Context :

Aims : We investigate the molecular gas properties of the deeply obscured luminous infrared galaxy NGC 4418 .
We address the excitation of the complex molecule HC _ { 3 } N to determine whether its unusually luminous emission is related to the nature of the buried nuclear source .

Methods : We use IRAM 30m and JCMT observations of rotational and vibrational lines of HC _ { 3 } N to model the excitation of the molecule by means of rotational diagrams .

Results : We report the first confirmed extragalactic detection of vibrational lines of HC _ { 3 } N. 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 v _ { 6 } and v _ { 7 } , 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 HC _ { 3 } N abundance , of the order of 10 ^ { -7 } .

Conclusions : The excitation of the HC _ { 3 } N 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 HC _ { 3 } N line emission is a result of both high abundances and excitation .
The properties of the HC _ { 3 } N emitting gas are similar to those found for hot cores in Sgr B2 , which implies that the nucleus ( < 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 .