Vibrationally excited HC₃N in NGC 4418

Aims: We investigate the molecular gas properties of the deeply obscured luminous infrared galaxy NGC 4418. We address the excitation of the complex molecule HC₃N to determine whether its unusually luminous emission is related to the nature of the buried nuclear source. Methods: We use IRAM 30 m and JCMT observations of rotational and vibrational lines of HC₃N to model the excitation of the molecule by means of rotational diagrams. Results: We report the first confirmed extragalactic detection of vibrational lines of HC₃N. We detect 6 different rotational transitions ranging from J = 10-9 to J = 30-29 in the ground vibrational state and obtain a tentative detection of the J = 38-37 line. We also detect 7 rotational transitions of the vibrationally excited states v₆ and v₇, with angular momenta ranging from J = 10-9 to 28-27. The energies of the upper states of the observed transitions range from 20 to 850 K. In the optically thin regime, we find that the rotational transitions of the vibrational ground state can be fitted for two temperatures, 30 K and 260 K, while the vibrationally excited levels can be fitted for a rotational temperature of 90 K and a vibrational temperature of 500 K. In the inner 300 pc of NGC 4418, we estimate a high HC₃N abundance, of the order of 10$^{-7}$. Conclusions: The excitation of the HC₃N molecule responds strongly to the intense radiation field and the presence of warm, dense gas and dust at the center of NGC 4418. The intense HC₃N line emission is a result of both high abundances and excitation. The properties of the HC₃N emitting gas are similar to those found for hot cores in Sgr B2, which implies that the nucleus (łt}300 pc) of NGC 4418 is reminiscent of a hot core. The potential presence of a compact, hot component (T = 500 K) is also discussed.