## 4. An energetic explanation for the occurrence of the PW in different galaxy modelsThe origin of PWs, and of the flows of a different nature obtained
by CDPR, can be explained qualitatively in terms of the where and where is the total potential, is the one-dimensional isotropic velocity dispersion obtained by solving the Jeans equations for the adopted mass model (e.g., Binney & Tremaine 1987), and is the specific mass return rate. The global is obtained by replacing the differentials in Eq. (2) with their integrals over the whole galaxy. The differences in the flow behavior of JH models and models with a
central constant density region can be understood with the aid of
Fig. 6, where is shown for four
representative one-component models. These have the same global
, and so from an energetical point of view are
globally equivalent. Three of them are -models,
with , i.e., they are the Jaffe model, the
Hernquist model, and a model with a core. The fourth density
distribution is described in the Appendix, and is flatter at the
center than the model, but for large
It is apparent from Fig. 6 that the steeper the density
profile, the stronger is the variation of
across the galaxy, and the higher are the values that it reaches at
the center. This explains why a strong decoupling can be present in
the flow of highly concentrated systems: a
significantly higher than unity in the central regions produces a
central inflow, while in the external parts a degassing is
energetically possible because . A second effect on is produced by the dark halo. If this halo is more diffuse than the stellar mass, increasing makes more bound the external regions, when is kept constant, and so the radius at which moves outward (see Fig. 7 for the case of JH models).
The trend of for models with a core explains the results of the numerical simulations for King models plus diffuse quasi-isothermal dark halos obtained by CDPR and Pellegrini & Fabbiano (1994). The flow phases found by CDPR with were all global; with low , instead, the flow can be decoupled again, as found by Pellegrini & Fabbiano (1994) in their detailed modeling of the X-ray properties of two ellipticals. This because with high the region with constant is very large (of the order of the core radius of the dark halo), while with low this region is of the order of the stellar core radius. In summary, the combined effect of SNIa's and dark matter is more varied for models with a core than for JH models: for a large range of values, the latter keep in PWs with a varying , while the former can be in wind, PWs, outflows or inflows. © European Southern Observatory (ESO) 1998 Online publication: April 20, 1998 |