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Black holes are controversial. (Just browse reader comments from partisans of various sorts of "alternative" astrophysical theories – which can be found at the end of many articles dealing with black holes that allow commenting by the general public.)

Nevertheless, very solid evidence has been accumulated over the years for the existence of two types of black holes: stellar-mass black holes with masses from 3 to several tens of solar masses (M_⊙), and supermassive black holes, which are vastly larger – generally millions to billions M_⊙. Concerning some of the evidence, see here.

Stellar-mass black holes are easy to explain as supernova remnants, while supermassive black holes seem to be an inseparable concomitant of the development of all galaxies.

Perhaps surprisingly, however, there has been very little evidence for the existence of black holes of intermediate mass. If such black holes exist at all, the processes that form them must be rather more unusual. Evidence for the existence of intermediate mass black holes has been reported in the past. (Theres some discussion here of possible black holes of mass less than a million M_⊙.)

But because black holes, by their nature, are difficult to observe directly, and so their existence must be inferred indirectly, it has been difficult to come up with relatively unambiguous evidence. Now we have announcements of better evidence in two cases.

New Class Of Black Holes Discovered (7/1/09)

A new class of black hole, more than 500 times the mass of the Sun, has been discovered by an international team of astronomers.

The finding in a distant galaxy approximately 290 million light years from Earth is reported today in the journal Nature.

Until now, identified black holes have been either super-massive (several million to several billion times the mass of the Sun) in the centre of galaxies, or about the size of a typical star (between three and 20 Solar masses).

The new discovery is the first solid evidence of a new class of medium-sized black holes.

Important discoveries often dont come by themselves. Other researchers and teams tend to report related results at the same time. And this is no exception. The above reports concern a candidate object in a galaxy (ESO 243-49) about 290 million light-years away. But theres also a report of an object much closer, in the globular cluster M54 (more here), which is only about 87,000 light-years away. Its thought to belong, actually, not to the Milky Way itself, but rather to the Saggitarius Dwarf Elliptical Galaxy, a satellite of the Milky Way.

Density and kinematic cusps in M54 at the heart of the Sagittarius dwarf galaxy: evidence for a 10⁴ M_⊙ Black Hole?

We report the detection of a stellar density cusp and a velocity dispersion increase in the center of the globular cluster M54, located at the center of the Sagittarius dwarf galaxy (Sgr). The central line of sight velocity dispersion is 20.2 +/- 0.7 km/s, decreasing to 16.4 +/- 0.4 km/s at 2.5" (0.3 pc). Modeling the kinematics and surface density profiles as the sum of a King model and a point-mass yields a black hole (BH) mass of ~ 9400 M_⊙. However, the observations can alternatively be explained if the cusp stars possess moderate radial anisotropy.

M54

Further reading (ESO 243-49 candidate object):

Finally, an Average Black Hole (7/1/09) – ScienceNOW

New Candidates for Midsize Black Holes (7/3/09) – Sky and Telescope

An intermediate-mass black hole of over 500 solar masses in the galaxy ESO – Nature research article

XMM-Newton discovers a new class of black holes (7/1/09) – ESA press release

New Observations Suggest Mid-Size Black Holes Exist (7/1/09) – Space.com

Black holes: now available in size M (7/2/09) – Cosmos magazine

New Class of Black Hole Found? (7/1/09) – National Geographic

Astronomers Discover Medium-Sized Class of Black Holes (7/1/09) – Universe Today

Intermediate-mass black hole (7/1/09) – Science Centric

Astronomers sniff intermediate mass black hole (7/2/09) – The Register

Astronomers Size Up a Candidate for Midsize Black Hole (7/1/09) – Scientific American

New Class of Black Holes Discovered (7/1/09) – Wired

X-rays are smoking gun for middleweight black holes (7/1/09) – New Scientist

A New Kind of Black Hole (7/2/09) – Smithsonian.com

Team May Have Found Intermediate Black Hole (7/6/09) – New York Times

Further reading (M54 candidate object):

Density and kinematic cusps in M54 at the heart of the Sagittarius dwarf galaxy: evidence for a 10^4 M_sun Black Hole? – Astrophysical Journal research article

Tags: black holes

It may come as a surprise to lay readers to learn that even as recently as 10 years ago there were prominent physicists who still doubted the existence of black holes.

For example, theres Sir John Maddox, a trained chemist and physicist who was editor of Nature for 22 years. In a book published in 1998 (What Remains to be Discovered), he wrote, "The concept of black holes raises serious difficulties of a philosophical character." (p. 43) (Remember what C. F. Gauss said about philosophers.) And, "The habit of others in referring to black holes as "putative" seems to imply a collective uneasiness about the concept." (p. 112)

But recent skeptics of black holes are in good company. Einstein, for one, vigorously objected to the idea. Around 1935 Subrahmanyan Chandrasekhar, later a Nobel Prize winner and now celebrated, but only in his mid-20s at the time, had the audacity to argue that black holes might form from collapsed stars only 1.44 times as heavy as the Sun – and nearly had his career wrecked from Sir Arthur Eddingtons sharp criticism.

Even very recently one still sees theoretical studies – such as reported here, here, here, here, here, here – that offer alternatives to the standard relativisic model of black holes.

Nevertheless, to the consternation of skeptics, the evidence for the correctness of the standard model of black holes just continues to pile up:

Quasar tests general relativity to the limit

[T]eam leader Mauri Valtonen of Tuorla Observatory in Finland claims the work provides the first hard evidence for black holes, which are so massive that space–time is predicted to completely curve in on itself: "People refer to the concept of black holes all the time, but strictly speaking one first has to prove that general relativity holds in strong gravitational fields before we can be sure that black holes exist," he told physicsworld.com.

And actually, the more significant part of that research is its validation of general relativity in very strong gravitational fields:

Astronomers have obtained the most compelling evidence yet that massive objects dramatically warp space–time, as predicted by Einsteins general theory of relativity. Although the geometric nature of gravity was first demonstrated in 1919, when Arthur Eddington famously detected the subtle warping effect of the Sun on the light from distant stars, the new results provide the first test of Einsteins theory in much stronger gravitational fields.

Tags: black holes, general relativity

Astronomers see inside a quasar for the first time

For the first time, astronomers have looked inside quasars -- the brightest objects in the universe -- and have seen evidence of black holes.

The study lends further confirmation to what scientists have long suspected -- that quasars are made up of super-massive black holes and the super-heated disks of material that are spiraling into them.

The suspicion that quasars are powered by supermassive black holes has, until now, been based only on the fact that astrophysicists couldnt think of any other plausible explanation. The new evidence that supports this hypothesis is that it has been possible to observe what looks like a black hole accretion disk inside two quasars:

[The researchers] were able to measure the size of the so-called accretion disk around the black hole inside each quasar.

In each, the disk surrounded a smaller area that was emitting X-rays, as if the disk material was being heated up as it fell into the black hole in the center.

Further supporting their conclusion was the fact that the hightest-energy emissions (X-rays) occurred near the center of the quasar, while optical light originated much farther out. (Since gas and dust would be heated most near a central black hole.) It was possible to make these observations on two quasars only because they were magnified by gravitational lensing due to intervening massive objects, and only by by combining results from ground-based optical telescopes and the orbiting Chandra X-Ray Observatory.

Other references:

Black Holes Power the Brightest Cosmic Objects – Space.com

Quasars Under the Lens – ScienceNow (subscription rqd)

Tags: astrophysics, quasars, black holes