Electron-ion recombination measurements of Fe7+, Fe8+, Fe13+ motivated by active galactic nuclei x-ray absorption features

Abstract

Recent spectroscopic models of active galactic nuclei have indicated that the recommended electron-ion recombination rate coeficients for iron ions with partially filled Mshells are incorrect in the temperature range where these ions form in photoionized plasmas. We have investigated this experimentally for Fe7+ forming Fe6+, Fe8+ forming Fe7+, and Fe13+ forming Fe12+. The recombination rate coeficient was measured employing the electron-ion merged beams method at the Heidelberg heavy-ion storage-ring TSR. The measured energy range encompassed at least all dielectronic recombination (DR) resonances associated with core excitations within the M-shell of the parent ions. Already in our first measurement, that is for Fe13+, we find unusually strong DR resonances at low electron-ion collision energies leading to low temperature plasma DR rate coeficients orders of magnitude larger than the recommended rate coeficient.