2023ApJ...953..113L

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 ${T}_{{\rm{d}},\mathrm{outflow}}^{\mathrm{PI}}\sim 45$ K and an emissive index of ${\beta }_{\mathrm{outflow}}^{\mathrm{PI}}\sim 2.3$. The disks are characterized by low ${T}_{{\rm{d}},\mathrm{disk}}^{\mathrm{PI}}=[24,31]$ K and ${\beta }_{\mathrm{disk}}^{\mathrm{PI}}\sim 1$. 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.