We have analyzed ASCA data of about 30 early type galaxies, and studied their X-ray emitting ISM (InterStellar Medium) properties. Among these targets, the X-ray luminous galaxy NGC 4636 has been observed for extremely long time, ~240 ks, has played a particularly important role because of superior statistics of its data.
One particular difficulty in estimating the ISM abundance was that the uncertainty in Fe-L atomic physics. In order to solve the problem, we have carefully examined abundance ratios among elements when fitting the Fe-L dominant plasma. We found that the effects of line overlapping and free-bound continuum cause strong coupling among plasma emission codes should be caused by the couling. In contrast, when abundance of α-elements is fixed to a given value, the different plasma emission codes give similar Fe-abundances only within 20-30%. In order to untie the coupling, we included systematic errors in Fe-L region of the spectra. Then, in X-ray luminous galaxies, the derived Aα and AFe both became ~1 solar within a factor of 2. Therefore, the severe discrepancy between the ISM and stellar abundance has been relaxed. The ISM metallicity of X-ray fainter galaxies are uncertain, but at least SNe Ia contribution to the ISM abundance is smaller than in the X-ray luminous ones.
We have also discovered that X-ray emissions from X-ray luminous galaxies are very extended, and expressed with two beta models of different angular scales. This means that the X-ray luminous ellipticals are central galaxies of some larger-scale potential structures. We show that this presence/absence of the larger-scale potential can consistently account for both the scatter of X-ray luminosity and difference of SNe Ia contribution to the ISM abundance.