The radio continuum perspective on cosmic-ray transport in external galaxies

Abstract

Radio continuum observations of external galaxies provide us with an excellent outside view on the distribution of cosmic-ray electrons in the disc and halo. In this review, we summarise the current state of what we have learned from modelling such observations with cosmic-ray transport, paying particular attention to the question to what extent we can exploit radio haloes when studying galactic winds. We have developed the user-friendly framework spinnaker to model radio haloes with either pure advection or diffusion, allowing us to study both diffusion coefficients and advection speeds in nearby galaxies. Using these models, we show that we can identify galaxies with winds using both morphology and radio spectral indices of radio haloes. Advective radio haloes are ubiquitous, indicating that already fairly low values of the star formation rate (SFR) surface density ($\Sigma_{ \mathrm{SFR}}$ Σ SFR ) can trigger galactic winds. The advection speeds scale with SFR, $\Sigma_{\mathrm{SFR}}$ Σ SFR , and rotation speed as expected for stellar feedback-driven winds. Accelerating winds are in agreement with our radio spectral index data, but this is sensitive to the magnetic field parametrisation, so that constant wind speeds cannot be ruled out either. The question to what extent cosmic rays can be a driving force behind winds is still an open issue and we discuss only in passing how a simple iso-thermal wind model could fit our data. Nevertheless, the comparison with inferences from observations and theory looks promising with radio continuum offering a complementary view on galactic winds. We finish with a perspective on future observations and challenges lying ahead.

Constraints on annihilating dark matter in the Omega Centauri cluster

“…For the diffusion index δ, some models have predicted the possible range of δ [46,47] and we have examined a wider range δ = 0.3 − 0.7 to minimize the systematic uncertainty. For the diffusion coefficient D 0 , theory can predict its order of magnitude [27,41,42] and the prediction is consistent with the observed range of D 0 in the Milky Way [43,44]. Based on the theoretical prediction, we expect that the value of D 0 for ω-Cen should be ≤ 10 26 cm 2 /s.…”

“…Generally speaking, D 0 depends on the scale of structures and there is a physical range for galaxies. Theoretical model suggests that D 0 ∼ LV [27,41,42], where L is the injection scale and V is the turbulent velocity. In our Milky Way, since L ∼ 1 kpc and V ∼ 100 km/s, we have D 0 ∼ 10 28 cm 2 /s.…”

Editorial: Plasma, particles, and photons: ISM physics revisited

“…In this topical collection we collect reviews and new results in ISM research with an emphasis on multi-messenger Astrophysics. There are contributions on the production of cosmic ray (CR) electrons (Alsaberi et al 2019) and their transport (Heesen 2021) as deduced from observations, the generation of X-rays and cosmic γ -rays in interstellar shocks (Sano and Fukui 2021), the mechanism of particle acceleration via diffuse shock acceleration (Urošević et al 2019), the thermal and non-thermal X-ray emission from superbubbles (Kavanagh 2020), and the characteristics of non-equilibrium ionisation plasmas (Breitschwerdt and de Avillez 2021). Next, there are contributions on simulations of cosmic ray propagation (Mertsch 2020), on their detection (Albrecht et al 2022) and what we can learn from that, on interstellar radioactive isotopes (Diehl 2021), and on observations of neutrinos (Kheirandish 2020).…”

“…These different scenarios can for once be distinguished by the different energy dependencies that enter the final cosmic ray spectrum. Moreover, the shape of the outflows observed in edge-on galaxies can discriminate between different transport mechanisms such as diffusion or advection (Heesen 2021). However, as the observed spectra are entangling acceleration, transport, and loss processes, the interpretation remains difficult and is in need of a large set of observables while taking into account the level of (an)isotropy of cosmic rays, the magnetic field structure, gas distribution, as well as the medium's composition (electrons, protons, heavier nuclei).…”

Energetic Particle Perpendicular Diffusion: Simulations and Theory in Noisy Reduced Magnetohydrodynamic Turbulence

“…The present work focuses on perpendicular diffusion, which is directly relevant to understanding the distribution of cosmic rays in the heliosphere (e.g., Strauss et al 2012), relating to a key health hazard for extended human space missions (Knipp 2011), as well as solar modulation of the cosmic ray flux according to the ∼11 yr solar activity cycle (Forbush 1954) and ∼22 yr solar magnetic cycle (Jokipii & Thomas 1981). Perpendicular and parallel diffusion are also key to diffusive shock acceleration of energetic particles (Axford et al 1977;Krymskii 1977;Bell 1978;Drury 1983) and are relevant to the distribution of cosmic rays in our Galaxy (Recchia et al 2016) and other galaxies (Heesen 2021).…”