Metallicity Gradients in the Intracluster Gas of Abell 496

Abstract

Analysis of spatially resolved ASCA spectra of the intracluster gas in Abell 496 confirms there are mild metal abundance enhancements near the center, as previously found by White et al. (1994) in a joint analysis of Ginga LAC and Einstein SSS spectra. Simultaneous analysis of spectra from all ASCA instruments (SIS + GIS) shows that the iron abundance is 0.36 +- 0.03 solar 3-12' from the center of the cluster and rises ~50% to 0.53 +- 0.04 solar within the central 2'. The F-test shows that this abundance gradient is significant at the >99.99% level. Nickel and sulfur abundances are also centrally enhanced. We use a variety of elemental abundance ratios to assess the relative contribution of SN Ia and SN II to the metal enrichment of the intracluster gas. We find spatial gradients in several abundance ratios, indicating that the fraction of iron from SN Ia increases toward the cluster center, with SN Ia accounting for ~50% of the iron mass 3-12' from the center and ~70% within 2'. The increased proportion of SN Ia ejecta at the center is such that the central iron abundance enhancement can be attributed wholly to SN Ia; we find no significant gradient in SN II ejecta. These spatial gradients in the proportion of SN Ia/II ejecta imply that the dominant metal enrichment mechanism near the center is different than in the outer parts of the cluster. We show that the central abundance enhancement is unlikely to be due to ram pressure stripping of gas from cluster galaxies, or to secularly accumulated stellar mass loss within the central cD. We suggest that the additional SN Ia ejecta near the center is the vestige of a secondary SN Ia-driven wind from the cD (following a more energetic protogalactic SN II-driven wind phase), which was partially smothered in the cD due to its location at the cluster center.