Astron. Astrophys. 333, 433-444 (1998)

5. Conclusions

Using evolutionary hydrodynamical simulations, we have investigated the properties of hot gas flows in elliptical galaxies described by a Jaffe stellar density profile, and containing various amounts of dark matter, distributed as a Hernquist law; the rate of SNIa explosions is one-fourth of the Tammann's rate, or lower. The results can be summarized as follows.

The simulations show that most of the gas flows are strongly decoupled, i.e., they develop an inflow in the central region of the galaxy, while the external parts are still degassing ¹. The stagnation radius of these flows (PWs) may range from a small fraction of the optical effective radius to many . This situation is present even when the global energy balance of the flow indicates that the energy inputs from SNIa's and from the thermalization of the stellar random motions is high enough to unbind all the gas (). Alternatively, the gas can be outflowing from the outer part of the galaxy even when the global energy balance indicates that the energy available is less than needed to unbind it all (). So, is not a good indicator of the flow phase; larger values, though, correspond to larger stagnation radii, for a fixed . Global inflows are produced when the dark matter content is so high to bind the gas over the whole galaxy.

The X-ray luminosity of PWs is higher when is lower, because the inflow region is larger; for global inflows is higher when is also higher, because the gas is hotter. The key factor causing large variations in JH models is the size of the central inflow region. The lowest values observed can be easily reproduced by PWs; high dark matter contents are required to approach the highest observed.

The shape of the X-ray surface brightness profile of PWs is close to that of global inflows, over most of the optical image, if the stagnation radius is a few ; it is externally less peaked than that, if is a fraction of , i.e., when the gas is hotter than in the inflow case.

The strong decoupling of the flow is explained by the radial trend of the local energy balance , that is very peaked for steep mass density profiles as in the JH models. Previously used density profiles which were flat at the center - such as the King models plus quasi-isothermal dark halos - showed a larger tendency to have global flow phases, because also their is flatter. So, the change of the mass profile has an important effect: both numerical simulations and analytical calculations show how peaked density profiles preferentially develop decoupled gas flows (in the sense that the region of the parameter space populated by PWs is large compared to that of inflows or winds).

The results of our simulations compare with the observations in two main aspects. The first is that most of the observed spread in shown by the data is easily reproduced: the highest are again, as in CDPR, associated with global inflows, while the bulk of the galaxies are now in PW, rather than in outflow; global winds are present only in the smallest galaxies.

The second is that the presence of cold gas at the center of Es, and a peaked X-ray surface brightness profile, cannot be unequivocally associated to a cooling flow: a PW could be present as well, with a significant part of the galaxy degassing. Particularly, a galaxy can have and still a non negligible amount of cold matter at the center. In addition, since varies over a wide range of radii, PWs can accumulate largely varying quantities of cold gas.

© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998
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