Chemically Distinct Nuclei and Outflowing Shocked Molecular Gas in Arp
  220
  We present the results of interferometric spectral line observations of Arp
220 at 3.5mm and 1.2mm from the Plateau de Bure Interferometer (PdBI), imaging
the two nuclear disks in H$^{13}$CN$(1 - 0)$ and $(3 - 2)$, H$^{13}$CO$^+(1 -
0)$ and $(3 - 2)$, and HN$^{13}$C$(3 - 2)$ as well as SiO$(2 - 1)$ and $(6 -
5)$, HC$^{15}$N$(3 - 2)$, and SO$(6_6 - 5_5)$. The gas traced by SiO$(6 - 5)$
has a complex and extended kinematic signature including a prominent P Cygni
profile, almost identical to previous observations of HCO$^+(3 - 2)$. Spatial
offsets $0.1''$ north and south of the continuum centre in the emission and
absorption of the SiO$(6 - 5)$ P Cygni profile in the western nucleus (WN)
imply a bipolar outflow, delineating the northern and southern edges of its
disk and suggesting a disk radius of $\sim40$ pc, consistent with that found by
ALMA observations of Arp 220. We address the blending of SiO$(6 - 5)$ and
H$^{13}$CO$^+(3 - 2)$ by considering two limiting cases with regards to the
H$^{13}$CO$^+$ emission throughout our analysis. Large velocity gradient (LVG)
modelling is used to constrain the physical conditions of the gas and to infer
abundance ratios in the two nuclei. Our most conservative lower limit on the
[H$^{13}$CN]/[H$^{13}$CO$^+$] abundance ratio is 11 in the WN, cf. 0.10 in the
eastern nucleus (EN). Comparing these ratios to the literature we argue on
chemical grounds for an energetically significant AGN in the WN driving either
X-ray or shock chemistry, and a dominant starburst in the EN.
