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B. Canzian (U.S. Naval Observatory)
NGC4079 is a two-armed, grand design Sbc spiral galaxy. Its spiral structure is very regular and extensive, winding outward in continuous and smooth fashion from nearby the nucleus to large radius. The spiral arms are narrow near the nucleus, but become broad at large radius. The transition from narrow to broad spiral arms is interpreted here as the result of the difference in dynamics between regions interior to and exterior to the corotation resonance.
VLA HI spectral line observations of NGC4079 have been processed into an HI intensity map and velocity field. The galaxy's HI mass and extent and rotation curve are measured using the VLA maps. The geometric phase method applied to the spiral velocity perturbations locates the corotation resonance, verifying that the transition from narrow to broad spiral arms occurs in its vicinity. Other physical properties of the disk can be deduced using the rotation curve and knowledge of the corotation resonance location.
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