TY - JOUR A1 - Pfrommer, Christoph A1 - Werhahn, Maria A1 - Pakmor, Rudiger A1 - Girichidis, Philipp A1 - Simpson, Christine M. T1 - Simulating radio synchrotron emission in star-forming galaxies BT - small-scale magnetic dynamo and the origin of the far-infrared-radio correlation JF - Monthly notices of the Royal Astronomical Society N2 - In star-forming galaxies, the far-infrared (FIR) and radio-continuum luminosities obey a tight empirical relation over a large range of star-formation rates (SFR). To understand the physics, we examine magnetohydrodynamic galaxy simulations, which follow the genesis of cosmic ray (CR) protons at supernovae and their advective and anisotropic diffusive transport. We show that gravitational collapse of the proto-galaxy generates a corrugated accretion shock, which injects turbulence and drives a small-scale magnetic dynamo. As the shock propagates outwards and the associated turbulence decays, the large velocity shear between the supersonically rotating cool disc with respect to the (partially) pressure-supported hot circumgalactic medium excites Kelvin-Helmholtz surface and body modes. Those interact non-linearly, inject additional turbulence and continuously drive multiple small-scale dynamos, which exponentially amplify weak seed magnetic fields. After saturation at small scales, they grow in scale to reach equipartition with thermal and CR energies in Milky Way-mass galaxies. In small galaxies, the magnetic energy saturates at the turbulent energy while it fails to reach equipartition with thermal and CR energies. We solve for steady-state spectra of CR protons, secondary electrons/positrons from hadronic CR-proton interactions with the interstellar medium, and primary shock-accelerated electrons at supernovae. The radio-synchrotron emission is dominated by primary electrons, irradiates the magnetized disc and bulge of our simulated Milky Way-mass galaxy and weakly traces bubble-shaped magnetically loaded outflows. Our star-forming and star-bursting galaxies with saturated magnetic fields match the global FIR-radio correlation (FRC) across four orders of magnitude. Its intrinsic scatter arises due to (i) different magnetic saturation levels that result from different seed magnetic fields, (ii) different radio synchrotron luminosities for different specific SFRs at fixed SFR, and (iii) a varying radio intensity with galactic inclination. In agreement with observations, several 100-pc-sized regions within star-forming galaxies also obey the FRC, while the centres of starbursts substantially exceed the FRC. KW - dynamo KW - MHD KW - methods: numerical KW - cosmic rays KW - galaxies: formation KW - radio KW - continuum: galaxies Y1 - 2022 U6 - https://doi.org/10.1093/mnras/stac1808 SN - 0035-8711 SN - 1365-2966 VL - 515 IS - 3 SP - 4229 EP - 4264 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Werhahn, Maria A1 - Pfrommer, Christoph A1 - Girichidis, Philipp T1 - Cosmic rays and non-thermal emission in simulated galaxies - III. Probing cosmic-ray calorimetry with radio spectra and the FIR-radio correlation JF - Monthly notices of the Royal Astronomical Society N2 - An extinction-free estimator of the star formation rate (SFR) of galaxies is critical for understanding the high-redshift universe. To this end, the nearly linear, tight correlation of far-infrared (FIR), and radio luminosity of star-forming galaxies is widely used. While the FIR is linked to massive star formation, which also generates shock-accelerated cosmic-ray (CR) electrons and radio synchrotron emission, a detailed understanding of the underlying physics is still lacking. Hence, we perform three-dimensional magnetohydrodynamical (MHD) simulations of isolated galaxies over a broad range of halo masses and SFRs using the moving-mesh code AREPO, and evolve the CR proton energy density self-consistently. In post-processing, we calculate the steady-state spectra of primary, shock-accelerated and secondary CR electrons, which result from hadronic CR proton interactions with the interstellar medium. The resulting total radio luminosities correlate with the FIR luminosities as observed and are dominated by primary CR electrons if we account for anisotropic CR diffusion. The increasing contribution of secondary emission up to 30 per cent in starbursts is compensated by the larger bremsstrahlung and Coulomb losses. CR electrons are in the calorimetric limit and lose most of their energy through inverse Compton interactions with star light and cosmic microwave background (CMB) photons while less energy is converted into synchrotron emission. This implies steep steady-state synchrotron spectra in starbursts. Interestingly, we find that thermal free-free emission flattens the total radio spectra at high radio frequencies and reconciles calorimetric theory with observations while free-free absorption explains the observed low-frequency flattening towards the central regions of starbursts. KW - MHD KW - methods: numerical KW - cosmic rays KW - galaxies: magnetic fields KW - galaxies: starburst KW - radio continuum: galaxies Y1 - 2021 U6 - https://doi.org/10.1093/mnras/stab2535 SN - 0035-8711 SN - 1365-2966 VL - 508 IS - 3 SP - 4072 EP - 4095 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Werhahn, Maria A1 - Pfrommer, Christoph A1 - Girichidis, Philipp A1 - Puchwein, Ewald A1 - Pakmor, Rüdiger T1 - Cosmic rays and non-thermal emission in simulated galaxies BT - I. Electron and proton spectra compared to Voyager-1 data JF - Monthly notices of the Royal Astronomical Society N2 - Current-day cosmic ray (CR) propagation studies use static Milky Way models and fit parametrized source distributions to data. Instead, we use three-dimensional magnetohydrodynamic (MHD) simulations of isolated galaxies with the moving-mesh code arepo that self-consistently accounts for hydrodynamic effects of CR protons. In post-processing, we calculate their steady-state spectra, taking into account all relevant loss processes. We show that this steady-state assumption is well justified in the disc and generally for regions that emit non-thermal radio and gamma rays. Additionally, we model the spectra of primary electrons, accelerated by supernova remnants, and secondary electrons and positrons produced in hadronic CR proton interactions with the gas. We find that proton spectra above 10 GeV only weakly depend on galactic radius, while they acquire a radial dependence at lower energies due to Coulomb interactions. Radiative losses steepen the spectra of primary CR electrons in the central galactic regions, while diffusive losses dominate in the outskirts. Secondary electrons exhibit a steeper spectrum than primaries because they originate from the transported steeper CR proton spectra. Consistent with Voyager-1 and AMS-02 data, our models (i) show a turnover of proton spectra below GeV energies due to Coulomb interactions so that electrons start to dominate the total particle spectra and (ii) match the shape of the positron fraction up to 10 GeV. We conclude that our steady-state CR modelling in MHD CR galaxy simulations is sufficiently realistic to capture the dominant transport effects shaping their spectra, arguing for a full MHD treatment to accurately model CR transport in the future. KW - astroparticle physics KW - MHD KW - methods: numerical KW - cosmic rays KW - local KW - interstellar matter Y1 - 2021 U6 - https://doi.org/10.1093/mnras/stab1324 SN - 0035-8711 SN - 1365-2966 VL - 505 IS - 3 SP - 3273 EP - 3294 PB - Oxford University Press CY - Oxford ER -