SN Ia Enrichment in Virgo Early-Type Galaxies from ROSAT
and ASCA Observations
by Finoguenov A.(IKI) and Jones C.(CfA)
We analyzed nine X-ray bright Virgo early-type galaxies observed
by both the 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
the new evidence for iron abundance gradients in NGC4472 and NGC4649 and
confirm the previous results on NGC4636. Mg and Si abundance gradients
are on average flatter compared to those of iron and correspond to underabundance
of alpha-process elements at high Fe values. For galaxies with low metallicities
there is some evidence toward overabundance of alpha-process elements relative
to iron. Compared to the stars in our Galaxy, the combined results on Virgo
early-types reveal a somewhat larger heavy element contribution from SNe
Ia at higher metallicities. We explain the observed trend by the metallicity
dependence of SN Ia metal production and present constraints on the available
theoretical modeling of low-metallicity inhibition of SNe Ia (Kobayashi
et al. 1998). We demonstrate the ability of this model to explain the abundance
measurements and to reconcile the high SN Ia rates found in optical searches
with the relatively low X-ray upper limits. Our estimates for the SN Ia
rate at the centers of the brightest galaxies in the sample give value
of ~0.05 h75^3 SNU. The rates inferred from optical searches values should
be corrected for the presence of the "faint'' SN Ia events, since
they release little metals and therefore are not detected at X-rays. With
this correction the present-day SN Ia rate in early-type galaxies is 0.10+/-0.06
h75^2 SNU (Capellaro et al. 1997) and is therefore comparable with the
X-ray estimates.
