PRODUCTION AND CIRCULATION OF IRON IN ELLIPTIC GALAXIES AND CLUSTERS OF GALAXIES

We introduce the concept of cluster iron mass-to-light ratio (IMLR) and estimate its value for both the intracluster medium and the stars, finding the amount of iron to be nearly the same in the two cluster components. We discuss under which conditions the past supernova activity can account for the observed IMLR and provide evidence that either the past average rate of Type Ia supernovae was at least a factor of approximately 10 higher than the present rate in ellipticals, or massive stars in clusters formed with a very flat initial mass function (IMF). A solution in which approximately 1/4 of the observed iron in clusters of galaxies was contributed by Type II supernovae, and approximately 3/4 by Type Ia supernovae, presents several advantages, as it would allow to preserve the current scenario for the establishment of element abundance ratios in the Galactic halo, bulge, and disk and would leave open the possibility of a universal IMF. We consider the iron abundance in the gas flow of individual elliptical galaxies, emphasize a macroscopic discrepancy that has recently emerged between the predicted iron abundance in the flow and that derived from X-ray observations, and discuss possible solutions of the discrepancy. We argue that the iron in the intracluster medium (ICM) of rich clusters was ejected from individual galaxies by internal forces generated by supernovae or active galactic nuclei (AGNs), rather than stripped by external forces such as ram pressure. We go further in considering the supernova heating and the presupernova light production which are implied by the observed IMLR and discuss their implications for elliptical galaxy formation. We suggest a possible test for the proposed scenarios, based on the predicted difference in elemental ratios between the ICM and cluster elliptical galaxies, with alpha-elements to iron ratio being below solar in the ICM and above solar in the stellar component of galaxies.