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A new semi-analytical treatment of the effect of supernovae on ULIRG spectral energy distributions

This work presents a method for generating synthetic spectra of Ultra-Luminous Infrared Galaxies (ULIRGS) using AGN, HII region and supernovae source functions. The AGN element represents the far-infrared contribution to the ULIRG spectrum from an energetic central engine. It is modelled using a quasar source embedded in an axi-symmetric dusty torus. The radiative transfer of flux (RT) is then simulated and the AGN emergent spectral energy distributions (SEDs) generated. The HII region solution is then developed. A stellar evolutionary synthesis code is used to generate instantaneous burst (ISB) source functions which decay in time. The evolution of the gas and dust density in a spherically-symmetric, dense GMC, under the influence of a time-dependent ionizing source flux, is derived. Having irradiated the dust distribution with the source cluster SED, the RT is calculated and the HII region SEDs obtained. The impact of supernovae energy on a GMC already ionized by stellar flux is then considered. Using the standard pressure-driven expansion model of e.g., Weaver et al. (1977) the radial evolution of a superbubble expanding under the influence of a continuous super novae energy function is derived. The superbubble is modelled in both an adiabatic rapid expansion phase and in an isothermal momentum-conserving phase. As the superbubble expands, upstream gas is swept into a thin shell trapped on its surface and the gas density enhancement is modelled using simple shock physics. Having generated expressions for the evolution of the shell gas temperature, it is linked to the dust density via a temperature dependent condensation factor. Finally expressions are developed to quantify the evolution of the optical depth along a line of sight. It is found that the star formation efficiency (SFE) has a profound effect on the radial evolution of the optical depth distributions in GMCs generating marked differences in behaviour between and high and low SFEs. Low SFE models have shells below the dust condensation temperature at the GMC boundary R2w and the extinction, having initially been in decline, recovers to more substantial values in a dust reformation scenario. These systems tend to be optically thick for most of their evolution. Those models classified high SFE have supershell temperatures in excess of the dust condensation temperature at R2w and the extinction distribution drops precipitously and reaches very low values (< 1) at R2w- It then remains low for some time before a small recovery in extinction occurs as the shell dust condenses out. These systems tend to be optically thin for most of their life-times. It is the more powerful supernovae source functions included in the modelled space which generate the high SFE extinction behaviour and vice versa. Having derived the dust density distributions they are irradiated by the appropriate central source cluster SED and the RT simulated to generate the emergent SEDs. These are similarly categorised as low and high SFE. The low SFE model SEDs appear to be representative of systems where the dust acts as an enshrouding bolometer and most it not all UV and optical radiation is reprocessed and re-emitted into the infrared. Conversely, the high SFE model SEDs are optically revealed and exhibit substantial, only mildly attenuated source flux at short wavelengths for the majority of their evolution. The emergent AGN and starburst (ISB HII region and supernovae) SEDs are then combined in pairs to form a ULIRG SED Library. These SEDs are matched to the published data for a sample of six nearby (redshift z < 1) ULIRGS. No model ULIRG SED is found to have a better than 40% probability of belonging to the same population distribution as the published data. This is found to be most likely the result of using an ISB source function. The starburst SED library is therefore extended to approximate constant star formation (CSFRA) using a time decay parameterization and the ULIRGs refitted. In each case a model ULIRG SED was found to match the observations with a better than 5% probability of non-random fit, which suggests that a constant rather than ISB star formation mode is perhaps more appropriate in ULIRGs. Using the CSFRA component of the best-fitting model ULIRG SED, estimates are made for the star formation rate, starburst age and the implied merger and interaction state for each ULIRG. In all case these quantities agree favourably with the literature. Each ULIRG was fitted with a CSFRA SED element originating in the high SFE group of models. This was found to be a direct result of fitting the upper limits to the short wavelength (A < 3m) flux data points, as it is the high SFE SEDs that are optically revealed.
Date January 2008
CreatorsJenner, Clare Elizabeth
PublisherUniversity College London (University of London)
Source SetsEthos UK
Detected LanguageEnglish
TypeElectronic Thesis or Dissertation

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