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 \sim 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 \pm 0.06 h _ { 75 } ^ { 2 } SNU ( Cappellaro et al .
1997 ) and is therefore comparable with the X-ray estimates .
A simple comparison shows 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 .