Cosmic ray transport in starburst galaxies

Abstract

Starburst galaxies are efficient γ-ray producers, because their high supernova rates generate copious cosmic ray (CR) protons, and their high gas densities act as thick targets off which these protons can produce neutral pions and thence γ-rays. In this paper we present a first-principles calculation of the mechanisms by which CRs propagate through such environments, combining astrochemical models with analysis of turbulence in weakly ionised plasma. We show that CRs cannot scatter off the strong large-scale turbulence found in starbursts, because efficient ion-neutral damping prevents such turbulence from cascading down to the scales of CR gyroradii. Instead, CRs stream along field lines at a rate determined by the competition between streaming instability and ion-neutral damping, leading to transport via a process of field line random walk. This results in an effective diffusion coefficient that is nearly energyindependent up to CR energies of ∼ 1 TeV. We apply our computed diffusion coefficient to a simple model of CR escape and loss, and show that the resulting γ-ray spectra are in good agreement with the observed spectra of the starbursts NGC 253, M82, and Arp 220. In particular, our model reproduces these galaxies' relatively hard GeV γ-ray spectra and softer TeV spectra without the need for any fine-tuning of advective escape times or the shape of the CR injection spectrum.

Star-forming galaxies dominate the diffuse, isotropic γ-ray background

“…However, Ref. [11] recently introduced a model for f cal (E), based on the idea that the rate at which CRs diffuse through the magnetised ISM, and thus the fraction that escape before colliding, is determined by the balance between driving of turbulence by the CR streaming instability and ion-neutral damping. This model successfully reproduces the γ-ray spectra of NGC 253, M82, and Arp 220, and we therefore adopt it for this work.…”

“…We employ the model of Ref. [11] for CR transport and calorimetry in SFGs. The basic premise of the model is that, in the neutral phase that dominates the mass of the ISM and thus the set of available targets for γ-ray production, CR transport is primarily by streaming along magnetic field lines.…”

“…the ISM is molecule-dominated, ii) the main ionised species is C + , and iii) the magnetic field is set by a turbulent dynamo, we adopt the fiducial parameters of Ref. [11] appropriate for such galaxies. Specifically, we take χ = 10 −4 , µ i = 12, M A = 2, V Ai = u LA /χ 1/2 M A , and u LA = σ g / √ 2, where σ g is the gas velocity dispersion of the galaxy.…”

Cosmic ray feedback across the sequence of star-forming galaxies

“…In this Proceedings article we report the results of our three recent papers [2,3,7] that incorporate a realistic model of cosmic ray (CR) transport through partially-ionised, but largely neutral, star-forming gas in galaxies. Our model for transport incorporates insights developed by Xu, Lazarian and co-workers in their recent studies [8,[12][13][14] of CR transport in such partiallyionised media.…”

“…There is, thus, an absence of extrinsic turbulence at the gyro-radius scale. This is because of the phenomenon of ion-neutral damping which, for ionisation fractions ∼ 10 −2 − 10 −4 (typical of star-formation-hosting gas: [7]) leads to dissipation of the turbulence at a scale significantly larger than the ∼ GeV gyro-radius scale.…”