Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...476..560h&link_type=abstract
Astrophysical Journal v.476, p.560
Astronomy and Astrophysics
Astronomy
21
Galaxies: Abundances, X-Rays: Galaxies
Scientific paper
Significantly subsolar metallicities have been reported in X-rays for a variety of sources, often in disagreement with theoretical expectations or with measurements at other wavelengths. As a test of the reliability of the X-ray abundances in systems dominated by thermal emission from hot gas in collisional ionization equilibrium, we compare Fe abundances determined independently from the Fe L and Fe K emission regions in simple one- and two-temperature thermal fits to ASCA X-ray spectra 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 were severe problems with the atomic physics or with the instrument calibrations. 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 the Fe K emission arises primarily from Fe XXV. There is also a contribution from less ionized gas at lower temperatures in M87. For M87, the abundances from Fe K are systematically lower by 30%--50% than those from Fe L, and the differences are attributable to the complicated temperature structure in the central cooling flow. Allowing for the possibility of excess absorption of the emission from the cooling flow increases the systematic offset of the Fe L and Fe K abundances to 40%--80% in the simple model we consider. The cluster spectra, on the other hand, are for nearly isothermal regions outside any central cooling flow and the abundances have no systematic offset. The consistency between the 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 uncertainties exist in the current codes are not likely to render Fe abundance measurements unreliable within the ~30%--50% agreement we find for the systems we examined. The results using various thermal emission codes differ by 15%--20%, showing the level of scatter due just to different assumptions about the atomic physics. The atomic rates for H- and He-like ions, which are relevant for the determination of abundances in elements other than Fe, are more accurately known than those for less ionized species that dominate the complicated Fe L emission. In light of the support our results give to the abundances derived from ASCA data for clusters and bright elliptical galaxies, we consider briefly the plausibility of subsolar abundances in elliptical galaxies.
Fukazawa Yashushi
Hwang Una
Loewenstein Michael
Markert Thomas H.
Matsumoto Haru
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