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Synchrotron radiation from cosmic rays is a key observational probe of the galactic magnetic field. Interpreting synchrotron emission data requires knowledge of the cosmic ray number density, which is often assumed to be in energy equipartition (or otherwise tightly correlated) with the magnetic field energy. However, there is no compelling observational or theoretical reason to expect such tight correlation to hold across all scales. We use test particle simulations, tracing the propagation of charged particles (protons) through a random magnetic field, to study the cosmic ray distribution at scales comparable to the correlation scale of the turbulent flow in the interstellar medium ($simeq 100,{rm pc}$ in spiral galaxies). In these simulations, we find that there is no spatial correlation between the cosmic ray number density and the magnetic field energy density. In fact, their distributions are approximately statistically independent. We find that low-energy cosmic rays can become trapped between magnetic mirrors, whose location depends more on the structure of the field lines than on the field strength.
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Interpretations of synchrotron observations often assume a tight correlation between magnetic and cosmic ray energy densities. We examine this assumption using both test-particle simulations of cosmic rays and MHD simulations which include cosmic rays as a diffusive fluid. We find no spatial correlation between the cosmic rays and magnetic field energy densities at turbulent scales. Moreover, the cosmic ray number density and magnetic field energy density are statistically independent. Nevertheless, the cosmic ray spatial distribution is highly inhomogeneous, especially at low energies because the particles are trapped between random magnetic mirrors. These results can significantly change the interpretation of synchrotron observations and thus our understanding of the strength and structure of magnetic fields in the Milky Way and nearby spiral galaxies.
In this white paper we introduce the IMAGINE Consortium and its scientific background, goals and structure. Our purpose is to coordinate and facilitate the efforts of a diverse group of researchers in the broad areas of the interstellar medium, Galactic magnetic fields and cosmic rays, and our goal is to develop more comprehensive insights into the structures and roles of interstellar magnetic fields and their interactions with cosmic rays. To achieve a higher level of self-consistency, depth and rigour can only be achieved by the coordinated efforts of experts in diverse areas of astrophysics involved in observational, theoretical and numerical work. We present our view of the present status of this topic, identify its key unsolved problems and suggest a strategy that will underpin our work. The backbone of the consortium is the Interstellar MAGnetic field INference Engine, a publicly available Bayesian platform that employs robust statistical methods to explore the multi-dimensional likelihood space using any number of modular inputs. It provides an interpretation and modelling framework that has the power and flexibility to include a variety of observational, theoretical and numerical lines of evidence into a self-consistent and comprehensive picture of the thermal and non-thermal interstellar media. An important innovation is that a consistent understanding of the phenomena that are directly or indirectly influenced by the Galactic magnetic field, such as the deflection of ultra-high energy cosmic rays or extragalactic backgrounds, is made an integral part of the modelling. The IMAGINE Consortium, which is informal by nature and open to new participants, hereby presents a methodological framework for the modelling and understanding of Galactic magnetic fields that is available to all communities whose research relies on a state-of-the-art solution to this problem. (Abridged.)
The propagation of cosmic rays in turbulent magnetic fields is a diffusive process driven by the scattering of the charged particles by random magnetic fluctuations. Such fields are usually highly intermittent, consisting of intense magnetic filaments and ribbons surrounded by weaker, unstructured fluctuations. Studies of cosmic ray propagation have largely overlooked intermittency, instead relying on Gaussian random magnetic fields. Using test particle simulations, we investigate cosmic ray diffusivity in intermittent, dynamo-generated magnetic fields. The results are compared with those obtained from non-intermittent magnetic fields having identical power spectra. The presence of magnetic intermittency significantly enhances cosmic ray diffusion over a wide range of particle energies. We demonstrate that the results can be interpreted in terms of a correlated random walk.
We derive the magnitude of fluctuations in total synchrotron intensity in the Milky Way and M33, from both observations and theory under various assumption about the relation between cosmic rays and interstellar magnetic fields. Given the relative magnitude of the fluctuations in the Galactic magnetic field (the ratio of the rms fluctuations to the mean magnetic field strength) suggested by Faraday rotation and synchrotron polarization, the observations are inconsistent with local energy equipartition between cosmic rays and magnetic fields. Our analysis of relative synchrotron intensity fluctuations indicates that the distribution of cosmic rays is nearly uniform at the scales of the order of and exceeding $100p$, in contrast to strong fluctuations in the interstellar magnetic field at those scales. A conservative upper limit on the ratio of the the fluctuation magnitude in the cosmic ray number density to its mean value is 0.2--0.4 at scales of order 100,pc. Our results are consistent with a mild anticorrelation between cosmic-ray and magnetic energy densities at these scales, in both the Milky Way and M33. Energy equipartition between cosmic rays and magnetic fields may still hold, but at scales exceeding 1,kpc. Therefore, we suggest that equipartition estimates be applied to the observed synchrotron intensity smoothed to a linear scale of kiloparsec order (in spiral galaxies) to obtain the cosmic ray distribution and a large-scale magnetic field. Then the resulting cosmic ray distribution can be used to derive the fluctuating magnetic field strength from the data at the original resolution. The resulting random magnetic field is likely to be significantly stronger than existing estimates.
We present a suite of high-resolution cosmological simulations, using the FIRE-2 feedback physics together with explicit treatment of magnetic fields, anisotropic conduction and viscosity, and cosmic rays (CRs) injected by supernovae (including anisotropic diffusion, streaming, adiabatic, hadronic and Coulomb losses). We survey systems from ultra-faint dwarf ($M_{ast}sim 10^{4},M_{odot}$, $M_{rm halo}sim 10^{9},M_{odot}$) through Milky Way masses, systematically vary CR parameters (e.g. the diffusion coefficient $kappa$ and streaming velocity), and study an ensemble of galaxy properties (masses, star formation histories, mass profiles, phase structure, morphologies). We confirm previous conclusions that magnetic fields, conduction, and viscosity on resolved ($gtrsim 1,$pc) scales have small effects on bulk galaxy properties. CRs have relatively weak effects on all galaxy properties studied in dwarfs ($M_{ast} ll 10^{10},M_{odot}$, $M_{rm halo} lesssim 10^{11},M_{odot}$), or at high redshifts ($zgtrsim 1-2$), for any physically-reasonable parameters. However at higher masses ($M_{rm halo} gtrsim 10^{11},M_{odot}$) and $zlesssim 1-2$, CRs can suppress star formation by factors $sim 2-4$, given relatively high effective diffusion coefficients $kappa gtrsim 3times10^{29},{rm cm^{2},s^{-1}}$. At lower $kappa$, CRs take too long to escape dense star-forming gas and lose energy to hadronic collisions, producing negligible effects on galaxies and violating empirical constraints from $gamma$-ray emission. But around $kappasim 3times10^{29},{rm cm^{2},s^{-1}}$, CRs escape the galaxy and build up a CR-pressure-dominated halo which supports dense, cool ($Tll 10^{6}$ K) gas that would otherwise rain onto the galaxy. CR heating (from collisional and streaming losses) is never dominant.
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