Luminous HC3N line and the HCN/HCO+ ratio in NGC 4418 -: Buried AGN or nascent starburst?

Aims. We investigate the properties of the nuclear molecular gas and address the nature of the deeply buried source driving the IR emission of NGC 4418. Methods. We present IRAM 30 m observations and basic non-LTE, single component radiative transport modelling of HNC, HCN, HCO+, CN, HC3N, and H2CO. Results. We find that NGC 4418 has a rich molecular chemistry - including unusually luminous HC3N J = 10 - 9, 16-15, and 25-24 line emission - compared to the other high density tracers. We furthermore detect: ortho-H2CO 2-1, 3-2; CN 1-0, 2-1; HCO+, 1-0. 3-2, HCN 3-2, HNC 1-0, 3-2, and tentatively OCS 12-11. The HCN, HCO+, H2CO, and CN line emission can be fitted to densities of n = 5 x 10(4) - 10(5) cm(-3) and gas temperatures T-k = 80 - 150 K. Both HNC and HC3N are, however, significantly more excited than the other species, which requires higher gas densities or radiative excitation through mid-IR pumping. The HCN line intensity is fainter than that of HCO+ and HNC for the 3-2 transition, in contrast to previous findings for the 1-0 lines, where the HCN emission is the most luminous. Assuming all line emission is emerging from the same gas, abundances of the observed species are estimated to be similar to each other within factors of 2-5. The most noteworthy of these is the high abundance of HC3N and a small-to-moderate abundance ratio between HCN and HCO+. Conclusions. We tentatively suggest that the observed molecular line emission is consistent with a young starburst, where the emission can be understood as emerging from dense, warm gas with an additional PDR component. We find that X-ray chemistry is not required to explain the observed mm-line emission, including the HCN/HCO+ 1-0 and 3-2 line ratios. The luminous HC3N line emission is an expected signature of dense, starforming gas. A deeply buried AGN cannot be excluded, but its impact on the surrounding molecular medium is then suggested to be limited. However, detailed modelling of HC3N abundances in X-ray dominated regions (XDRs) should be carried out. The possibility of radiative excitation should also be investigated further.