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We discuss the dense molecular gas in central regions of nearby Seyfert galaxies, and report new arcsec resolution observations of HCN (1-0) and HCO+ (1-0) for three objects. In NGC 3079, the lines show complex profiles as a result of self-absorption and saturated continuum absorption. H13CN reveals the continuum absorption profile, with a peak close to the galaxy's systemic velocity that traces disc rotation, and a second feature with a blue wing extending to -350 km s-1 that most likely traces a nuclear outflow. The morphological and spectral properties of the emission lines allow us to constrain the dense gas dynamics. We combine our kinematic analysis for these three objects, as well as another with archival data, with a previous comparable analysis of four other objects, to create a sample of eight Seyferts. In seven of these, the emission line kinematics imply thick disc structures on radial scales of ~100 pc, suggesting such structures are a common occurrence. We find a relation between the circum-nuclear LHCN and Mdyn that can be explained by a gas fraction of 10 per cent and a conversion factor αHCN ~ 10 between gas mass and HCN luminosity. Finally, adopting a different perspective to probe the physical properties of the gas around active galactic nuclei, we report on an analysis of molecular line ratios which indicates that the clouds in this region are not self-gravitating. [ABSTRACT FROM AUTHOR]

Galactic gas-gas collisions involving a turbulent multiphase interstellar medium (ISM) share common ISM properties: dense extraplanar gas visible in CO, large linewidths (≳50 km s-1), strong mid-infrared H2 line emission, low star formation activity, and strong radio continuum emission. Gas-gas collisions can occur in the form of ram pressure stripping caused by the rapid motion of a spiral galaxy within the intracluster medium, galaxy head-on collisions, compression of the intragroup gas and/or galaxy ISM by an intruder galaxy which flies through the galaxy group at a high velocity, or external gas accretion on an existing gas torus in a galactic center. We suggest that the common theme of all these gas-gas interactions is adiabatic compression of the ISM leading to an increase of the turbulent velocity dispersion of the gas. The turbulent gas clouds are then overpressured and star formation is quenched. Within this scenario we developed a model for turbulent clumpy gas disks where the energy to drive turbulence is supplied by external infall or the gain of potential energy by radial gas accretion within the disk. The cloud size is determined by the size of a continuous (C-type) shock propagating in dense molecular clouds with a low ionization fraction at a given velocity dispersion. We give expressions for the expected volume and area filling factors, mass, density, column density, and velocity dispersion of the clouds. The latter is based on scaling relations of intermittent turbulence whose open parameters are estimated for the circumnuclear disk in the Galactic center. The properties of the model gas clouds (∼0.1 pc, ∼100 M⊙, Δv ≳ 6 kms-1) and the external mass accretion rate necessary for the quenching of the star formation rate due to adiabatic compression (Ṁ ∼ 1-10 M⊙ yr-1) are consistent with those derived from high-resolution H2 2.12 μm line observations. Based on these findings, a scenario for the evolution of gas tori in galactic centers is proposed and the implications for star formation in the Galactic center are discussed. [ABSTRACT FROM AUTHOR]

Recently, high-resolution observations with the help of the near-infrared adaptive optics integral field spectrograph Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) at the Very Large Telescope proved the existence of massive and young nuclear star clusters in the centres of a sample of Seyfert galaxies. With the help of three-dimensional high-resolution hydrodynamical simulations with the Pluto code, we follow the evolution of such clusters, especially focusing on stellar mass loss feeding gas into the ambient interstellar medium and driving turbulence. This leads to a vertically wide distributed clumpy or filamentary inflow of gas on large scales (tens of parsec), whereas a turbulent and very dense disc builds up on the parsec scale. In order to capture the relevant physics in the inner region, we treat this disc separately by viscously evolving the radial surface density distribution. This enables us to link the tens of parsec-scale region (accessible via SINFONI observations) to the (sub-)parsec-scale region (observable with the mid-infrared interferometer instrument and via water maser emission). Thereby, this procedure provides us with an ideal testbed for data comparison. In this work, we concentrate on the effects of a parametrized turbulent viscosity to generate angular momentum and mass transfer in the disc and additionally take star formation into account. Most of the input parameters are constrained by available observations of the nearby Seyfert 2 galaxy NGC 1068, and we discuss parameter studies for the free parameters. At the current age of its nuclear starburst of 250 Myr, our simulations yield disc sizes of the order of 0.8–0.9 pc, gas masses of and mass transfer rates of through the inner rim of the disc. This shows that our large-scale torus model is able to approximately account for the disc size as inferred from interferometric observations in the mid-infrared and compares well to the extent and mass of a rotating disc structure as inferred from water maser observations. Several other observational constraints are discussed as well. [ABSTRACT FROM AUTHOR]