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DISENTANGLING THE CIRCUMNUCLEAR ENVIRONS OF CENTAURUS A: GASEOUS SPIRAL ARMS IN A GIANT ELLIPTICAL GALAXY

We report the existence of spiral arms in the recently formed gaseous and dusty disk of the closest giant elliptical, NGC 5128 (Centaurus A), using high-resolution 12CO(2–1) observations of the central 3′ (3 kpc) obtained with the Submillimeter Array. This provides evidence that spiral-like features can develop within ellipticals if enough cold gas exists. We elucidate the distribution and kinematics of the molecular gas in this region with a resolution of 4.″4 × 1.″9 (80 pc × 40 pc). The spiral arms extend from the circumnuclear gas at a radius of 200 pc to at least 1 kpc. The general properties of the arms are similar to those in spiral galaxies: they are trailing, the width is ∼500 ± 200 pc, and the pitch angle is 20°. From independent estimates of the time when the H i-rich galaxy merger occurred, we infer that the formation of spiral arms happened on a timescale of less than ∼108 yr. The formation of spiral arms increases the gas density and thus the star formation efficiency in the early stages of the formation of a disk.