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We present high resolution (0."4) IRAM PdBI and ALMA mm and submmobservations of the (ultra) luminous infrared galaxies ((U)LIRGs)IRAS17208-0014, Arp220, IC860 and Zw049.057 that reveal intense line emissionfrom vibrationally excited ($\nu_2$=1) J=3-2 and 4-3 HCN. The emission isemerging from buried, compact (r<17-70 pc) nuclei that have very high impliedmid-infrared surface brightness $>$$5\times 10^{13}$ L$_{\odot}$ kpc$^{-2}$.These nuclei are likely powered by accreting supermassive black holes (SMBHs)and/or hot (>200 K) extreme starbursts. Vibrational, $\nu_2$=1, lines of HCNare excited by intense 14 micron mid-infrared emission and are excellent probesof the dynamics, masses, and physical conditions of (U)LIRG nuclei when H$_2$column densities exceed $10^{24}$ cm$^{-2}$. It is clear that these lines openup a new interesting avenue to gain access to the most obscured AGNs andstarbursts. Vibrationally excited HCN acts as a proxy for the absorbedmid-infrared emission from the embedded nuclei, which allows for reconstructionof the intrinsic, hotter dust SED. In contrast, we show strong evidence thatthe ground vibrational state ($\nu$=0), J=3-2 and 4-3 rotational lines of HCNand HCO$^+$ fail to probe the highly enshrouded, compact nuclear regions owingto strong self- and continuum absorption. The HCN and HCO$^+$ line profiles aredouble-peaked because of the absorption and show evidence of non-circularmotions - possibly in the form of in- or outflows. Detections of vibrationallyexcited HCN in external galaxies are so far limited to ULIRGs and early-typespiral LIRGs, and we discuss possible causes for this. We tentatively suggestthat the peak of vibrationally excited HCN emission is connected to a rapidstage of nuclear growth, before the phase of strong feedback.

Probing highly obscured, self-absorbed galaxy nuclei with vibrationally excited HCN