This enables us to measure the mass of the 'invisible' companion. In most cases, their masses turn out to be several solar masses, beyond that for neutron stars. The black hole pulls out matter from its companion star; this matter forms an accretion disc around the hole and is slowly sucked in, the vast graviational energy is converted into intense X-ray and other forms of electromagnetic radiation. More matter is pulled in when the star is closer in orbit around the hole, so that the high energy radiation (Eg - X-Rays) exhibits a periodicity corresponding to the binary period. This is the signature for a black hole.
Doughnut Around a Black Hole
An international team of astronomers have found new evidence that massive black holes are surrounded by a torus (aka a doughnut) of gas and dust that can block our view if seen edge-on. The latest observations were made using the Integral and XMM-Newton space observatories, which looked at NGC 4388; and edge-on spiral galaxy located 65 million light years away. The team was able to determine the thickness and composition of the torus by looking through it at the radiation coming out of the supermassive black hole.
(Not your usual Krispy Kreme)
Using ESA’s Integral and XMM-Newton observatories, an international team of astronomers has found more evidence that massive black holes are surrounded by a doughnut-shaped gas cloud, called a torus. Depending on our line of sight, the torus can block the view of the black hole in the centre. The team looked `edge on’ into this doughnut to see features never before revealed in such a clarity.
Black holes are objects so compact and with gravity so strong that not even light can escape from them. Scientists think that `supermassive’ black holes are located in the cores of most galaxies, including our Milky Way galaxy. They can contain the mass of thousands of millions of suns, confined within a region no larger than our Solar System. They appear to be surrounded by a hot, thin disk of accreting gas and, farther out, the thick doughnut-shaped torus.
Depending on the inclination of the torus, it can hide the black hole and the hot accretion disc from the line of sight. Galaxies in which a torus blocks the light from the central accretion disc are called `Seyfert 2’ types and are usually faint to optical telescopes. Another theory, however, is that these galaxies appear rather faint because the central black hole is not actively accreting gas and the disc surrounding it is therefore faint.
An international team of astronomers led by Dr Volker Beckmann, Goddard Space Flight Center (Greenbelt, USA) has studied one of the nearest objects of this type, a spiral galaxy called NGC 4388, located 65 million light years away in the constellation Virgo. Since NGC 4388 is relatively close, and therefore unusually bright for its class, it is easier to study.
Astronomers often study black holes that are aligned face-on, thus avoiding the enshrouding torus. However, Beckmann's group took the path less trodden and studied the central black hole by peering through the torus. With XMM-Newton and Integral, they could detect some of the X-rays and gamma rays, emitted by the accretion disc, which partially penetrate the torus. "By peering right into the torus, we see the black hole phenomenon in a whole new light, or lack of light, as the case may be here," Beckmann said.
Beckmann's group saw how different processes around a black hole produce light at different wavelengths. For example, some of the gamma rays produced close to the black hole get absorbed by iron atoms in the torus and are re-emitted at a lower energy. This in fact is how the scientists knew they were seeing `reprocessed’ light farther out. Also, because of the line of sight towards NGC 4388, they knew this iron was from a torus on the same plane as the accretion disk, and not from gas clouds `above’ or `below’ the accretion disk.
This new view through the haze has provided valuable insight into the relationship between the black hole, its accretion disc and the doughnut, and supports the torus model in several ways.
Gas in the accretion disc close to the black hole reaches high speeds and temperatures (over 100 million degrees, hotter than the Sun) as it races toward the void. The gas radiates predominantly at high energies, in the X-ray wavelengths.
According to Beckmann, this light is able to escape the black hole because it is still outside of its border, but ultimately collides with matter in the torus. Some of it is absorbed; some of it is reflected at different wavelengths, like sunlight penetrating a cloud; and the very energetic gamma rays pierce through. "This torus is not as dense as a real doughnut or a true German Krapfen, but it is far hotter - up to a thousand degrees - and loaded with many more calories," Beckmann said.
The new observations also pinpoint the origin of the high-energy emission from NGC 4388. While the lower-energy X-rays seen by XMM-Newton appear to come from a diffuse emission, far away from the black hole, the higher-energy X-rays detected by Integral are directly related to the black hole activity.
The team could infer the doughnut’s structure and its distance from the black hole by virtue of light that was either reflected or completely absorbed. The torus itself appears to be several hundred light years from the black hole, although the observation could not gauge its diameter, from inside to outside.
The result marks the clearest observation of an obscured black hole in X-ray and gamma-ray `colours’, a span of energy nearly a million times wider than the window of visible light, from red to violet. Multi-wavelength studies are increasingly important to understanding black holes, as already demonstrated earlier this year. In May 2004, the European project known as the Astrophysical Virtual Observatory, in which ESA plays a major role, found 30 supermassive black holes that had previously escaped detection behind masking dust clouds.
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