Astronomers from Nanjing University in China have detected gamma ray emission from two star-forming galaxies designated M33 and Arp 299. The finding, which is detailed in a paper published March 17 on arXiv.org, could be useful in improving knowledge about the origin of very high-energy emission in galaxies.
According to Phys.org, it is thought that gamma rays in galaxies are the result of interaction of cosmic rays with the interstellar gas. Star-forming galaxies are huge reservoirs of cosmic rays and therefore could be crucial for studies of extragalactic gamma ray emission. However, the list of known star-forming galaxies detected in gamma rays is still relatively short, hence finding new ones and studying them in detail is of high importance for astronomers.
Now, a team of astronomers led by Shao-Qiang Xi reports the detection of two star-forming galaxies in gamma rays. The discovery was made as part of a search for possible gamma ray emission from galaxies in the IRAS Revised Bright Galaxies Sample, using data from NASA’s Fermi spacecraft.
“We selected our sample galaxies from the IRAS Revised Bright Galaxies Sample, excluding the 15 IR-bright galaxies that have been detected in gamma rays with Fermi–LAT and listed in Fermi–LAT Fourth Source Catalog. We performed the standard sequence of analysis steps for each galaxy, resulting in the detection of two new gamma ray sources that are, respectively, spatially coincident with M33 and Arp 299,” the astronomers wrote in the paper.
M33, or Messier 33 (also known as the Triangulum Galaxy) is the third largest galaxy in the Local Group. It is located some 2.73 million light years from the Earth in the constellation Triangulum. The researchers measured a flux of approximately 1.28 perg/cm2/s in the energy range 0.1 − 100 GeV. This value indicates a luminosity of about 1.1 duodecillion erg/s.
“Our findings improve understanding of particle acceleration in jets of microquasars, which also sheds light on jet physics in much larger and more powerful extragalactic jets in quasars,” said co-lead author Dr. Hao Zhou, a researcher in the Physics and Theoretical Divisions at Los Alamos National Laboratory.
Dr. Zhou and colleagues gathered data from (HAWC) High-Altitude Water Cherenkov Gamma-Ray Observatory, which is a mountain-top detector in Mexico that observes gamma ray emission from supernova remnants, rotating dense stars called pulsars, and quasars.
They examined the HAWC data taken from more than 1,017 days of the SS 433 observation and saw evidence that gamma rays were coming from the ends of the microquasar’s jets, rather than the central part of the star system. Based on their analysis, the astronomers concluded that electrons in the jets attain energies that are about 1,000 times higher than can be achieved using Earth-bound particle accelerators, such as the Large Hadron Collider.
The jet electrons collide with the low-energy microwave background radiation that permeates space, resulting in gamma ray emission. It is a newly observed mechanism for getting high-energy gamma rays out of this kind of system and is different from what scientists have observed when the jets are aimed at Earth.