The brightest gamma-ray emission has been detected from a merging galaxy called Arp 220 by a team of scientists at the University of Oklahoma. Arp 220 is the closest ultraluminous infrared galaxy from us, about 250 million light years away from Earth.

The gamma-ray emission is believed to have come from the nucleus of Arp 220, where most activities of star formation occur. This data collected from the National Aeronautics and Space Administration’s (NASA) Fermi Gamma-Ray Space Telescope sheds light on the extreme energetic mechanisms in galaxies.

“We are very excited about this discovery,” says Xinyu Dai, a professor in the Homer L. Dodge Department of Physics and Astronomy at the University of Oklahoma. “The gamma-ray light unveils the population of extreme energetic particles in galaxies, and this discovery shows that the cosmic ray content is proportional to the luminosity of galaxies, even for the most luminous one.”


Hubble Space Telescope image of Arp 220. Credits: University of Oklahoma and NASA

Apparently, the explosion from a supernova causes the cosmic particles to accelerate into rays that collide with each other, giving way to gamma-ray emissions. The interaction of cosmic rays also gives rise to light as well as other particles.

Ultraluminous infrared galaxies and luminous infrared galaxies, such as Arp 220, are the results of galaxies combining. These two types of galaxies are considered to be the brightest among all galaxies.

“These galaxies are different because of their immense star formation and extra dust that scatters the light and makes them luminous in the infrared,” points out Rhiannon Griffin, a graduate assistant at the University of Oklahoma. “With this detection, we are expanding the range of energies used to study these galaxies.”

The centre of Arp 220 consists of over 200 massive star clusters. The biggest star cluster contains material with weight the equivalent of 10 million suns, a number twice as big as any star cluster found in the Milky Way.