Nearby galaxies in the LOFAR Two-metre Sky Survey

Abstract

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.

Galactic Magnetic Fields. I. Theoretical Model and Scaling Relations

“…With these values, we obtain the values in Table 7. These rough estimates are consistent, at the order-of-magnitude level, with those of Heesen et al (2023), who estimate a range 4.7-7.4 μG for  B . Moreover, we generally find…”

“…This can be taken as further evidence that turbulence driving in nearby galaxies is dominated by SN Notes. Fitting data for the total magnetic field strength in multiple galaxies (estimated assuming energy equipartition with cosmic rays), Heesen et al (2023) obtain  B in the range 4.7-7.4 μG. Our model coefficients are generally of the same order of magnitude as theirs.…”

“…There has also been a flurry of very recent work. Heesen et al (2023) find j = 0.182 ± 0.004, but with considerable scatter, or j = 0.22 when they use the mean values of each of the 12 galaxies considered. However, when they account for cosmic-ray transport, they infer a value of j = 0.284 ± 0.006 instead of 0.182 ± 0.004.…”

ViCTORIA project: The LOFAR view of environmental effects in Virgo cluster star-forming galaxies

“…Two notable outliers with a large transport length are NGC 4254 and NGC 4548 with l CRe > 5 kpc. Those values are larger than the values ≤3.8 found for field galaxies at 144 MHz in Heesen et al (2023b). This is likely due to the significant contribution of RPS (and in the case of NGC 4254, also the tidal perturbation, Vollmer et al 2005a) to the CRe-transport, a scenario also suggested by Ignesti et al (2022b).…”

Diffusion of cosmic-ray electrons in M 51 observed with LOFAR at 54 MHz

“…We now calculate the electron lifetime taking into account synchrotron and inverse Compton losses. For that we use the average total (mostly random) equipartition magnetic field strength of B = 12.0 ± 2.6 µG (Heesen et al 2023). The radiation energy density U rad is the sum of two parts: the CMB energy density, which can be measured directly, and the total energy density U TIR with an additional contribution of stellar light.…”