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

Abstract

We present ALMA Cycle-0 observations of the CO (6-5) line emission (rest-frame frequency = 691.473 GHz) and of the 435$\hat{I}¼m$ dust continuum emission in the nuclear region of NGC 34, a local luminous infrared galaxy (LIRG) 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 ($0."26\times 0."23$), with an integrated flux of ${\mathrm{f}}_{\mathrm{CO}(6-5)}=1004(\pm 151)\mathrm{Jy}\mathrm{km}{\mathrm{s}}^{-1}$. 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 red-shifted wing of the line profile at the frequency of the ${\mathrm{H}}^{13}\mathrm{CN}(8-7)$ line, with an integrated flux of $17.7\pm 2.1(\mathrm{random})\pm 2.7(\mathrm{sysmatic})\mathrm{Jy}\mathrm{km}{\mathrm{s}}^{-1}$. However, it cannot be ruled out that the feature is due to an outflow of warm dense gas with a mean velocity of $400\mathrm{km}{\mathrm{s}}^{-1}$. The continuum is resolved into an elongated configuration, and the observed flux corresponds to a dust mass of ${\mathrm{M}}_{\mathrm{dust}}=10^{6.97\pm 0.13}{\mathrm{M}}_{\mathrm{sun}}$. An unresolved central core ($\mathrm{radius}\simeq 50\mathrm{pc}$) 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 ($>=10^4{\mathrm{M}}_{\mathrm{sun}}\mathrm{p}{\mathrm{c}}^{-2}$) in the nuclear region at $\mathrm{radius}<=100\mathrm{pc}$.

10 pages, 13 figures, accepted for publication in Astrophysical Journal