free debate

May 3, 2010

Have Astronomers Found the Infamous Intermediate Black Holes?

Image credit: X-ray: NASA/CXC/UA/J.
Irwin et al. Optical: NASA/STScI
When a large star several times the size of our sun explodes as a supernova, it can create a stellar mass black hole with about the same mass as that star. At the center of galaxies, we find strong evidence for supermassive black holes that are millions or billions of times the mass as our own sun. The theory has been that stellar mass black holes may merge together over time to create the supermassive black holes, but there's one problem. If this theory is correct, shouldn't there be lots of intermediate sized black holes wandering around? So the hunt has been on and now a team of astronomers think they may have found some.

First a little more background. X-ray light is produced by high energy processes. Stars can produce some X-rays, supernova can produce large bursts of X-rays, and supermassive black holes can be some of the brightest objects in the X-ray spectrum. This is because they can form a disk of material that backs up on its way into the black hole, like water backing up on its way down a drain pipe. The particles in that disk of material, called an accretion disk, start bumping into each other creating heat and X-rays (as well as other forms of radiation). ULX stands for Ultraluminous X-ray source. A ULX puts of X-rays in amounts between what we see from stars and what we see from supermassive black holes at the centers of galaxies.

What causes these ULX is a mystery. It would be easy to assume that they are the long searched for intermediate black holes, but to make that assumption without other evidence would be an argument from ignorance. So some researchers have been looking for the smoking gun that will connect intermediate black holes to these ULX. Now a team says that they have evidence of a intermediate black hole creating a ULX by tidally shredding an old star.

The team took a spectrum of this ULX that was located in a globular cluster, a very old dense collection of stars on the edge of the galaxy. The elements they observed in the spectrum were unusual for globular clusters. Large amounts of oxygen and nitrogen were detected, but a serious lack of hydrogen. "We think these unusual signatures can be explained by a white dwarf that strayed too close to a black hole and was torn apart by the extreme tidal forces," said coauthor Joel Bregman of the University of Michigan. A white dwarf is a star around the same mass as our sun at the end of its life. That would explain the large amounts of oxygen and go a long way in explaining the lack of hydrogen. The presence of nitrogen still remains a mystery.

So is this the smocking gun evidence that astrophysicists have been looking for? I would say not yet. This is very suggestive and needs further study. If the researchers are correct then this object should begin to fade over time as the star is consumed. If the astronomers are correct they have found what is almost certainly a very rare object. I still want to know where all this nitrogen is coming from. This is an extremely exciting result for one or our greatest questions in astrophysics, but I don't think we should close the book on this one quite yet.

For more information I recommend the NASA page