Physical properties of dense molecular gas in centres of Seyfert
  galaxies
  We present new ~1" resolution data of the dense molecular gas in the central
50-100 pc of four nearby Seyfert galaxies. PdBI observations of HCN and, in 2
of the 4 sources, simultaneously HCO+ allow us to carefully constrain the
dynamical state of the dense gas surrounding the AGN. Analysis of the
kinematics shows large line widths of 100-200 km/s FWHM that can only partially
arise from beam smearing of the velocity gradient. The observed morphological
and kinematic parameters (dimensions, major axis position angle, red and blue
channel separation, and integrated line width) are well reproduced by a thick
disk, where the emitting dense gas has a large intrinsic dispersion (20-40
km/s), implying that it exists at significant scale heights (25-30% of the disk
radius). To put the observed kinematics in the context of the starburst and AGN
evolution, we estimate the Toomre Q parameter. We find this is always greater
than the critical value, i.e. Q is above the limit such that the gas is stable
against rapid star formation. This is supported by the lack of direct evidence,
in these 4 Seyfert galaxies, for on-going star formation close around the AGN.
Instead, any current star formation tends to be located in a circumnuclear
ring. We conclude that the physical conditions are indeed not suited to star
formation within the central ~100 pc.
