SUPPORT FOR X-RAY DEDUCED ABUNDANCES IN GALAXIES AND CLUSTERS - THE CONSISTENCY OF IRON ABUNDANCES FROM L AND K EMISSION

Significantly subsolar metallicities have been reported in X-rays for a variety of sources, often in disagreement with theoretical expectati ons or with measurements at other wavelengths. As a test of the reliab ility of the X-ray abundances in systems dominated by thermal emission from hot gas in collisional ionization equilibrium, we compare Fe abu ndances determined independently from the Fe L and Fe K emission regio ns in simple one- and two-temperature thermal fits to ASCA X-ray spect ra of the elliptical galaxy M87 and five galaxy clusters. We find that the Fe L and Fe K abundances are consistent within or near their 90% confidence errors, which would not necessarily be the case if there we re severe problems with the atomic physics or with the instrument cali brations. Our data cover a range of plasma temperatures kT = 2-4 keV, where the Fe L emission arises primarily from Fe XXIII and XXIV and th e Fe K emission arises primarily from Fe XXV. There is also a contribu tion from less ionized gas at lower temperatures in M87. For M87, the abundances from Fe K are systematically lower by 30%-50% than those fr om Fe L, and the differences are attributable to the complicated tempe rature structure in the central cooling flow. Allowing for the possibi lity of excess absorption of the emission from the cooling flow increa ses the systematic offset of the Fe L and Fe K abundances to 40%-80% i n the simple model we consider. The cluster spectra, on the other hand , are for nearly isothermal regions outside any central cooling flow a nd the abundances have no systematic offset. The consistency between t he Fe K and Fe L abundances, which is also seen in other clusters and in the corona of Ar Lac, indicates that whatever atomic physics uncert ainties exist in the current codes are not likely to render Fe abundan ce measurements unreliable within the similar to 30%-50% agreement we find for the systems we examined. The results using various thermal em ission codes differ by 15%-20%, showing the level of scatter due just to different assumptions about the atomic physics. The atomic rates fo r H- and He-like ions, which are relevant for the determination of abu ndances in elements other than Fe, are more accurately known than thos e for less ionized species that dominate the complicated Fe L emission . In light of the support our results give to the abundances derived f rom ASCA data for clusters and bright elliptical galaxies, we consider briefly the plausibility of subsolar abundances in elliptical galaxie s.