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Astron. Astrophys. 360, 76-84 (2000)

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5. A giant elliptical galaxy

In the first simulation we set a giant elliptical of gaseous mass of [FORMULA] to form through the collapse of a spherical DM halo of [FORMULA]. Each gas particle has an initial mass of [FORMULA].

Looking at Fig. 1 and Fig. 2, baryons slowly infall towards the center of the potential well, gas condenses, cools and then finally, stars begin to form at a rate of [FORMULA]. The strong episode of star formation lasts for about 2 Gyr and it is marked by a large rate of production of SNae of type II. These, however, are not able to expel from the galaxy a relevant fraction of the gas, given its strong gravitational field. At the end of this period, it has been processed into stars 80[FORMULA] of the original material. The remaining 20[FORMULA] is left partly in the outermost regions of the dark halo, partly out of the virial radius.

[FIGURE] Fig. 1. The formation of a giant elliptical. From the top to the bottom, snapshots refer to 1, 2, 3 and 9 Gyrs.

[FIGURE] Fig. 2. SF for the giant elliptical as a function of time. From the top to the bottom the same model is shown at increasing number of particles, i.e at increasing resolution.

The collapse of baryons in the DM potential well can be realized by noting that, at [FORMULA], in the innermost 10 kpc of the protogalaxy, the DM halo is 10 times denser than the gaseous component but at the end of the formation of the spheroid, the stellar component reaches a "central" density 30 times that of the dark component.

Not surprisingly, the final distribution of stars does not follow that of the DM. In fact, as the infall proceeds, the DM remains [FORMULA] scale-free, while the baryons develop a [FORMULA] scale [FORMULA] of about one tenth of the virial radius. More precisely, the dissipative zero-angular momentum infall of the (out-to-250 kpc) scale-free baryonic material produces a half-mass scale-length of [FORMULA] kpc. The final stellar distribution closely follows a Hernquist profile (see Fig. 3) with effective radius [FORMULA] slightly larger than that of ellipticals of the same baryonic mass. However, the simulated value of [FORMULA] depends on the assumed DM density and on the prescriptions of star formation. Actually, one could use the observed [FORMULA] vs stellar mass relationship to fine tune the semi-analytical parts of the code (Buonomo et al. 2000).

[FIGURE] Fig. 3. Final stellar density profile of the giant galaxy. A Hernquist profile for [FORMULA] kpc is superimposed.

[FIGURE] Fig. 4. Time evolution of the density and dispersion profiles. Solid circles stand for DM, open triangles for gas and open squares for stars. From the top to the bottom profiles refer to 1, 3, 3.5, 4.5 and 9 Gyr.

During the galaxy assembly a coupling between the baryons and the DM is created. The initial halo distribution [FORMULA] first is slightly contracted by the baryonic infall and then expanded by a (limited) supernovae-driven outflow. As a result, the final DM distribution is not too different from the primordial one. On the other hand, the DM potential controls the star formation rate and efficiency, including the fate of stellar ejecta.

Even in the present simplified scenario, the time-evolution of the mass distribution is not that of an adiabatic process (Blumenthal et al. 1986). In fact, (see Fig. 5) the adiabatic invariant [FORMULA] varies with time. In the innermost parts [FORMULA] kpc, it increases with time while, for larger radii, [FORMULA] kpc, it decreases as the collapse proceeds. The baryonic infall develops shell crossings as an effect of the strong radial dependence of the the energy (un)balance among cooling, heating and release of gravitational energy.

[FIGURE] Fig. 5. The formation of a dwarf elliptical. From the top to the bottom, snapshots refer to 1, 2, 3 and 9 Gyrs.

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000