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

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6. A dwarf elliptical galaxy

The second simulation concerns a proto-elliptical of gaseous mass of [FORMULA] forming through the monolithic collapse of a spherical virialized DM halo of [FORMULA]. In this case a gas particle has an initial mass of of [FORMULA].

Despite the fact that the virial radius is now much smaller ([FORMULA] kpc), the gas is not more rapid in condensating, cooling and forming stars. Note that both the free-fall time and the cooling time are smaller.

The time evolution of the SFR in this case cannot be described as a a single burst, but rather as a series of several episodes, each one of about [FORMULA], lasting for over 4 Gyr (see Fig. 6 and Fig. 7), a result which is in close agreement with observations of dwarf galaxies in the local group (Mateo 1998).

[FIGURE] Fig. 6. SF history for the dwarf elliptical as a function of time. The two panels show the same model at increasing particle number.

[FIGURE] Fig. 7. Baryons final density profile. Superimposed is a Hernquist profile for [FORMULA] kpc.

At the end of this period only [FORMULA] of the original materia it has been processed into long-living starsl. The remaining [FORMULA] has been thrown out from the galaxy or left, unprocessed, behind, in the outermost regions of the DM halo.

In the central regions, the baryonic density increases as the collapse proceeds to reach "at the center" 30 times the density of the dark component. The stellar scale-length is [FORMULA] kpc, which is not too different from that of ellipticals of the same baryonic mass (see Fig. 8). Also in this case, there is a clear coupling between baryons and DM during the galaxy assembly.

[FIGURE] Fig. 8. Adiabatic invariant evolution for the dwarf galaxy in three different galaxy regions. [FORMULA] is in code units.

However, in this case, due to the weaker gravitational field, most of the gas gets expelled from the galaxy. The halo density evolves during the baryonic infall, in a non-adiabatic way, as shown in Fig. 9. The final DM distribution is not too different from the primordial one (see Fig. 10).

[FIGURE] Fig. 9. Evolution of density and dispersion profiles for the Dwarf Elliptical Galaxy. Solid circles stand for DM, open triangles for gas and open squares for stars. From the top to the bottom profiles refer to 1.5, 2, 3.5, 5 and 9 Gyr.

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000