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Found: A Galaxy’s Missing Gamma Rays

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Arp 220
Recent observations have detected high-energy gamma-ray emission for the first time from the massive, star-forming galaxy Arp 220 (shown here in optical wavelengths, as imaged by Hubble). [NASA/ESA/C. Wilson (McMaster University)]

Recent reanalysis of data from the Fermi Gamma-ray Space Telescope has resulted in the first detection of high-energy gamma rays emitted from a nearby galaxy. This discovery reveals more about how supernovae interact with their environments.

Colliding Supernova Remnant

After a stellar explosion, the supernova’s ejecta expand, eventually encountering the ambient interstellar medium. According to models, this generates a strong shock, and a fraction of the kinetic energy of the ejecta is transferred into cosmic rays — high-energy radiation composed primarily of protons and atomic nuclei. Much is still unknown about this process, however. One open question is: what fraction of the supernova’s explosion power goes into accelerating these cosmic rays?

In theory, one way to answer this is by looking for gamma rays. In a starburst galaxy, the collision of the supernova-accelerated cosmic rays with the dense interstellar medium is predicted to produce high-energy gamma rays. That radiation should then escape the galaxy and be visible to us.

Pass 8 to the Rescue

Observational tests of this model, however, have been stumped by Arp 220. This nearby ultraluminous infrared galaxy is the product of a galaxy merger ~700 million years ago that fueled a frenzy of starbirth. Due to its dusty interior and extreme levels of star formation, Arp 220 has long been predicted to emit the gamma rays produced by supernova-accelerated cosmic rays. But though we’ve looked, gamma-ray emission has never been detected from this galaxy … until now.

In a recent study, a team of scientists led by Fang-Kun Peng (Nanjing University) reprocessed 7.5 years of Fermi observations using the new Pass 8 analysis software. The resulting increase in resolution revealed the first detection of GeV emission from Arp 220!

Acceleration Efficiency

scaling relation

Gamma-ray luminosity vs. total infrared luminosity for LAT-detected star-forming galaxies and Seyferts. Arp 220’s luminosities are consistent with the scaling relation. [Peng et al. 2016]

Peng and collaborators argue that this emission is due solely to cosmic-ray interactions with interstellar gas. This picture is supported by the lack of variability in the emission, and the fact that Arp 220’s gamma-ray luminosity is consistent with the scaling relation between gamma-ray and infrared luminosity for star-forming galaxies. The authors also argue that, due to Arp 220’s high gas density, all cosmic rays will interact with the gas before escaping.

Under these two assumptions, Peng and collaborators use the gamma-ray luminosity and the known supernova rate in Arp 220 to estimate how efficiently cosmic rays are accelerated by supernova remnants in the galaxy. They determine that 4.2 ± 2.6% of the supernova remnant’s kinetic energy is used to accelerate cosmic rays above 1 GeV.

This is the first time such a rate has been measured directly from gamma-ray emission, but it’s consistent with estimates of 3-10% efficiency in the Milky Way. Future analysis of other ultraluminous infrared galaxies like Arp 220 with Fermi (and Pass 8!) will hopefully reveal more about these recent-merger, starburst environments.

Citation

Fang-Kun Peng et al 2016 ApJ 821 L20. doi:10.3847/2041-8205/821/2/L20