Chemical complexity in NGC 1068

Abstract

We aimed to study the chemistry of the circumnuclear molecular gas of NGC 1068, and to compare it with those of the starburst galaxies M 82 and NGC 253. Using the IRAM-30 m telescope, we observed the inner 2kpc of NGC 1068 between 86.2 GHz and 115.6 GHz. We identified 35 spectral features, corresponding to 24 different molecular species. Among them, HC3N, SO, N2H+, CH3CN, NS, 13CN, and HN13C are detected for the first time in NGC 1068. Assuming local thermodynamic equilibrium (LTE), we calculated the column densities of the detected molecules, as well as the upper limits to the column densities of some undetected species. The comparison among the chemistries of NGC 1068, M 82, and NGC 253, suggests that, apart from X-rays, shocks also determine the chemistry of NGC 1068. We propose the column density ratio between CH3CCH and HC3N as a prime indicator of the imprints of starburst and AGN environments in the circumnuclear interstellar medium. This ratio is, at least, 64 times larger in M 82 than in NGC 1068, and, at least, 4 times larger in NGC 253 than in NGC, 1068. Finally, we used the UCL_CHEM and UCL_PDR chemical codes to constrain the origin of the species, as well as to test the influence of UV radiation fields and cosmic rays on the observed abundances.