In a spiral galaxy, disks of stars and of magnetized interstellar medium (ISM) are dynamically coupled by gravity. The theory of magnetohydrodynamic (MHD) density waves in such a composite rotating disk system provides a basic framework for studying large-scale dynamics and multiwavelength diagnostics of galactic structures. As swirllike, trailing spiral structures with broad, fuzzy arms (regular or irregular) manifest in neutral hydrogen H Ⅰ disks that are usually larger than optical spiral patterns in disk galaxies, spiral MHD density waves in H Ⅰ gas disks should persist within and beyond optical patterns. Sporadic diffusions of relativistic cosmic-ray electrons beyond optical patterns may cause radio continuum arms to occasionally extend across optical spiral patterns and correlate with H Ⅰ spiral arms at larger radii. We here report the first H Ⅰ observations of the southern spiral galaxy NGC 2997 to support the fast MHD density wave (FMDW) scenario and to confirm the prior prediction that the isolated, polarized radio continuum arm discovered in the southeast quadrant is indeed associated with a broad segment of the H Ⅰ arm. For multiwavelength observations of spiral galaxies, we emphasize the important perspective that large-scale magnetic fields of MHD density waves play the key role in organizing correlated spiral structures of the various underlying ISM components. Meanwhile, we briefly discuss the nature of circumnu-clear structures of NGC 2997 in reference to those of NGC 1097.
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