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Register a new userCosmic-Ray Transport in Simulations of Star-forming Galactic Disks
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Cosmic ray transport on galactic scales depends on the detailed properties of the magnetized, multiphase interstellar medium (ISM). In this work, we post-process a high-resolution TIGRESS magnetohydrodynamic simulation modeling a local galactic disk patch with a two-moment fluid algorithm for cosmic ray transport. We consider a variety of prescriptions for the cosmic rays, from a simple purely diffusive formalism with constant scattering coefficient, to a physically-motivated model in which the scattering coefficient is set by critical balance between streaming-driven Alfven wave excitation and damping mediated by local gas properties. We separately focus on cosmic rays with kinetic energies of $sim 1$ GeV (high-energy) and $sim 30$~MeV (low-energy), respectively important for ISM dynamics and chemistry. We find that simultaneously accounting for advection, streaming, and diffusion of cosmic rays is crucial for properly modeling their transport. Advection dominates in the high-velocity, low-density, hot phase, while diffusion and streaming are more important in higher density, cooler phases. Our physically-motivated model shows that there is no single diffusivity for cosmic-ray transport: the scattering coefficient varies by four or more orders of magnitude, maximal at density $n_mathrm{H} sim 0.01, mathrm{cm}^{-3}$. Ion-neutral damping of Alfven waves results in strong diffusion and nearly uniform cosmic ray pressure within most of the mass of the ISM. However, cosmic rays are trapped near the disk midplane by the higher scattering rate in the surrounding lower-density, higher-ionization gas. The transport of high-energy cosmic rays differs from that of low-energy cosmic rays, with less effective diffusion and greater energy losses for the latter.
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Recently, there has been an increased interest in the study of the generation of low-energy cosmic rays (CRs; < 1 TeV) in shocks situated on the surface of a protostar or along protostellar jets. These locally accelerated CRs offer an attractive explanation for the high levels of non-thermal emission and ionisation rate, $zeta$, observed close to these sources. The high $zeta$ observed in some protostellar sources is generally attributed to shock-generated UV photons. The aim of this article is to show that when synchrotron emission and a high $zeta$ are measured in the same spatial region, a locally shock-accelerated CR flux is sufficient to explain both phenomena. We assume that relativistic particles are accelerated according to the first-order Fermi acceleration mechanism and compute $zeta$ and the non-thermal emission at cm wavelengths. We then apply our model to the star-forming region OMC-2 FIR 3/FIR 4. Using a Bayesian analysis, we constrain the parameters of the model and estimate the spectral indices of the non-thermal radio emission. We demonstrate that the local CR acceleration model makes it possible to simultaneously explain the synchrotron emission along the HOPS 370 jet within the FIR 3 region and $zeta$ observed near the FIR 4 protocluster. Our model constrains the magnetic field strength (~250-450$~mu$G), its turbulent component (~20-40$~mu$G), and the jet velocity in the shock reference frame for the three non-thermal sources of the HOPS 370 jet (~350-1000 km s$^{-1}$). Beyond the modelling of the OMC-2 FIR 3/FIR 4 system, we show how the combination of continuum observations at cm wavelengths and molecular transitions is a powerful new tool for the analysis of star-forming regions: these two types of observations can be simultaneously interpreted by invoking only the presence of locally accelerated CRs, without having to resort to shock-generated UV photons.
It has been hypothesized that photons from young, massive star clusters are responsible for maintaining the ionization of diffuse warm ionized gas seen in both the Milky Way and other disk galaxies. For a theoretical investigation of the warm ionized medium (WIM), it is crucial to solve radiation transfer equations where the ISM and clusters are modeled self-consistently. To this end, we employ a Solar neighborhood model of TIGRESS, a magnetohydrodynamic simulation of the multiphase, star-forming ISM, and post-process the simulation with an adaptive ray tracing method to transfer UV radiation from star clusters. We find that the WIM volume filling factor is highly variable, and sensitive to the rate of ionizing photon production and ISM structure. The mean WIM volume filling factor rises to ~0.15 at |z|~1 kpc. Approximately half of ionizing photons are absorbed by gas and half by dust; the cumulative ionizing photon escape fraction is 1.1%. Our time-averaged synthetic H$alpha$ line profile matches WHAM observations on the redshifted (outflowing) side, but has insufficient intensity on the blueshifted side. Our simulation matches the Dickey-Lockman neutral density profile well, but only a small fraction of snapshots have high-altitude WIM density consistent with Reynolds Layer estimates. We compute a clumping correction factor C = <n_e>/sqrt<n_e^2>~0.2 that is remarkably constant with distance from the midplane and time; this can be used to improve estimates of ionized gas mass and mean electron density from observed H$alpha$ surface brightness profiles in edge-on galaxies.
Galaxies experiencing intense star-formation episodes are expected to be rich in energetic cosmic rays (CRs). These CRs undergo hadronic interactions with the interstellar gases of their host to drive $gamma$-ray emission, which has already been detected from several nearby starbursts. Unresolved $gamma$-ray emission from more distant star-forming galaxies (SFGs) is expected to contribute to the extra-galactic $gamma$-ray background (EGB). However, despite the wealth of high-quality all-sky data from the Fermi-LAT $gamma$-ray space telescope collected over more than a decade of operation, the exact contribution of such SFGs to the EGB remains unsettled. We investigate the high-energy $gamma$-ray emission from SFGs up to redshift $z=3$ above a GeV, and assess the contribution they can make to the EGB. We show the $gamma$-ray emission spectrum from a SFG population can be determined from just a small number of key parameters, from which we model a range of possible EGB realisations. We demonstrate that populations of SFGs leave anisotropic signatures in the EGB, and that these can be accessed using the spatial power spectrum. Moreover, we show that such signatures will be accessible with ongoing operation of current $gamma$-ray instruments, and detection prospects will be greatly improved by the next generation of $gamma$-ray observatories, in particular the Cherenkov Telescope Array.
In recent years, $gamma$-ray emission has been detected from star-forming galaxies (SFGs) in the local universe, including M82, NGC 253, Arp 220 and M33. The bulk of this emission is thought to be of hadronic origin, arising from the interactions of cosmic rays (CRs) with the interstellar medium of their host galaxy. Distant SFGs are presumably also bright in $gamma$-rays. Although they would not be resolvable as point sources, distant unresolved SFG populations contribute $gamma$-rays to the extra-galactic $gamma$-ray background (EGB). Despite the wealth of high-quality all-sky EGB data collected over more than a decade of operation with the textit{Fermi}-LAT $gamma$-ray space telescope, the exact contribution of SFGs to the EGB remains unsettled. In this study, we model the $gamma$-ray emission from SFG populations and demonstrate that such emission can be characterized by just a small number of physically-motivated parameters. We further show that source populations would leave anisotropic signatures in the EGB, which could be used to yield information about the underlying properties, dynamics and evolution of CR-rich SFGs.
We present new developments on the Cosmic--Ray driven, galactic dynamo, modeled by means of direct, resistive CR--MHD simulations, performed with ZEUS and PIERNIK codes. The dynamo action, leading to the amplification of large--scale galactic magnetic fields on galactic rotation timescales, appears as a result of galactic differential rotation, buoyancy of the cosmic ray component and resistive dissipation of small--scale turbulent magnetic fields. Our new results include demonstration of the global--galactic dynamo action driven by Cosmic Rays supplied in supernova remnants. An essential outcome of the new series of global galactic dynamo models is the equipartition of the gas turbulent energy with magnetic field energy and cosmic ray energy, in saturated states of the dynamo on large galactic scales.
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