The unbearable opaqueness of Arp 220
  We explore the potential of imaging vibrationally excited molecular emission
at high angular resolution to better understand the morphology and physical
structure of the dense gas in Arp~220 and to gain insight into the nature of
the nuclear powering sources. Vibrationally excited emission of HCN is detected
in both nuclei with a very high ratio relative to the total $L_{FIR}$, higher
than in any other observed galaxy and well above what is observed in Galactic
hot cores. HCN $v_2=1f$ is observed to be marginally resolved in
$\sim60\times50$~pc regions inside the dusty $\sim100$~pc sized nuclear cores.
Its emission is centered on our derived individual nuclear velocities based on
HCO$^+$ emission ($V_{WN}=5342\pm4$ and $V_{EN}=5454\pm8$~\kms, for the western
and eastern nucleus, respectively). With virial masses within $r\sim25-30$~pc
based on the HCN~$v_2=1f$ line widths, we estimate gas surface densities (gas
fraction $f_g=0.1$) of $3\pm0.3\times10^4~M_\odot~\rm pc^{-2}$ (WN) and
$1.1\pm0.1\times10^4~M_\odot~\rm pc^{-2}$ (EN). The $4-3/3-2$ flux density
ratio could be consistent with optically thick emission, which would further
constrain the size of the emitting region to $>15$~pc (EN) and $>22$~pc (WN).
The absorption systems that may hide up to $70\%$ of the HCN and HCO$^+$
emission are found at velocities of $-50$~\kms~(EN) and $6$, $-140$, and
$-575$~\kms (WN) relative to velocities of the nuclei. Blueshifted absorptions
are the evidence of outflowing motions from both nuclei. The bright vibrational
emission implies the existence of a hot dust region radiatively pumping these
transitions. We find evidence of a strong temperature gradient that would be
responsible for both the HCN $v_2$ pumping and the absorbed profiles from the
vibrational ground state as a result of both continuum and self-absorption by
cooler foreground gas.
