Theoretical studies and numerical simulations

People involved at OAB: Cappi, Ciotti, D'Ercole, Lanzoni, Londrillo, Zamorani.

The main results in this field are summarized as follows:
$\bullet$ By combining the $M_{bh} -\sigma$ relation with the Fundamental Plane of elliptical galaxies, it was examined the possibility that the dissipationless merging is important in the formation and evolution of elliptical galaxies. One of the main results of this work is that a major role of dissipation in galaxy formation is strongly required.
$\bullet$ The question of whether a systematic non-homology could be partly responsible for the correlations that define the Fundamental Plane was reexamined. It is found that elliptical galaxies should not be considered homologous dynamical systems and that neither the strict homology nor the constant stellar mass-to-light solution are a satisfactory explanation of the observed Fundamental Plane.
$\bullet$ In the context of the physical interpretation of the Fundamental Plane of elliptical galaxies, the relations between the Fundamental Plane thinness and tilt, and the amount of radial orbital anisotropy, has been studied by using N-body simulations of galaxy models characterized by observationally motivated density profiles, and also allowing for the presence of massive dark matter halos. The main results of this work are that galaxy models that are radially anisotropic enough to be found outside the observed Fundamental Plane (with their isotropic parent models lying on the Fundamental Plane) are unstable, and their end-products fall back on the Fundamental Plane itself. In addition, it was also shown that a systematic increase of radial orbit anisotropy with galaxy luminosity cannot explain by itself the whole tilt of the Fundamental Plane, becoming the galaxy models unstable at moderately high luminosities: at variance with the previous case their end-products are found well outside the Fundamental Plane itself.
$\bullet$ From the theoretical point of view it has been studied how the presence of abundance gradients in the intracluster medium affect commonly adopted estimates of the average abundance, assuming various plausible ICM density and temperature profiles. It has been found that, by adopting the observed abundance gradients, the true average mass weighted abundance is less than (although not largely deviating from) the commonly used emission weighted abundance.
$\bullet$ An evolutive scenario which links the X-ray evolution of elliptical galaxies with the QSO evolution and activity has been developed. In this scenario the solution of the puzzling question of the lack of strong AGN activity in the large majority of elliptical galaxies containing a massive black hole at their center (activity naturally predicted in the standard cooling flow scenario) is due to an accretion mechanism modulated by radiation feedback. The feedback is due to Compton scattering of the radiation emitted by the accreting material on the electrons of the galactic hot gaseous halo. This scenario has been explored in great detail by using a specific numerical hydrocode which takes into account several aspects of radiative transfer. Follwing this work, a robust method to derive the duty cycle of QSO activity based on the empirical QSO luminosity function and on the present-day linear relation between the masses of supermassive black holes and those of their spheroidal host stellar systems has been developed. It application showed that the duty cycle is substantially less than unity, with characteristic values in the range $3-6\times 10^{-3}$, in excellent agreement with the theoretical results presented above.
$\bullet$ Finally, numerical and analytical techniques have been developed to construct axysimmetric dynamical models of elliptical galaxies.

These works are carried on by Ciotti in collaboration with prof. T.S. van Albada (University of Groningen, The Netherlands), G. Bertin (University of Milano and Scuola Normale Superiore, Pisa) and M. Del Principe (graduate student of SNS-Pisa), S. Pellegrini and C. Nipoti (Dip. Astron. BO), J.P. Ostriker (Cambridge University - UK and Princeton University - USA ) and Z. Haiman (Princeton University), and Londrillo, Lanzoni, Cappi, Zamorani.

D'Ercole, in collaboration with F. Matteucci, S. Recchi (University of Trieste) and Tosi continued the studies about the effects of SN explosions in starburst galaxies. In particular, 2D simulations have been performed studying the effects of two instantaneous starbursts, separated by a quiescent period, on the dynamical and chemical evolution of Blue Compact Dwarf galaxies. The simulations followed the evolution of a first weak burst of star formation followed by a second more intense one occurring after several hundred million years. The evolution of several chemical abundances and the contribution of both SNII and SNI have been considered. A comparison with IZw18 shows that this starburst galaxy must have experienced two bursts of star formation, one occurred about 300 Myr ago and a recent one with an age between 4-7 Myr.

In the same vein, D'Ercole extended the hydrocode to 3D in order to study problems where the axis-simmetry does not hold. In particular, it is now under study the effect of the ICM ram-pressure on the galactic winds of starburst galaxies.