Exploring the physics behind the non-thermal emission from star-forming galaxies detected in gamma rays

Abstract

Context. Star-forming galaxies emit non-thermal radiation from radio to γ-rays. Observations show that their radio and γ-ray luminosities scale with their star formation rates, supporting the hypothesis that non-thermal radiation is emitted by cosmic rays produced by their stellar populations. However, the nature of the main cosmic-ray transport processes that shape the emission in these galaxies is still poorly understood, especially at low star formation rates. Aims. We aim to investigate the main mechanisms of global cosmic-ray transport and cooling in star-forming galaxies. The way they contribute in shaping the relations between non-thermal luminosities and star formation rates could shed light onto their nature, and allow to quantify their relative importance at different star formation rates. Methods. We develop a model to compute the cosmic-ray populations of star-forming galaxies, taking into account their production, transport and cooling. The model is parameterised only through global galaxy properties, and describes the non-thermal emission in both radio (at 1.4 GHz and 150 MHz) and γ-rays (in the 0.1-100 GeV band). We focus on the role of diffusive and advective transport by galactic winds, either driven by turbulent or thermal instabilities. We compare model predictions to observations, for which we compile a homogeneous set of luminosities in these radio bands, and update those available in γ rays. Results. Our model reproduces reasonably well the observed relations between the γ-ray or 1.4 GHz radio luminosities and the star formation rate, assuming a single power-law scaling of the magnetic field with the latter with index β = 0.3, and winds blowing either at Alfvenic speeds (∼ tens of km s −1 , for 5 M yr −1 ) or typical starburst wind velocities (∼ hundreds of km s −1 , for 5 M yr −1 ). Escape of cosmic rays is negligible for 30 M yr −1 . A constant ionisation fraction of the interstellar medium fails to reproduce the 150 MHz radio luminosity throughout the whole star formation rate range. Conclusions. Our results reinforce the idea that galaxies with high star formation rate are cosmic-ray calorimeters, and that the main mechanism driving proton escape is diffusion, whereas electron escape also proceeds via wind advection. They also suggest that these winds should be cosmic-ray or thermally-driven at low and intermediate star formation rates, respectively. Our results globally support that magnetohydrodynamic turbulence is responsible for the dependence of the magnetic field strength on the star formation rate and that the ionisation fraction is strongly disfavoured to be constant throughout the whole range of star formation rate.

Observable Signatures of Cosmic Rays Transport in Starburst Galaxies on Gamma-ray and Neutrino Observations

“…The phenomenology arising from these differences in the gamma-ray and neutrino fluxes is the fundamental topic of this paper. Since the starburst activity lasts for long time (∼ 10 8 yr (Peretti et al 2021)), a steady state is reached between the cosmic-ray cooling, transport, and injection phenomena. In particular, the distribution F p (E) of high-energy protons with energy E can be written as…”

“…In the plot, the integrated luminosity is computed assuming a reference value of the spectral index (Γ = 2.2) for all the galaxies as in Refs. Kornecki et al (2021); Roth et al (2021). However, it is worth noticing that L γ depends only marginally on Γ.…”

“…(H. E. S. S. Collaboration et al 2018) collaborations have shown their capability to produce gamma-rays (Ajello et al 2020) with a luminosity correlated to their star formation rate (SFR). This suggests that the non-thermal emission of these galaxies can be strictly dependent on their star-forming activity (Ackermann et al 2012;Ajello et al 2020;Kornecki et al 2020Kornecki et al , 2021. Indeed, supernova remnants (SNRs) shocks are supposed to accelerate protons up to PeV energies (Sveshnikova 2003;Murase et al 2013;Tamborra et al 2014).…”

“…This is because it depends on the interplay between different phenomena like diffusion, wind advections and energy losses, which are difficult to reliably model. Different CR transport scenarios have been used to describe the observed gammaray emission from nearby SFGs and SBGs, trying to disentangle a calorimetric behaviour from a diffusion-dominated regime (τ diff τ loss , τ adv ) (Peretti et al 2019;Krumholz et al 2020;Ha et al 2020;Müller et al 2020;Shimono et al 2021;Xiang et al 2021;Owen et al 2021b,a;Werhahn et al 2021a,b;Sudoh et al 2018;Kornecki et al 2020Kornecki et al , 2021. The major uncertainty factor is due to distinctive timescale modelling.…”