The black holes of the Caldwell 5 spiral galaxy

Posted by Thomas Anderson

NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) recently spotted the brilliant glow of two black holes lurking inside spiral galaxy IC342.

Astronomers also refer to the galaxy as Caldwell 5, which is located approximately 7 million light-years away in the constellation Camelopardalis (the Giraffe). 

Previous X-ray observations of the galaxy from NASA's Chandra X-ray Observatory revealed the presence of two blinding black holes, known as ultraluminous X-ray sources (ULXs).

Nevertheless, how ULXs can shine so brilliantly is an ongoing mystery that has baffled astronomers for decades. Indeed, while these black holes are not as powerful as the supermassive black hole at the hearts of galaxies, they are at least 10 times brighter than the stellar-mass black holes peppered among the stars in our own galaxy.

Astronomers theorize that ULXs could be less common intermediate-mass black holes, with a few thousand times the mass of our sun, or smaller stellar-mass black holes in an unusually bright state. Yet a third possibility is that these black holes don't fit neatly into either category.

"High-energy X-rays hold a key to unlocking the mystery surrounding these objects," explained Fiona Harrison, NuSTAR principal investigator at the California Institute of Technology in Pasadena. "Whether they are massive black holes, or there is new physics in how they feed, the answer is going to be fascinating."

As you can see in the image above, the two bright spots that appear entangled in the arms of the IC342 galaxy are the black holes. High-energy X-ray light has been translated into the color magenta, while the galaxy itself is shown in visible light.

"Before NuSTAR, high-energy X-ray pictures of this galaxy and the two black holes would be so fuzzy that everything would appear as one pixel," said Harrison.

The second image features the well-known, historical supernova remnant Cassiopeia A, located 11,000 light-years away in the constellation Cassiopeia.

The color blue indicates the highest-energy X-ray light seen by NuSTAR, while red and green signify the lower end of NuSTAR's energy range. The blue region is where the shock wave from the supernova blast is slamming into material surrounding it, accelerating particles to nearly the speed of light.

As the particles speed up, they emit a type of light known as synchrotron radiation. NuSTAR should be able to determine how energetic the particles are, while addressing the mystery of what causes them to reach such great speeds.

"Cas A is the poster child for studying how massive stars explode and also provides us a clue to the origin of the high-energy particles, or cosmic rays, that we see here on Earth," said Brian Grefenstette of Caltech, a lead researcher on the observations. "With NuSTAR, we can study where, as well as how, particles are accelerated to such ultra-relativistic energies in the remnant left behind by the supernova explosion."

Launched last June, NuSTAR is the first orbiting telescope capable of focusing high-energy X-ray light. It is capable of viewing objects in considerably greater detail than previous telescopes operating at similar wavelengths.

The mission has already examined a wide range of extreme, high-energy objects, including black holes near and far, and the incredibly dense cores of dead stars. In addition, NuSTAR has begun black hole searches in the inner region of the Milky Way galaxy and in distant galaxies in the universe.