Δisobars in finite nuclei and nuclear matter

Random phase approximation calculations using a microscopic residual interaction (G matrix) are performed for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$. The influence of Δ isobars on isovector magnetic states is investigated and found to be small since many-body renormalization effects for the coupling of ph to Δh states ($g_{\mathrm{N}{\mathit{\Delta }}^{\mathcal{’}}}$) are considerably smaller in finite nuclei than in nuclear matter. Energies of isovector states and collective isoscalar states are described reasonably well. The electromagnetic transition strength is in general overestimated for both magnetic and electric transitions.