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Abstract:

Active galactic nuclei are clearly heating gas in `cooling flows'. The effectiveness and spatial distribution of the heating are controversial. We use three-dimensional simulations on adaptive grids to study the impact on a cooling flow of weak, subrelativistic jets. The simulations show cavities and vortex rings as in the observations. The cavities are fast-expanding dynamical objects rather than buoyant bubbles as previously modelled, but shocks still remain extremely hard to detect with X-rays. At late times the cavities turn into overdensities that strongly excite the cluster's g-modes. These modes damp on a long timescale. Radial mixing is shown to be an important phenomenon, but the jets weaken the metallicity gradient only very near the centre. The central entropy density is modestly increased by the jets. We use a novel algorithm to impose the jets on the simulations.

Abstract:

We present results from a detailed dynamical analysis of five high surface brightness, late-type spiral galaxies, NGC 3810, NGC 3893, NGC 4254, NGC 5676, and NGC 6643, which were studied with the aim of quantifying the luminous-to-dark matter ratio inside their optical radii. The galaxies' stellar light distribution and gas kinematics have been observed and compared to hydrodynamic gas simulations that predict the gasdynamics arising in response to empirical gravitational potentials, which are combinations of differing stellar disk and dark halo contributions. The gravitational potential of the stellar disk was derived from near-infrared photometry, color corrected to yield a constant stellar mass-to-light ratio (M/L); for the dark halo, the mass density distribution of an axisymmetric isothermal sphere with a core was chosen. Hydrodynamic gas simulations were performed for each galaxy for a sequence of five different mass fractions of the stellar disk and for a wide range of spiral pattern speeds. These two parameters mainly determine the modeled gas distribution and kinematics. The agreement between the simulated and observed gas kinematics permitted us to conclude that the galaxies with the highest rotation velocities tend to possess very massive stellar disks that dominate the gasdynamics within the optical radius. In less massive galaxies, with a maximal rotation velocity of less than 200 km s-1, the mass of the dark halo at least equals the stellar mass within 2-3 disk scale lengths. The maximal disk stellar mass-to-light ratio in the K band was found to lie at about M/LK ≈ 0.6. Furthermore, the gasdynamic simulations provide a powerful tool for accurately determining the dominant spiral pattern speed for galaxies, independent of a specific density wave theory. It was found that the location of the corotation resonance falls into a narrow range of around three exponential disk scale lengths for all galaxies from the sample. The corotation resonance encloses the strong part of the stellar spiral in all cases. Based on the experience gained from this project, the use of a color correction to account for local stellar population differences is strongly encouraged when properties of galactic disks are studied that rely on their stellar mass distributions.