ALMA Observation of NGC 5135: The Circumnuclear CO (6–5) and Dust Continuum Emission at 45 pc Resolution*

Abstract

We present high-resolution (0.″17 × 0.″14) Atacama Large Millimeter/submillimeter Array (ALMA) observations of the CO (6–5) line and 435 μ m dust continuum emission within a ∼9″ × 9″ area centered on the nucleus of the galaxy NGC 5135. NGC 5135 is a well-studied luminous infrared galaxy that also harbors a Compton-thick active galactic nucleus (AGN). At the achieved resolution of 48 × 40 pc, the CO (6–5) and dust emissions are resolved into gas “clumps” along the symmetrical dust lanes associated with the inner stellar bar. The clumps have radii in the range of ∼45–180 pc and CO (6–5) line widths of ∼60–88 . The CO (6–5) to dust continuum flux ratios vary among the clumps and show an increasing trend with the [Fe ii ]/Br γ ratios, which we interpret as evidence for supernova-driven shocked gas providing a significant contribution to the CO (6–5) emission. The central AGN is undetected in continuum, nor is it detected in CO (6–5) if its line velocity width is no less than ∼ 40 . We estimate that the AGN contributes at most 1% of the integrated CO (6–5) flux of 512 ± 24 Jy within the ALMA field of view, which in turn accounts for ∼32% of the CO (6–5) flux of the whole galaxy.