The Magnetic Fields of Starburst Galaxies. I. Identification and Characterization of the Thermal Polarization in the Galactic Disk and Outflow

Abstract

Far-infrared polarized emission by means of magnetically aligned dust grains is an excellent tracer of the magnetic fields (B-fields) in the cold phase of the galactic outflows of starburst galaxies. We present a comprehensive study of the B-fields in three nearby (3.5–17.2 Mpc) starbursts (M82, NGC 253, and NGC 2146) at 5 pc–1.5 kpc resolutions using publicly available 53–890 μm imaging polarimetric observations with Stratospheric Observatory for Infrared Astronomy/HAWC+, James Clerk Maxwell Telescope/POL-2, and ALMA. We find that the polarized spectral energy distributions (SEDs) of the full galaxies are dominated by the polarized SEDs of the outflows with dust temperatures of T d , outflow PI ∼ 45 K and an emissive index of β outflow PI ∼ 2.3 . The disks are characterized by low T d , disk PI = [ 24 , 31 ] K and β disk PI ∼ 1 . We show that disk- and outflow-dominated galaxies can be better distinguished by using polarized SEDs instead of total SEDs. We compute the 53–850 μm polarization spectrum of the disk and outflow, and find that dust models of the diffuse ISM can reproduce the fairly constant polarization spectrum of the disk, 〈 P disk〉 = 1.2% ± 0.5%. The dust models of heterogenous clouds and two-temperature components are required to explain the polarization spectrum of the outflow (2%–4% at 53 μm, ∼1% at 850 μm, and a minimum within 89–154 μm). We conclude that the polarized dust grains in the outflow arise from a dust population with higher dust temperature and emissivities than those from the total flux. The B-fields of the outflows have maximum extensions within 89–214 μm reaching heights of ∼4 kpc, and have flatter polarized fluxes than total fluxes. The extension of the B-field permeating the circumgalactic medium increases as the star formation rate increases.

Extragalactic Magnetism with SOFIA (SALSA Legacy Program). VII. A Tomographic View of Far-infrared and Radio Polarimetric Observations through MHD Simulations of Galaxies

“…In this context, magnetohydrodynamical (MHD) simulations of galaxy formation emerge as a powerful method to unravel the complexity of magnetic fields and their connection with galaxy properties and observables. During the last decade, multiple MHD high-resolution studies have been able to simulate realistic galaxies with magnetic properties in broad agreement with those inferred by global properties of observed galaxies (e.g., Dubois & Teyssier 2010;Pakmor et al 2017;Hopkins et al 2020;Martin-Alvarez et al 2021, 2023. These simulations have been able to capture both the turbulent and large-scale dynamo amplification processes.…”

“…This result may suggest that these two wavelength ranges do not capture a considerable amount of magnetic energy, likely due to the decrease of number density with the height of both electronic CRs and dust. Furthermore, with magnetic field lines above the galactic disk likely oriented along the direction of winds and outflows (Lopez-Rodriguez et al 2021;Lopez-Rodriguez 2023), synchrotron, and dust emission will now be preferentially oriented toward LOSs with some degree of inclination.…”

“…Theoretical and observational studies predict that the magnetic energy budget is comparable to that of the thermal and turbulent components in the ISM (Beck 2007;Bernet et al 2008;Mao et al 2017;Pakmor et al 2017;Martin-Alvarez et al 2020;Lopez-Rodriguez et al 2021;Geach et al 2023;Lopez-Rodriguez 2023), and dominates over the thermal component in cold molecular clouds (Crutcher 1999). Magnetic fields impact the ISM, due to their significant strengths, with typical values in the ∼3-7 μG range for the large-scale ordered component and are typically on the order of ∼17 μG for the total field strength (Fletcher 2010;Beck et al 2019).…”

“…Some of their effects are the co-regulation of star formation (McKee & Ostriker 2007), modifying the gas distribution across ISM phases (Iffrig & Hennebelle 2017;Hennebelle & Inutsuka 2019), the fragmentation of gas (Inoue & Yoshida 2019), and influencing the formation of molecular clouds (Inoue et al 2018;. Their effects expand beyond small galactic scales, as magnetic fields are likely to modify gas mixing in halos and outflows (Cottle et al 2020;Lopez-Rodriguez et al 2021;Van De Voort et al 2021;Buie et al 2022;Lopez-Rodriguez 2023), general properties of galaxies (Pillepich et al 2018;Martin-Alvarez et al 2020), and even be non-negligible during galaxy hierarchical growth (Whittingham et al 2021). One of the few galaxy parameters that appear to not be directly affected by the predicted magnetic fields of ∼μG are galaxy stellar masses (Su et al 2017;Martin-Alvarez et al 2020), except for dwarf galaxies, where the effects of magnetic fields are still not fully understood (Martin-Alvarez et al 2023;Whitworth et al 2023;Sanati et al 2024).…”