SN Ia enrichment in Virgo early-type galaxies from ROSAT and ASCA observations

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 profiles and abundances of Mg, Si, and Fe for six galaxies. The temperature profiles are consistent with isothermal temperatures outside of cooler regions at the galaxies' centers. We present new evidence for iron abundance gradients in NGC 4472 and NGC 4649 and confirm the previous results on NGC 4636. Mg and Si abundance gradients on average are flatter than those of iron and correspond to an underabundance of a-process elements at high Fe values, while at low iron the element ratios favor enrichment by Type II supernovae (SNe). We explain the observed trend using the metallicity dependence of SN Ia metal production and present constraints on the available theoretical modeling for low-metallicity inhibition of SNe Ia. Our results imply a cutoff 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 similar to 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 and is therefore comparable with the X-ray estimates. A simple comparison shows that the X-ray abundances we derive are still discrepant from optically determined values. We attribute this difference to the low spatial resolution of our X-ray measurements, radial gradients in the abundances, and the importance of hydrodynamical effects, particularly the inflow of cooling gas, on the measured X-ray abundances.