Circumnuclear Spiral Arms and Starburst Rings in Magnetized Barred Spiral Galaxies

Abstract

The Seyfert galaxy NGC 1097 has an extended neutral hydrogen disk, a companion, a prominent bar and a luminous circumnuclear starburst "ring". Magnetic fields as revealed by nonthermal radio-continuum emissions correlate well with the optical barred spiral structure on large scales, have a gross enhancement overlapping with the optical/infrared "ring", and show a trailing swirl around and within the "ring". We propose a scenario of bar-excited long-trailing fast magnetohydrodynamic (MHD) density waves at the modified inner Lindblad resonance (mILR), physically identified with the outer rim of the "ring". These sustained outgoing long-waves are bounced back by the $Q_M$-barrier in the form of incoming short-trailing waves. The damping of these waves deposits a {\it negative} angular momentum into the magnetized circumnuclear gas disk. Thus, gas materials spiral inward, bring in frozen-in magnetic flux, and accumulate inside the mILR to create a circular zone of high density and magnetic flux vulnerable to massive star formation. Depending on the wave damping efficiency, this process may simultaneously sustain a net mass inflow across the "ring" and toward the nucleus. A wavelet analysis on a Hubble Space Telescope image of central NGC 1097 shows a distinct two-arm spiral structure extended down to the nucleus as a strong evidence for circumnuclear MHD density waves. We predict that magnetic-field observations with improved sensitivity and resolution would reveal a specific correspondence between circumnuclear optical and magnetic field spirals much as those known to exist on large scales in nearby spiral galaxies, including NGC 1097.