Title: An in-depth study of Gamma rays from the Starburst Galaxy M 82 with VERITAS

Authors: The VERITAS Collaboration

First Author’s institution: Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA 02138, USA

Status: Accepted to ApJ [open access]

Starbursts: not just a candy!

Cosmic rays are charged particles (electrons, protons, and nuclei) that we’ve known about for over a century, but notoriously zig and zag around the Universe, due to magnetic fields that attract them and then shoot them off in whichever direction the field goes. This makes it really hard to figure out what’s making them – especially because we detect them up to really high energies (PeV-scale) – over thousands of times more energetic than the particles accelerated at accelerators like the LHC! The mystery of what out there in the Universe is making cosmic rays at these high energies has plagued astronomers ever since we discovered that cosmic rays exist in the first place.

Luckily, there are other ways to figure out where these enigmatic particles are made, including using plain old light – namely gamma-rays, which are the particles that correspond to the highest energy light out there. Cosmic rays produce gamma rays in all sorts of different interactions with the material in the sources where they’re born (i.e., a cosmic ray source should also be a gamma-ray source!). By looking at how the gamma rays from a given astronomical object are distributed in brightness and energy (called a spectral energy distribution), we can figure out what sort of cosmic rays make the gamma rays we observe and identify the big cosmic ray factories of the Universe. 

Starburst galaxies (often just called “starbursts”), as their names imply, are galaxies that are forming stars very quickly – at rates 10-30 times higher than the Milky Way! This is probably just a phase though, since these galaxies will quickly use up all their star-forming material (i.e., gas and dust) and will need to wait for this material to be re-released in supernovae or planetary nebulae when these stars eventually die. 

We’ve long thought that starbursts might be these cosmic ray factories, as they host several high energy processes such as supernovae and stellar winds launched by massive stars, hitting the dense material in star-forming regions. Since these processes don’t occur on anywhere near the same scales in our own Milky Way galaxy, we need to look elsewhere to understand if these processes are responsible for cosmic ray production!

Today’s authors look at the nearby starburst galaxy M 82 (also known as the Cigar galaxy for its distinctive oblong shape; see Figure 1) using 335 hours of data taken over fifteen years of observations with the VERITAS gamma-ray telescopes (Figure 2). They dive deeper into M 82’s gamma rays than ever before and try to piece together what sorts of cosmic rays might be making them. If starbursts are cosmic ray factories, it’s also possible that the galaxy’s brightness can tell us directly the number of cosmic rays it’s made during star formation episodes, allowing cosmic rays to be studied much more easily than ever before!

Long time, no gammas

Today’s paper follows up on a previous attempt to study M 82 with VERITAS in 2009, which frustratingly resulted in the weakest detection VERITAS has ever reported – even after 137 hours of observations! Over a decade later, they’re back with a solid detection of gamma-ray emission from M 82 (see Figure 3) and enough data to give M 82 the attention it deserves!

It’s all in the spectrum!

Even though M 82 is twelve million light years away (or over a billion trillion kilometres), we can still learn an incredible amount of information about the subatomic particles that come from it, which are over half a million times smaller than a single human hair! The authors accomplish this by looking at the spectral energy distribution (SED; see Figure 4); how M 82’s brightness changes with the energy (or frequency) of the gamma rays and any other light we observe.

The authors use simulations of how the gamma-ray SED would look if all the gamma-rays were produced by only electrons, only protons/nuclei (together called hadrons), or a mix of both – to create models to fit the observed data. The reason for the distinction between particles is that electrons are the much less exciting cousin to protons and nuclei (sorry electrons). Electrons are light and fairly easy to accelerate but lose their energy pretty quickly, meaning that they can’t make up most of the high-energy cosmic ray population that the authors are interested in. We don’t know of any astronomical sources that produce the hadrons we detect on Earth and had, so nailing down M 82 as a hadronic source would be huge for tracking down these pesky cosmic ray factories. 

What gets a little bit hairy is that hadronic particle interactions (protons/nuclei hitting things, accelerating, decaying, etc.) will also make a bunch of electrons (these are called secondary electrons) in the process, making it so that the authors are unlikely to see the proton/nucleus only scenario. The spectrum is more likely to look more like a blend of both electrons and hadrons, even if electrons aren’t initially involved. These electrons are created after sudden bursts of star formation produce a bunch of cosmic rays, which will go on to make these secondary electrons that will stay within the galaxy over much longer timescales, leaving a lasting imprint of the cosmic ray production that can be measured long after the star formation stops. 

The authors compare leptonic (electron-only) and hadronic models to the observed gamma-ray data, to see which particles are responsible for making the gamma-rays. They find that at least some hadronic component is needed to fit the observed gamma-ray data from Fermi-LAT and VERITAS (see Figure 4), meaning that M 82 is probably making cosmic rays that are protons and nuclei. They can also infer properties of the cosmic rays, such as the maximum energy of 60 TeV, which is really impressive, but doesn’t quite make it to the ~PeV benchmark of the mystery cosmic ray population (1000x the energy of M82’s particles). We know supernovae can near this benchmark, but still don’t know what’s producing the higher energy particles.

Star light, star bright, starburst!

Starbursts are promising sources that might be producing the elusive high-energy cosmic rays that astronomers have been hunting for over a century. They also give us a larger sample size of star-powered sources that may allow us to gain more understanding about the smaller populations of supernova remnants and massive stars that exist in our own Milky Way. With the next-generation gamma-ray telescopes coming online soon, like CTAO, which can detect gamma-ray sources in 1/10th the time it takes current-generation telescopes, like VERITAS, hopefully studying these challenging and gamma-ray-dim sources will only be easier and more fruitful from this point forward!

Disclaimer: Today’s author was an author on the paper as a member of the VERITAS collaboration but was not directly involved in this project.

Featured image adapted from: Chandra X-ray Observatory 

Edited by: Pranav Satheesh