Dense circum-nuclear molecular gas in starburst galaxies
  We present results from a study of the dense circum-nuclear molecular gas of
starburst galaxies. The study aims to investigate the interplay between
starbursts, active galactic nuclei and molecular gas. We characterise the dense
gas traced by HCN, HCO$^{+}$ and HNC and examine its kinematics in the
circum-nuclear regions of nine starburst galaxies observed with the Australia
Telescope Compact Array. We detect HCN (1$-$0) and HCO$^{+}$ (1$-$0) in seven
of the nine galaxies and HNC (1$-$0) in four. Approximately 7 arcsec resolution
maps of the circum-nuclear molecular gas are presented. The velocity integrated
intensity ratios, HCO$^{+}$ (1$-$0)/HCN (1$-$0) and HNC (1$-$0)/HCN (1$-$0),
are calculated. Using these integrated intensity ratios and spatial intensity
ratio maps we identify photon dominated regions (PDRs) in NGC 1097, NGC 1365
and NGC 1808. We find no galaxy which shows the PDR signature in only one part
of the observed nuclear region. We also observe unusually strong HNC emission
in NGC 5236, but it is not strong enough to be consistent with X-ray dominated
region (XDR) chemistry. Rotation curves are derived for five of the galaxies
and dynamical mass estimates of the inner regions of three of the galaxies are
made.
