2015A&A...584A..42A

We present high resolution (0.4'') IRAM PdBI and ALMA mm and submm observations of the (ultra) luminous infrared galaxies ((U)LIRGs) IRAS17208-0014, Arp220, IC860 and Zw049.057 that reveal intense line emission from vibrationally excited (ν₂=1) J=3-2 and 4-3 HCN. The emission is emerging from buried, compact (r<17-70pc) nuclei that have very high implied mid-infrared surface brightness >5x10¹³ L_☉kpc^–2. These nuclei are likely powered by accreting supermassive black holes (SMBHs) and/or hot (>200K) extreme starbursts. Vibrational, ν₂=1, lines of HCN are excited by intense 14µm mid-infrared emission and are excellent probes of the dynamics, masses, and physical conditions of (U)LIRG nuclei when H₂ column densities exceed 10²⁴cm^–2. It is clear that these lines open up a new interesting avenue to gain access to the most obscured AGNs and starbursts. Vibrationally excited HCN acts as a proxy for the absorbed mid-infrared emission from the embedded nuclei, which allows for reconstruction of the intrinsic, hotter dust SED. In contrast, we show strong evidence that the ground vibrational state (ν=0), J=3-2 and 4-3 rotational lines of HCN and HCO⁺ fail to probe the highly enshrouded, compact nuclear regions owing to strong self- and continuum absorption. The HCN and HCO⁺ line profiles are double-peaked because of the absorption and show evidence of non-circular motions - possibly in the form of in- or outflows. Detections of vibrationally excited HCN in external galaxies are so far limited to ULIRGs and early-type spiral LIRGs, and we discuss possible causes for this. We tentatively suggest that the peak of vibrationally excited HCN emission is connected to a rapid stage of nuclear growth, before the phase of strong feedback.