ASCA Study of Heavy Element Distribution in Clusters of Galaxies.

A. Finoguenov, L.P. David and T.J. Ponman

ApJ 2000 in press (Nov. issue).

ABSTRACT:

We perform a spatially resolved X-ray spectroscopic study of a set of 11

relaxed clusters of galaxies observed by the ROSAT/PSPC and ASCA/SIS.

Using a method which corrects for the energy dependent effects of the ASCA

PSF based on the ROSAT images, we constrain the spatial distribution of

Ne, Si, S and Fe in each cluster. Using theoretical prescriptions for the

chemical yields of Type Ia and II supernovae, we determine the Fe

enrichment from both types of supernovae as a function of radius within

each cluster. Using optical measurements from the literature, we also

determine the iron mass-to-light ratio (IMLR) separately for Fe synthesized

in both types of supernovae. For clusters with the best photon

statistics, we find that the total Fe abundance decreases significantly

with radius, while the Si abundance is either flat or decreases less

rapidly, resulting in an increasing Si/Fe ratio with radius. This result

indicates a greater predominance of Type II SNe enrichment at large radii

in clusters. On average, the IMLR for Fe synthesized within Type II SNe

increases with radius within clusters, while the IMLR for Fe synthesized

within Type Ia SNe decreases with radius. At a fixed radius of 0.4

$R_{virial}$ there is also a factor of 10 increase in the IMLR for Fe

synthesized by Type II SNe between groups and clusters. This suggests

that groups expelled (or weakly captured) as much as 90\% of the Fe

synthesized within type II SNe at early times. In addition, we find that

the Si/Fe ratios in the outer portions of some clusters are higher than

that expected based on the theoretical predictions of Type II SNe yields.

All of these results are consistent with a scenario in which the gas was

initially heated and enriched by Type II SNe driven galactic winds, during

which time, some of the gas and heavy elements were expelled from low

temperature systems. Any remaining gravitationally bound gas was then

enriched with elements synthesized by Type Ia supernovae as gas-rich

galaxies accreted onto clusters and were ram pressure stripped during

passage through the cluster core. We suggest that the high Si/Fe ratios in

the outskirts of rich clusters may arise from enrichment by Type II SNe

with low metallicity progenitors. This is consistent with our basic

scenario, since gas at the largest radii in clusters should have been

enriched at the earliest times by the lowest metallicity stars. Low

metallicity progenitor stars can also account for the low S/Fe ratios

observed in clusters.