Published online by Cambridge University Press: 10 November 2010
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).