ALMA observations of CS in NGC 1068: chemistry and excitation
  We present results from Atacama Large Millimeter/submillimeter Array (ALMA)
observations of CS from the nearby galaxy NGC 1068 ($\sim14$ Mpc). This Seyfert
2 barred galaxy possesses a circumnuclear disc (CND, $r\sim200$ pc) and a
starburst ring (SB ring, $r\sim1.3$ kpc). These high-resolution maps
($\sim0.5$", $\sim35$ pc) allow us to analyse specific sub-regions in the
galaxy and investigate differences in line intensity ratios and physical
conditions, particularly those between the CND and SB ring. Local thermodynamic
equilibrium (LTE) analysis of the gas is used to calculate CS densities in each
sub-region, followed by non-LTE analysis conducted using the radiative transfer
code RADEX to fit observations and constrain gas temperature, CS column density
and hydrogen density. Finally, the chemical code UCLCHEM is used to reconstruct
the gas, allowing an insight into its origin and chemical history. The density
of hydrogen in the CND is found to be $\geq10^5$ cm$^{-2}$, although exact
values vary, reaching $10^6$ cm$^{-2}$ at the AGN. The conditions in the two
arms of the SB ring appear similar to one another, though the density found
($\sim10^4$ cm$^{-2}$) is lower than in the CND. The temperature in the CND
increases from east to west, and is also overall greater than found in the SB
ring. These modelling methods indicate the requirement for multi-phase gas
components in order to fit the observed emission over the galaxy. A larger
number of high resolution transitions across the SLED may allow for further
constraining of the conditions, particularly in the SB ring.
