Application of the Multiphase Model to the Galactic Bulge

Summary

We present the application of the multiphase model to the Galactic bulge to assure that this model may be used in all regions. Results show a star formation rate that is higher in the bulge. The logical consequence is a higher metallicity due to the burst of star formation at early phases of the evolution. But when a comparison is made between models applied to an elliptical galaxy and a bulge, a different chemical evolution results: the relation [O/Fe] versus [Fe/H] is not the same for elliptical galaxies and bulges.

The multiphase model

The bulge is the central region of the Galaxy (Frogel, 1988) with a radius of 1–2 kpc and a total mass of ∼10¹⁰ M_⊙. In the outside region, the bulge connects with the spheroidal halo and with the adjacent disc. Characteristics for the stellar population of this bulge are obtained from observations: stars are old (Terndrup 1988; Lee 1992), metal rich (Gratton & Ortolani 1986; Rich 1988) and spatially distributed in two components (Rich, 1990; IRAS results). There is also a spheroidal metal-poor component without rotation and a more centrally concentrated and more metal-rich component with rotation.

The observed metallicity distribution is well reproduced by a simple “closed box” model. It implies no slow “infall” of gas in this region. Other models for the bulge are those of Arimoto & Yoshii (1987), based on their elliptical galaxy models, and Matteucci & Brocato (1990).