Circumnuclear Spiral Arms and Starburst Rings in Magnetized Barred Spiral Galaxies

Abstract

The Seyfert galaxy NGC 1097 has 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 QM-barrier in the form of incoming short-trailing waves. The damping of these waves deposits a 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 eventually reveal a specific correspondence between circumnuclear optical and magnetic field spirals similar to those known to exist on large scales in nearby spiral galaxies, including NGC 1097.