Extragalactic magnetism with SOFIA (Legacy Program) -- II: A magnetically-driven flow in the starburst ring of NGC 1097

Abstract

We present the properties of the magnetic field (B-field) in the starburst ring of the galaxy NGC 1097. Thermal polarized emission at 89 µm using HAWC+/SOFIA shows that the polarized flux is spatially located at the contact regions of the outer-bar with the starburst ring. The linear polarization decomposition analysis shows that the 89 µm and radio (3.5 and 6.2 cm) polarization traces two different modes, m, of the B-field: a constant B-field orientation and dominated by m = 0 at 89 µm, and a spiral B-field dominated by m = 2 at radio. The 12 CO(2-1) integrated emission line peaks in the inner ∼ 0.69 kpc, the FIR polarized flux peaks in the central ∼ 1.02 kpc, and the radio polarized flux peaks in the outer ∼ 1.39 kpc of the starburst ring. Dust temperature is higher, T 89 µm = 30.7 ± 0.4 K, at the region of the FIR polarized flux than at the radio's region, T 3.5 cm = 26.2 ± 1.7 K. We argue that the B-field at 89 µm is concentrated in the warmest region of a shock front caused by the galactic density wave in the contact regions between the outer-bar with the starburst ring. Radio polarization traces a superposition of the spiral B-field outside and within the starburst ring. According to Faraday rotation measures between 3.5 and 6.2 cm, the radial component of the B-field along the contact regions points toward the galaxy's center on both sides. Gas streams follow the B-field, which feeds the black hole with matter from the host galaxy.