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UBC Theses and Dissertations

Photo-chemical evolution of elliptical galaxies and the intergalactic medium Gibson, Bradley Kenneth


A model which allows the simultaneous monitoring of the chemical, photometric, and thermal evolution of the gaseous and stellar components of spheroidal star systems has been developed. Supernovae-driven superwinds are initiated once the magnitude of the interstellar medium (ISM) thermal energy exceeds that of its binding energy, causing the cessation of star formation and the ejection of the remaining, enriched ISM to the intergalactic medium. Wind models of this nature successfully reproduce many of the correlations amongst observable properties of ellipticals (e.g. colour and magnitude; mass and metallicity), and are generally accepted as the mechanism responsible for the observed galaxy cluster intracluster medium (1CM) heavy element abundances. A detailed accounting of the model sensitivity to the important input ingredients is provided (e.g. initial mass function (IMF); nucleosynthesis; adopted evolutionary scenario for the supernovae (SNe) remnant interior thermal energy). Previous models seem to have underestimated the energy per SNe, made available to the ISM for powering a galactic wind, by assuming that all remnants evolve and expand in isolation, ad infinitum. In reality, shells overlap with adjacent expanding remnants, radically altering each individual remnant’s dynamical evolution, as well as the ISM’s global thermal evolution, resulting in earlier “blow-out” epochs than previously encountered. We have also examined the role played by thermalised kinetic energy from massive star pre-SN mass-loss and find it is an important component for low mass dwarf galaxies, but can be readily neglected for giant ellipticals. The origin of the a-element-to-Iron ratio and the Iron mass-to-luminosity ratio (IMLR) in cluster ICMs is also investigated. The stellar initial mass function in cluster galaxies is shown to be a crucial parameter in determining the mass and abundance ratios of heavy elements, and in particular, we can exclude simple bimodal IMF/star formation models for ellipticals. Models incorporating IMFs mildly flatter than the Salpeter IMF are preferred, provided the intracluster medium IMLR and [α/Fe]>0 X-ray observational constraint, present-day elliptical galaxy colour-luminosity-metallicity relationships, and their stellar population’s Magnesium overabundance relative to Iron, are to be honoured. These models pollute the intracluster medium by means of early winds (i.e. before Iron enrichment from Type Ia SNe exceeds greatly that from Type II, thereby driving the [α/Fe] ratio to negative values), and retain the gas restored by stars afterwards. Conversely, models with Salpeter-like initial mass functions can reproduce the Iron mass-to-luminosity ratio only if they eject all of the stellar gas restored after the early winds (e.g. via ram pressure stripping), with the detrimental effect of yielding a negative [α/Fe] in disagreement with the observations. We demonstrate that the total mass of gas ejected by cluster ellipticals is insufficient to explain the bulk of the ICM X-ray emitting gas, lending support to the notion that the majority of it is of primordial origin. Our work is the first of its kind to couple self-consistently the photometric, chemical, and thermal evolution of the ISM and stellar populations, as well as the resultant effects upon the 1CM abundances and abundance ratios, all within the framework of the supernovae-driven galactic wind model for elliptical galaxies.

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