Chemical evolution of two-component galaxies I - an analytical approach

Equations for the chemical evolution of a two-component (spheroid plus disk) galaxy are derived for the instantaneous recycling approximation. The treatment, which minimizes the number of initial free parameters by introducing a new term, accounts for the collapse phase of the gas, which is assumed to have uniform density and composition. The morphological sequence of galaxies is simulated by varying the initial masses or by varying the final volume of the disk. The chemical evolution of the galactic halo is also considered. The results obtained by application of the treatment are found to be qualitatively consistent, but the approximation based on the instantaneous recycling hypothesis is generally rejected, and the need for improved dynamical models is noted.