Science and Reason: Intermediate mass black holes

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. (There's 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 don't 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 there's also a report of an object much closer, in the globular cluster M54 (more here), which is only about 87,000 light-years away. It's 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.