Mysterious Gamma Rays Traced to Galaxies Powered by Feeding Black Holes

A team of astronomers has traced hundreds of previously unidentified gamma-ray sources back to blazars, active galaxies with a feeding supermassive black hole at their center that fire jets of energy towards Earth.

The research, published in The Astronomical Journal, addresses one of the most significant challenges in high-energy gamma-ray astronomy; looking for the low-energy counterparts of unidentified gamma-ray sources.

These unidentified sources account for about a third of the objects found thus far by the Fermi Gamma-ray Space Telescope (Fermi), which was launched in 2008 and observes the sky in high energy radiation from a low-Earth orbit.

A blazar is created when gas around a supermassive black hole at the center of an active galaxy, known as an active galactic nucleus (AGN), is trapped by the gravitational influence of this object, which can have a mass as great as billions of times that of the sun.

This gas forms an accretion disk around the black hole as it is gradually "fed" to the massive central object. Before the gas crosses the event horizon of the black hole, the point at which not even light can escape, it generates so much electromagnetic radiation that AGNs are often brighter than the cumulative emissions of all the stars in the galaxy that homes it.

A blazar occurs when this highly energetic jet, including material blasted across space at 99 percent the speed of light, is angled in such a way that it is pointed directly at Earth.

"The word blazar is only a label for an active galaxy where the jet happens to point toward Earth," said University of Wisconsin-Madison professor of physics, Francis Halzen. "This of course means we are sitting right in the particle beam being spewed at us by the black hole."

The most common source of gamma rays has been identified as blazars, and this had led astronomers to believe unidentified gamma-ray sources could also be from these active galaxies which feature a central supermassive black hole greedily consuming material.

The problem is, to fully understand this connection and reveal the nature of these unidentified gamma-ray sources, astronomers have to look at candidates in visible light.

To do this a team of researchers, led by the National Institute of Astrophysics, Mexico, scientist Dr. Harold Peña Herazo, turned to the Optics, and Electronics (INAOE) Large Sky Area Multi-Object Fabre Spectroscopic Telescope (LAMOST) at the Xinglong Station in China, which has been surveying the Milky Way in visible and infrared light since 2011.

The team selected several blazar-like gamma-ray sources spotted by Fermi, known as Blazar Candidates of Uncertain type (BCUs), and searched for them in data collected by LAMOST. This allowed them to confirm many of these BCUs are indeed blazars.

"LAMOST data also permitted verifying the nature of hundreds of additional blazars by searching for emission or absorption lines used to determine their cosmological distances," Shanghai Astronomical Observatory of Chinese Academy of Sciences professor, Minfeng Gu, said in an institute press release.

Locating many of these blazars and pinpointing their distance from Earth may prove challenging, however. Astronomers usually use features in a cosmic object's spectra to obtain a distance measurement. The majority of these newly identified blazars are BL Lacertaes, a type of active galactic nucleus with featureless optical spectrums.

The Spectrum of a BL LAC
The featureless spectrum of a newly identified blazar makes it difficult to pinpoint. Fortunately, not all of the news blazars lacked signatures in their spectrums. Harold A. Peña Herazo/ESO

Fortunately, the LAMOST data reveals that a few of these newly identified blazars do have visible signatures in their optical spectrum. These features could be key to using LAMOST observations to measure the distances to these blazars

Because AGNs seem to influence the development of the galaxies in which they reside, studying blazars could be an important step towards understanding the evolution of the universe.

"I started working on this optical campaign and analyzing spectroscopic data in 2015, and nowadays, thanks to the observations available in LAMOST archive, we certainly made a significant step toward the identification of gamma-ray sources with blazars," Bologna University and INAF-OAS researcher Dr. Federica Ricci, said. "Future perspectives achievable thanks to LAMOST datasets will definitively reveal the nature of hundreds of new blazars in the years to come."

Active Galaxy
An illustration of an active galaxy as it blasts out a high energy jet. Astronomers have discovered that unexplained gamma ray sources could be these 'blazar' galaxies. M. Kornmesser/ESO