Molecular shock tracers in NGC 1068: SiO and HNCO
  SiO(3-2) and HNCO(6-5) emission has been imaged in NGC 1068 with the Plateau
de Bure Interferometer (PdBI). We perform an LTE and RADEX analysis to
determine the column densities and physical characteristics of the gas emitting
these two lines. We then use a chemical model to determine the origin of the
emission. There is a strong SiO peak to the East of the AGN, with weak
detections to the West. This distribution contrasts that of HNCO, which is
detected more strongly to the West. The SiO emission peak in the East is
similar to the peak of the molecular gas mass traced by CO. HNCO emission is
offset from this peak by as much as 80 pc ( 1"). We compare velocity integrated
line ratios in the East and West. We confirm that SiO emission strongly
dominates in the East, while the reverse is true in the West. We use RADEX to
analyse the possible gas conditions that could produce such emission. We find
that, in both East and West, we cannot constrain a single temperature for the
gas. We run a grid of chemical models of potential shock processes in the CND
and find that SiO is significantly enhanced during a fast (60 km/s) shock but
not during a slow (20 km/s) shock, nor in a gas not subjected to shocks at all.
We find the inverse for HNCO, whose abundance increases during slow shocks and
in warm non-shocked gas. High SiO and low HNCO indicated a fast shock, while
high HNCO and low SiO indicates either a slow shock or warm, dense, non-shocked
gas. The East Knot is therefore likely to contain gas that is heavily shocked.
From chemical modelling, gas in the West Knot may be non-shocked, or maybe
undergoing a much milder shock event. When taking into account RADEX results,
the milder shock event is the more likely of the two scenarios.
