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Gas and iron content of galaxy clusters
C. Chiosi *
Received 27 March 2000 / Accepted 5 October 2000
Up to now, many theoretical studies aimed at reproducing the total amount of iron and gas in the intra-cluster medium meet the embarrassing situation, in which if the iron content is reproduced, the gas is not. More precisely, at given iron mass, too little gas and too high Fe abundance in turn are obtained as compared to the observational data. Large dilution by primordial gas is then invoked to get rid of the difficulty. In this paper we present a new approach to this problem. Basic ingredients of the present analysis are: (i) The adoption of multi-zone models of elliptical galaxies in the framework of the super-nova driven galactic wind scheme. They yield a more realistic description of the galactic ejecta in which the effects of gradients in star formation and chemical enrichment are taken into account. (ii) The stellar initial mass function is let vary with the physical conditions of the star forming medium. More precisely, the typical mass scale of the initial mass function increases with the gas temperature. Since no cooling process exists decreasing the temperature of a galaxy's gas below the limit set by the current value of the cosmic background radiation, it immediately follows that the stellar initial mass function of proto-galaxies whose stellar activity began at high red-shift (when the CBR temperature was higher than the present-day mean temperature of molecular clouds) is different from the one in galaxies which did the same but at lower red-shift. Because of this, at given galaxy mass the ejecta are expected to depend on the red-shift. (iii) Finally, the basic assumption is made that at any time (red-shift) the mass distribution of proto-galaxies follows the Press-Schechter law holding for Dark Matter, however with masses comprised between suitable minimum and maximum values, and , respectively, that are also varying with time and/or red-shift. This is equivalent to assume a sort of continuously varying mass function for galaxies as well. When the same input physics (stellar initial mass function and galactic ejecta) is adopted, the new method recovers previous results in literature. However, when the above three ingredients are let work simultaneously, the total amount of iron and gas are reasonably matched, a point of major difficulty in previous studies. The absolute abundance of Fe and abundance ratio [O/Fe] are fully compatible with the observational determinations. However, even in this case some dilution (up to about 20%) by primordial gas (never cycled through galaxies) seems to be required. This is less than the older estimates of 65% to 90%. Finally, a simple model for the evolution of the intra-cluster medium abundances as a function of the red-shift is presented and compared with the observational data for clusters at high red-shift (up to ).
Key words: galaxies: clusters: general galaxies: intergalactic medium galaxies: evolution galaxies: abundances
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Online publication: January 29, 2001