SN Ia Enrichment in Virgo Early-Type Galaxies from ROSAT and ASCA Observations

Abstract

We analyzed nine X-ray bright Virgo early-type galaxies observed by both ASCA and ROSAT. Through spatially resolved spectroscopy, we determined the radial temperature profile and abundances of Mg, Si and Fe for six galaxies. The temperature profiles are consistent with isothermal temperatures outside of a cooler region at the galaxy center. We present new evidence for iron abundance gradients in NGC4472 and NGC4649 and confirm the previous results on NGC4636. Mg and Si abundance gradients on average are flatter compared to those of iron and correspond to an underabundance of alpha-process elements at high Fe values, while at low iron, the element ratios favor enrichment by type II SNe. We explain the observed trend by the metallicity dependence of SN Ia metal production and present constraints on the available theoretical modeling for low-metallicity inhibition of SNe Ia (Kobayashi et al. 1998). Our results imply a cut-off metallicity in the range 0.07-0.3 solar and require a lower limit of 0.3 solar on the Fe contribution of SN Ia. We estimate an SN Ia rate at the centers of the brightest galaxies in our sample of ~0.08 (h_75)^3 SNu (supernova units). The rates inferred from optical searches should be corrected for the presence of `faint' SN Ia events, since these release limited metals and therefore do not contribute significantly to the measured metallicity in the X-ray gas. With this correction the present-epoch SN Ia rate in early-type galaxies is 0.10+/-0.06 (h_75)^2 SNu (Cappellaro et al. 1997) and is therefore comparable with the X-ray estimates. Abridged.