Dielectronic recombination of Fe¹³⁺: benchmarking the M-shell

We have carried-out a series of multi-configuration Breit-Pauli AUTOSTRUCTURE calculations for the dielectronic recombination of Fe^{13+}. We present a detailed comparison of the results with the high-energy resolution measurements reported recently from the Heidelberg Test Storage Ring by Schmidt et al. Many Rydberg series contribute significantly from this initial 3s^2 3p M-shell ion, resulting in a complex recombination `spectrum'. While there is much close agreement between theory and experiment, differences of typically 50% in the summed resonance strengths over 0.1-10 eV result in the experimentally based total Maxwellian recombination rate coefficient being a factor of 1.52-1.38 larger than theory over 10^4-10^5 K, which is a typical temperature range of peak abundance for Fe^{13+} in a photoionized plasma. Nevertheless, this theoretical recombination rate coefficient is an order of magnitude larger than that used by modellers to-date. This may help explain the discrepancy between the iron M-shell ionization balance predicted by photoionization modelling codes such as ION and CLOUDY and that deduced from the iron M-shell unresolved-transition-array absorption feature observed in the X-ray spectrum of many active galactic nuclei. Similar data are required for Fe^{8+} through Fe^{12+} to remove the question mark hanging over the atomic data though.