ALMA Observations of Warm Molecular Gas and Cold Dust in NGC 34

Abstract

We present Atacama Large Millimeter Array (ALMA) Cycle-0 observations of the CO (6-5) line emission (rest-frame frequency = 691.473 GHz) and of the 435 μm dust continuum emission in the nuclear region of NGC 34, a local luminous infrared galaxy at a distance of 84 Mpc (1'' = 407 pc) which contains a Seyfert 2 active galactic nucleus (AGN) and a nuclear starburst. The CO emission is well resolved by the ALMA beam (0.''26 × 0.''23), with an integrated flux of f_(CO(6-5)) = 1004 (± 151) Jy km s^(–1). Both the morphology and kinematics of the CO (6-5) emission are rather regular, consistent with a compact rotating disk with a size of 200 pc. A significant emission feature is detected on the redshifted wing of the line profile at the frequency of the H^(13)CN (8-7) line, with an integrated flux of 17.7 ± 2.1(random) ± 2.7(systematic) Jy km s^(–1). However, it cannot be ruled out that the feature is due to an outflow of warm dense gas with a mean velocity of 400 km s^(–1). The continuum is resolved into an elongated configuration, and the observed flux corresponds to a dust mass of M_(dust) = 10^(6.97 ± 0.13) M☉. An unresolved central core (radius ≃50 pc) contributes 28% of the continuum flux and 19% of the CO (6-5) flux, consistent with insignificant contributions of the AGN to both emissions. Both the CO (6-5) and continuum spatial distributions suggest a very high gas column density (≳10^4 M☉ pc^(–2)) in the nuclear region at radius ≾100 pc.