Metal abundances in the hot interstellar medium in early-type galaxies observed with ASCA

Abstract

We have analyzed ASCA data of 27 early-type galaxies, and studied the properties of their X-ray emitting ISM (InterStellar Medium) in detail. We found that overlapping lines and free-bound continuum cause strong coupling in the derived abundances of various elements. The abundance determination is also difficult due to the uncertainties in the Fe-L atomic physics, because the Fe-L lines couple with the O and Ne K-lines. However, when the abundances of α-elements are fixed, all of the plasma codes give similar Fe abundances with a scattering of only 20-30%. To relax the strong coupling among the elements, we included 20% systematic errors in the Fe-L region of the spectra. Then, in X-ray luminous galaxies, the derived abundance of Fe and α-elements both became ∼ 1 solar within a factor of 2. This result relaxes the previous severe discrepancy between the ISM and stellar metallicities. The ISM metallicity in X-ray fainter galaxies is still uncertain, but we can at least constrain that the contribution from type-Ia SN to the ISM abundance is lower than in X-ray luminous systems. These results strongly suggest that a large fraction of SN Ia products have escaped into intergalactic space.