We are usually taught that there are two types of black hole, the "stellar mass" black holes that are created after the death of massive stars, and the supermassive black holes which are millions or billions of times more massive and reside in the centers of most large galaxies. The search is now on for intermediate mass black holes (IMBHs) - or the "middle child" of the black hole family - but they are proving difficult to find.
IMBHs would be hundreds to tens of thousands of times the mass of the sun. These are too large to be created by a typical supernova, but too small to be called "supermassive." The presence of these black holes has been hypothesized for over thirty years, but we have yet to find a definitive case.
Some models suggest that they should be hidden in the super-dense cores of globular clusters, but these are so crowded that it is hard to see very deep. Other models suggest that they would never survive there. Some galaxies that lack a central bulge of stars may have IMBHs residing in their centers, and two of these may have been detected with radio telescopes.
Feast your eyes on the infrared, optical, and x-ray image above of M82, a nearby galaxy undergoing a burst of star formation. The inset shows observations from NASA's Chandra X-Ray Observatory, which was used along with data from ESA's XMM Newton in two papers led by Hua Feng. The two brightest point sources may be elusive IMBHs.
Astronomers made this determination based on the variability of the x-ray emission, as well as the x-ray spectrum, or how the light is spread out in energy. For one of these candidates, the variability of the x-rays and the spectrum of the disk around the black hole both indicate tens of thousands of solar masses.
We don't know what could produce a black hole of such size, except that it may be representative of a "seed" black hole that was formed early in the universe's history. Many of these are thought to have been the "seeds" of the supermassive black holes in the nuclei of galaxies today.
The other candidate intermediate mass black hole is thought to be a few hundred times the mass of the sun. This could have formed from the runaway collisions of massive stars in a cluster, which formed an unusually powerful supernova, leaving behind a big black hole. Or, many stellar-mass black holes may have collided to form this intermediate monster. Or, it could be something entirely unexpected.
Astrophysics is a tricky business. You need a certain kind of ingenuity to work with sparse data and to constantly improve your models of what processes are at work. It never is as simple as going out to "weigh" an object in space, so sometimes the answers are hard to come by. But, as I've heard in at least one of my group meetings, "if it was simple, it would have been done already."
I don't usually post to a bunch of scientific papers, but I had to dig a bit deeper to understand this story. Thanks to arxiv.org, you can dig into these papers as well!
Image Credits: Inset: X-ray: NASA/CXC/Tsinghua Univ./H. Feng et al.; Full-field:
X-ray: NASA/CXC/JHU/D.Strickland; Optical: NASA/ESA/STScI/AURA/The Hubble Heritage Team; IR: NASA/JPL-Caltech/Univ. of AZ/C. Engelbracht