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 in 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 $53$-$890$ $\mu$m imaging polarimetric observations with SOFIA/HAWC+, JCMT/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 $T_{\mathrm{d},\mathrm{outflow}}^{\mathrm{PI}}\sim 45$ K and emissive index of ${\beta }_{\mathrm{outflow}}^{\mathrm{PI}}\sim 2.3$. The disks are characterized by low $T_{\mathrm{d},\mathrm{disk}}^{\mathrm{PI}}=[24,31]$ 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 $53$-$850$ $\mu$m 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, $⟨P_{\mathrm{disk}}⟩=1.2\pm 0.5$%. The dust models of heterogenous clouds and two temperature components are required to explain the polarization spectrum of the outflow ($2$-$4$% at $53$ $\mu$m, $\sim 1$% at $850$ $\mu$m, and a minimum within $89$-$154$ $\mu$m). 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 $89$-$214$ $\mu$m reaching heights of $\sim 4$ kpc, and flatter polarized fluxes than total fluxes. The extension of the B-field permeating the circumgalactic medium increases with increasing the star formation rate.

arXiv comment21 pages, 14 figures, 4 tables. Submitted to ApJ with positive review by the anonymous referee. Comments are welcome

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