Roles of SNIa and SNII in ICM Enrichment

Based on ASCA observations Mushotzky et al. (1996, ApJ 466, 686) have recently derived the relative-abundance ratios of $\alpha$-elements to iron, [\alpha/Fe] \simeq 0.2-0.3$, for four rich clusters, and have suggested that the origin of metals in an intra-cluster medium (ICM) is not a type-Ia supernovae (SNIa), but a type-II supernovae (SNII). However, these authors used the solar photospheric iron abundance for ASCA data reduction, while the meteoritic iron abundance is usually adopted in chemical-evolution studies. It is true that although the photospheric and meteoritic solar abundances are consistent for most of the elements, a serious discrepancy is known to exist for iron; indeed, the photospheric abundance of iron is $N_{Fe}/N_H = 4.68 10^{-5}$ by number, while the meteoritic value is $3.24 10^{-5}$. The argument concerning the relative roles of SNIa and SNII in ICM enrichment is quite sensitive to the precise values of [\alpha/Fe], and one should use an identical solar iron abundance in data reduction as well as in theoretical arguments. We therefore adopt the meteoritic iron abundance, which is consistent with chemical-evolution studies, and shift Mushotzky et al.'s ASCA data by $\Delta[\alpha/Fe] \simeq -0.16$ dex. By comparing the corrected [\alpha/Fe] values with theoretical nucleosynthesis prescriptions of SNIa and SNII, we reach a conclusion that an SNIa iron contribution of 50% or higher in the ICM enrichment could not be ruled out, and might indeed be favoured based on the ASCA spectra.