A multi-transition study of molecules toward NGC 1068 based on high-resolution imaging observations with ALMA

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Abstract We present 0.8-mm band molecular images and spectra obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) toward one of the nearest galaxies with an active galactic nucleus (AGN), NGC 1068. Distributions of CO isotopic species (13CO and C18O) J = 3–2, CN N = 3–2, and CS J = 7–6 are observed toward the circumnuclear disk (CND) and a part of the starburst ring with an angular resolution of ∼ 1${^{\prime\prime}_{.}}$3 × 1${^{\prime\prime}_{.}}$2. The physical properties of these molecules and shock-related molecules, such as HNCO, CH3CN, SO, and CH3OH, detected in the 3-mm band were estimated using rotation diagrams under the assumption of local thermodynamic equilibrium. The rotational temperatures of the CO isotopic species and the shock-related molecules in the CND are, respectively, 14–22 K and upper limits of 20–40 K. Although the column densities of the CO isotopic species in the CND are only from one-fifth to one-third of that in the starburst ring, those of the shock-related molecules are enhanced by a factor of 3–10 in the CND. We also discuss the chemistry of each species, and compare the fractional abundances in the CND and starburst ring with those of Galactic sources such as cold cores, hot cores, and shocked molecular clouds in order to study the overall characteristics. We find that the abundances of shock-related molecules are more similar to abundances in hot cores and/or shocked clouds than to cold cores. The CND hosts relatively complex molecules, which are often associated with shocked molecular clouds or hot cores. Because a high X-ray flux can dissociate these molecules, they must also reside in regions shielded from X-rays.