Turbulent dynamo in the two-phase interstellar medium

Seta, Amit; Federrath, Christoph

doi:10.48550/arxiv.2202.08324

Cited by 5 publications

(4 citation statements)

References 68 publications

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“…In the two cases where CR pressure feedback is weakest in the disc (κ ≥ 3 × 10 28 cm 2 s −1 isotropic diffusion), the growth of the average field stalls after the first rotation period and the largescale magnetic pressures saturate at 5%-10% of the turbulent pressure throughout the disc in Fig. 5.This saturation level compares well with the results obtained at higher resolution when forcing compressive or solenoidal turbulence in a multiphasic, resistive, and viscous medium (Seta & Federrath 2022). For the other cases with much stronger CR feedback, in particular for all the anisotropic diffusion cases, star formation proceeds at a reduced rate in the inner regions (see section 4), generating lower turbulent energy densities in these regions (see Fig.…”

Section: Large-scale Pressure Gradientssupporting

confidence: 81%

Cosmic-ray diffusion and the multi-phase interstellar medium in a dwarf galaxy. I. Large-scale properties and $γ$-ray luminosities

Nuñez-Castiñeyra,

Grenier,

Bournaud

et al. 2022

Preprint

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Context. Dynamically, cosmic rays with energies above about one GeV/nucleon may be important agents of galaxy evolution. Their pressure and pressure gradients compare with the thermal and magnetic ones to alter gas accretion onto a galaxy, drive fountains and massive galactic outflows, and alter the mass cycling between the diffuse gas reserves and the dense clouds where stars form. The feedback efficiency depends on the actual properties of cosmic-ray transport in the different media, so we crucially need theoretical clues and observational constraints on these properties in order to assess the impact of cosmic rays on galaxy evolution. Aims. We aim to study the dynamical role of cosmic rays in shaping the interstellar medium of a galaxy when changing their propagation mode. As a first step, we perform high-resolution simulations of the evolution of the same isolated galaxy and compare the impact of the simplest cosmic-ray transport assumption of uniform diffusion. We also compare the total γ-ray luminosity produced in the galaxy by hadronic interactions between cosmic rays and the gas as this observable encapsulates the convolution of the gas and cosmic-ray spatial distributions and it can be compared to Fermi LAT data. Methods. We have simulated the evolution of a gas-rich dwarf galaxy (∼10 11 M in total mass, forming about 1 M yr −1 of stars) with the magnetohydrodynamic adaptive-mesh-refinement code RAMSES, down to 9-pc resolution. Cosmic rays are advected by the gas and they diffuse either isotropically or preferentially along the magnetic field with uniform diffusion coefficients ranging from 3 × 10 27 to 10 29 cm 2 s −1 in order to bracket the average value inferred in the Milky Way. The simulations use gas cooling and heating functions that model the multiphasic structure of the gas down to a much lower resolution than used here. We have also updated the observational relation seen between the γ-ray luminosities and star-formation rates of galaxies using the latest detection and characterisations of Fermi LAT sources. Results. We focus this article on the impact of CR transport on the large-scale properties of the galaxy. We find that the radial and vertical distributions of the gas in the different phases, as well as the mass ranking between the phases, are marginally altered when changing cosmic-ray transport. We observe a positive feedback of cosmic rays on the amplification of the magnetic field in the inner half of the galaxy, except for fast isotropic diffusion. The increase in cosmic-ray pressure for slow or anisotropic diffusion can suppress star formation by up to 50%, but the dual effect of cosmic-ray pressure and magnetic amplification can reduce star formation by a factor 2.5. The global γ-ray luminosities and star-formation rates of the simulated galaxies are fully consistent with the best-fit trend seen in the observations in the case of anisotropic 10 27.5−29 cm 2 s −1 diffusion and for isotropic diffusion slower or equal to 3 × 10 28 cm 2 s −1 . These results therefore do not confirm clai...

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Section: Large-scale Pressure Gradientssupporting

confidence: 81%

Cosmic-ray diffusion and the multi-phase interstellar medium in a dwarf galaxy. I. Large-scale properties and $γ$-ray luminosities

Nuñez-Castiñeyra,

Grenier,

Bournaud

et al. 2022

Preprint

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“…We note that magnetic field-gas relation in multi-phase simulations is very complex and cannot be easily described by a single power-law relationship consistent with these simple gas compressions. In particular, the relation depends on the Alfvénic Mach number, where 𝜅 = 2/3 is found in high Mach number regions of the cold ISM dominated by molecular hydrogen (Seta & Federrath 2022). However, as such regions would likely occupy a small fraction of the ISM and H is mostly sub-to transsonic so that flux freezing does not hold.…”

Section: What Regulates Magnetic Fields In Galaxies?mentioning

confidence: 99%

Nearby galaxies in the LOFAR Two-metre Sky Survey

Heesen¹,

M.²,

Basu³

et al. 2022

A&A

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Context. Cosmic rays and magnetic fields are key ingredients in galaxy evolution, regulating both stellar feedback and star formation. Their properties can be studied with low-frequency radio continuum observations that are free from thermal contamination. Aims. We define a sample of 76 nearby (< 30 Mpc) galaxies with rich ancillary data in the radio continuum and infrared from the CHANG-ES and KINGFISH surveys, which will be observed with the LOFAR Two-metre Sky Survey (LoTSS) at 144 MHz. Methods. We present maps for 45 of them as part of the LoTSS data release 2 (LoTSS-DR2), where we measure integrated flux densities and study integrated and spatially resolved radio spectral indices. We investigate the radio–star formation rate (SFR) relation using SFRs derived from total infrared and Hα + 24-μm emission. Results. The radio–SFR relation at 144 MHz is clearly super-linear with L144 MHz ∝ SFR1.4−1.5. The mean integrated radio spectral index between 144 and ≈1400 MHz is ⟨α⟩= − 0.56 ± 0.14, in agreement with the injection spectral index for cosmic ray electrons (CREs). However, the radio spectral index maps show variation of spectral indices with flatter spectra associated with star-forming regions and steeper spectra in galaxy outskirts and, in particular, in extra-planar regions. We found that galaxies with high SFRs have steeper radio spectra; we find similar correlations with galaxy size, mass, and rotation speed. Conclusions. Galaxies that are larger and more massive are better electron calorimeters, meaning that the CRE lose a higher fraction of their energy within the galaxies. This explains the super-linear radio–SFR relation, with more massive, star-forming galaxies being radio bright. We propose a semi-calorimetric radio–SFR relation that employs the galaxy mass as a proxy for the calorimetric efficiency.

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“…We note that magnetic field-gas relation in multi-phase simulations is very complex and cannot be easily described by a single power-law relationship consistent with these simple gas compressions. In particular, the relation depends on the Alfvénic Mach number, where κ = 2/3 is found in high Mach number regions of the cold ISM dominated by molecular hydrogen (Seta & Federrath 2022). However, as such regions would likely occupy a small fraction of the ISM and H i is mostly sub-to trans-sonic, flux freezing does not hold.…”

Section: What Regulates Magnetic Fields In Galaxies?mentioning

confidence: 99%

Nearby galaxies in the LOFAR Two-metre Sky Survey

Heesen

Klocke

Brüggen

et al. 2022

A&A

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Context. Magnetic fields, which regulate stellar feedback and star formation in galaxies, are key to understanding galaxy evolution. Aims. We probe the origin of magnetic fields in late-type galaxies, measuring magnetic field strengths and exploring whether magnetic fields are only passive constituents of the interstellar medium or whether, being part of the local energy equilibrium, they are active constituents. Methods. We measure equipartition magnetic field strengths in 39 galaxies from LoTSS-DR2 using LOFAR observations at 144 MHz with 6 arcsec angular resolution (0.1-0.7 kpc). For a subset of nine galaxies, we obtain atomic and molecular mass surface densities using H i and CO(2-1) data from the THINGS and HERACLES surveys, respectively. These data are at 13 arcsec angular resolution, which corresponds to 0.3-1.2 kpc at the distances of our galaxies. We measure kinetic energy densities using H i and CO velocity dispersions.Results. We find a mean magnetic field strength of 3.6-12.5 µG with a mean of 7.9 ± 2.0 µG across the full sample. The magnetic field strength has the tightest and steepest relation with the total gas surface density, with B ∝ Σ 0.309±0.006 H I+H 2 . The relations with the starformation rate surface density and molecular gas surface density have significantly flatter slopes. After accounting for the influence of cosmic-ray transport, we find an even steeper relation of B ∝ Σ 0.393±0.009 H I+H 2 . Conclusions. These results suggest that the magnetic field is regulated by a B-ρ relation, which has its origin in the saturation of the small-scale dynamo. This is borne out by an agreement of kinetic and magnetic energy densities, although local deviations do exist, in particular in areas of high kinetic energy densities where the magnetic field is sub-dominant.

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Turbulent dynamo in the two-phase interstellar medium

Cited by 5 publications

References 68 publications

Cosmic-ray diffusion and the multi-phase interstellar medium in a dwarf galaxy. I. Large-scale properties and $γ$-ray luminosities

Cosmic-ray diffusion and the multi-phase interstellar medium in a dwarf galaxy. I. Large-scale properties and $γ$-ray luminosities

Nearby galaxies in the LOFAR Two-metre Sky Survey

Nearby galaxies in the LOFAR Two-metre Sky Survey

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