Title: Non-thermal emission in the central starburst region of M82
Authors: M. Persic, R. Rando, and Y. Rephaeli
First Author’s Institution: Istituto Nazionale di Astrofisica, Padova Astronomical Observatory, vicolo dell’Osservatorio 5, I-35122 Padova, Italy
Status: Accepted for A&A; available on Arxiv (open access)
Cosmic rays are particles accelerated to near-light speeds by some of the universe’s most energetic events, such as exploding stars and shock waves. Once created, they become an integral part of a galaxy’s interstellar medium (ISM), coexisting with thermal gas, turbulence, and magnetic fields. Their energy density, the amount of energy stored per unit volume, is comparable to that of both thermal gas and magnetic fields, making cosmic rays a major player in shaping the dynamics of the ISM. Yet despite their importance, their properties remain difficult to measure directly through observations.
While cosmic rays can be a variety of high-energy particles, the vast majority of them are cosmic ray protons. These protons can only be indirectly observed through gamma-ray emission, which is a faint and hard-to-detect signal. In contrast, cosmic ray electrons are easier to spot. As the electrons spiral around magnetic field lines, they emit radio waves through synchrotron emission, and when they collide with photons from background radiation fields, they can boost the photons to X-ray energies through a process called inverse-Compton scattering. The challenge is that these radio and X-ray signals don’t just trace the electrons themselves; they also carry the imprint of the surrounding magnetic field and the background radiation. Untangling these overlapping signals is a major obstacle for astronomers trying to pin down the true properties of cosmic ray electrons.
Although it is difficult to infer cosmic ray properties (for example, the slope and normalization of their spectrum) from observations at a single wavelength, combining data across multiple wavelengths can paint a more complete picture. In today’s paper, the authors do exactly that: they bring together gamma-ray, X-ray, far-infrared (FIR), and radio observations of the starburst galaxy M82 to model the spectra of both cosmic ray protons and electrons. Remarkably, they are also able to estimate the strength of the galaxy’s magnetic field—without relying on the common “equipartition” assumption, which assumes that the energy density of the magnetic fields and the cosmic rays are equal to one another.
M82, famous for its intense star formation, has long been a favorite target for studying cosmic ray emission, as they are accelerated by shocks from supernova remnants. Earlier work showed that M82’s FIR, radio, and gamma-ray emission maps all correlate with one another, since each of these signals is linked to the galaxy’s rate of star formation, and thus to the production of cosmic rays.
Now, thanks to the measurements from the Chandra X-ray Observatory, the authors have added another piece to the puzzle. The spatially resolved, X-ray spectroscopic map of M82 matches both the morphology and spectral slope of the radio emission, suggesting that Compton scattering of FIR photons by cosmic ray electrons is the source of these X-ray emissions. With this, the picture comes together: for the first time, we can piece together a full, multi-wavelength view of cosmic rays in M82.
The authors determine the properties of cosmic rays in the central starburst region of M82 through a step-by-step approach:
1. Fitting cosmic ray protons with gamma-ray emission.
When cosmic ray protons collide with surrounding gas, they produce pions. Neutral pions decay into gamma-ray photons, while charged pions decay into electrons and positrons. The authors assume the proton spectrum follows a power law and fit it to the gamma-ray observations, leaving the slope and normalization as free parameters.
2. Determining the spectrum of secondary electrons.
Secondary electrons arise from the decay of charged pions, rather than from direct acceleration (like “primary” electrons). Their spectrum is set by the proton spectrum, but then shaped further by energy losses, which depend on the local gas density and magnetic field. These secondary electrons produce X-rays and synchrotron radiation, just like primaries.
3. Modeling the ambient radiation field.
Because cosmic ray electrons scatter background photons up to X-ray energies, the intensity of the radiation field must also be known. The authors find that the field is dominated by light from local stars and dust, which they model with a Planck blackbody spectrum modified by a dust emissivity function. Using FIR data, they fix most parameters and leave only the dust emissivity index, β, as free parameter.
4. Fitting the X-ray data to obtain the primary electron spectrum.
With the ambient radiation field in hand, the authors fit the X-ray emission using a power-law electron spectrum, extracting the slope and normalization of the primary cosmic ray electrons.
5. Measuring the magnetic field.
With both primary and secondary electron spectra determined, they fit the resulting synchrotron emission around GHz frequencies to obtain the volume-weighted magnetic field strength.
Finally, the electron spectrum is also used to calculate how much cosmic ray electrons contribute to gamma rays via non-thermal bremsstrahlung and inverse Compton scattering. To ensure consistency, the authors repeat steps 1–5 iteratively until all pieces fit together.
Figure 1. The fitted gamma-ray and X-ray emission spectrum of M82. The x-axis presents the frequency, and the y-axis presents the gamma-ray/X-ray flux. Data points above 1022 Hz correspond to gamma-ray observations, while the single point near 1018 Hz marks the newly added X-ray continuum measurement. The solid line shows the total modeled emission. The dashed line indicates inverse-Compton scattering from cosmic ray electrons, the dotted line shows gamma rays from cosmic ray protons, and the dot-dashed line represents non-thermal bremsstrahlung emission from cosmic ray electrons.
The final fit to the gamma-ray and X-ray spectrum reveals that gamma-ray emission is dominated by cosmic ray protons (dotted line), while X-ray emission is dominated by cosmic ray electrons (dashed line), just as expected. Interestingly, a local dip appears around 1022 Hz, marking the transition from proton-dominated to electron-dominated emission. This feature shows up both in the observed gamma-ray data and in the model fit, confirming the consistency of the analysis.
Figure 2. The fitted radio emission spectrum of M82. Again, the x-axis represents frequency, and the y-axis represents flux in units of Jansky. The solid line shows the total modeled emission. The short dashed line corresponds to synchrotron emission from primary cosmic ray electrons, the dotted line to synchrotron emission from secondary cosmic ray electrons, and the long dashed line to free–free emission. The turnover at low frequencies is caused by thermal free–free absorption.
Fitting the radio spectrum shows that synchrotron emission from primary and secondary cosmic ray electrons are comparable in strength, with thermal emission playing only a minor role. From this fit, the authors estimate the magnetic field strength to be between 98 and 120 μG (depending on the assumed dust emissivity), without relying on the usual equipartition assumption between magnetic and cosmic ray energy. Interestingly, this estimate is consistent with earlier values derived from radio emission alone under the equipartition assumption. This agreement makes sense, as the authors find that the magnetic energy density implied by their result is very close to the total cosmic ray energy density.
This study provides one of the most complete pictures of cosmic rays in a dense starburst environment. By combining gamma-ray, X-ray, FIR, and radio data for M82, the authors disentangle the different emission mechanisms across the electromagnetic spectrum and pin down the contributions of both cosmic ray protons and electrons. Their detailed modeling offers a direct estimate of the galaxy’s magnetic field without assuming energy equipartition, and concludes a near-equipartition between cosmic ray and magnetic energy densities in the dense starburst environment. With 16 years of new gamma-ray observations, this work establishes key parameters that will help guide future studies of particle acceleration and transport in active galaxies.
Astrobite edited by Veronika Dornan
Featured image credit: Iwasawa et al. (2023)
