Astronomers studying black-hole-powered galaxies called blazars have uncovered a surprising link between two very different wavelengths of the light these extremely massive objects emit.
Blazars are the most common sources detected by NASA’s Fermi Gamma-ray Space Telescope. As matter falls toward the supermassive black hole at the galaxy’s center, some of it is accelerated outward at nearly the speed of light along jets pointed in opposite directions. When one of the jets happens to be aimed in the direction of Earth, as illustrated here, the galaxy appears especially bright and is classified as a blazar. Image credit: M. Weiss / CfA.
The discovery, which was accomplished by comparing data from NASA’s Wide-field Infrared Survey Explorer (WISE) and Fermi Gamma-ray Space Telescope, has enabled Dr. Francesco Massaro from the University of Turin in Italy and Dr. Raffaele D’Abrusco from the Harvard-Smithsonian Center for Astrophysics to uncover dozens of new blazar candidates.
The team shows for the first time that the mid-infrared colors of blazars in WISE data correlate to an equivalent measurement of their gamma-ray output.
“This connection links two vastly different forms of light over an energy range spanning a factor of 10 billion,” Dr. Massaro said.
“Ultimately, it will help us decipher how supermassive black holes in these galaxies manage to convert the matter around them into vast amounts of energy.”
Blazars constitute more than half of the discrete gamma-ray sources seen by Fermi.
At the heart of a blazar lies a supermassive black hole of millions solar masses surrounded by a disk of hot gas and dust.
As material in the disk falls toward the black hole, some of it forms dual jets that blast subatomic particles straight out of the disk in opposite directions at nearly the speed of light.
A blazar appears bright to Fermi for two reasons. Its jets produce many gamma rays, the highest-energy form of light, and we happen to be viewing the galaxy face on, which means one of its jets is pointing in our direction.
An analysis of blazar properties observed NASA’s WISE and Fermi space telescopes reveal a correlation in emissions from the mid-infrared to gamma rays, an energy range spanning a factor of 10 billion. When plotted by gamma-ray and mid-infrared colors, confirmed Fermi blazars (gold dots) form a unique band not shared by other sources beyond our galaxy. A blue line marks the best fit of these values. The relationship allows astronomers to identify potential new gamma-ray blazars by studying WISE infrared data. Image credit: NASA’s Goddard Space Flight Center / Francesco Massaro, University of Turin.
In 2010, WISE mapped the entire sky in four infrared wavelengths, cataloging more than half a billion sources.
A year later, the astronomers began using WISE data to investigate Fermi blazars.
“WISE made it possible to explore the mid-infrared colors of known gamma-ray blazars,” Dr. D’Abrusco said.
“We found that when we plotted Fermi blazars by their WISE colors in a particular way, they occupied a distinctly different part of the plot than other extragalactic gamma-ray sources.”
According to the team, the electrons, protons and other particles accelerated in blazar jets leave a specific ‘fingerprint’ in the infrared light they emit.
This same pattern is also clearly evident in their gamma rays.
The relationship effectively connects the dots for blazars across an enormous swath of the electromagnetic spectrum.
The findings were published in the August 10 issue of the Astrophysical Journal.
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F. Massaro & R. D’Abrusco. 2016. The Infrared-Gamma-Ray Connection: A WISE View of the Extragalactic Gamma-Ray Sky. ApJ 827, 67; doi: 10.3847/0004-637X/827/1/67
