Stability of thick two-component galactic discs

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Abstract
The crucial role that the cold interstellar gas can play in the dynamics and structure of early normal spiral galaxies has been shown in our previous works, where finite-thickness effects have not been taken into account. In view of the importance that such effects might have in the self-regulation mechanisms which are expected to operate in galactic discs and to be at the basis of their secular heating, we have tried to evaluate them. This can be done only after their vertical structure at equilibrium has been carefully investigated. A detailed analysis has thus been carried out to study the thickness-scales and the local parameters relevant to both the equilibrium and stability of two-component galactic discs in regimes of astrophysical interest. The results obtained, as regards the vertical structure at equilibrium of two-component galactic discs, are used to investigate their local linear stability properties. Under reasonable assumptions, finite-thickness corrections to the local dispersion relation can be expressed in terms of two reduction factors weakening the response of the two components or, equivalently, lowering their equilibrium surface densities. An ansatz for such reduction factors, justified by extending the analysis performed for one-component purely stellar discs, is made, and the corresponding two-fluid marginal stability curve is studied in standard star-dominated and peculiar gas-dominated regimes. It is found that the stabilizing role of finite-thickness effects can partially counterbalance the destabilizing role of the cold interstellar gas in linear regimes.

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