Mathematics – Logic
Scientific paper
Sep 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010phdt........32g&link_type=abstract
PhD thesis, Université Paris Diderot (Paris 7)
Mathematics
Logic
Low-Metallicity Galaxies, Interstellar Medium, Spectral Energy Distribution, Dust-To-Gas Mass Ratios
Scientific paper
This thesis aims to study the interstellar medium of nearby low-metallicity galaxies to characterize the physical properties of the gas and dust and study the influence of metal enrichment on the properties of galaxies. The Spectral Energy Distributions of low metallicity galaxies differ significantly from those of massive galaxies and the dust-to-gas mass ratio of the galaxy seems to depend on metallicity. Observations of low-metallicity galaxies also often led to the detection of an excess at submm wavelengths not always accounted for in usual Spectral Energy Distribution models. Further studies and observations had to be performed to better cover the far-IR to submm range and probe the coldest phase of dust. We adopt a multi-wavelength approach to model and analyze the Spectral Energy Distributions of low-metallicity galaxies observed with LABOCA at 870 μm. We estimated the fraction of cool dust to be significant compared to the total dust mass of the galaxies. Some dust-to-gas mass ratios are incompatible compared to what is expected from the current chemical evolution model, revealing possible reservoirs of gas not detected by current HI or CO observations. I enlarged the first sample to a wider range of metallicities and showed that submm measurements significantly affect the dust mass estimates of galaxies. i) For dustier galaxies, submm fluxes are crucial to constrain the position of the peak and the submm slope of their Spectral Energy Distribution. ii) For low-metallicity galaxies, the submm wavelength domain harbours an excess that may imply a large amount of very cold dust. Obtaining a more precise inventory of the cold dust and resolve the main actors of dust evolution in massive star forming regions and molecular clouds was the logical following step. We obtain LABOCA observations of a resolved star forming region of the Large Magellanic Cloud (LMC), the N158/N159/N160 complex. The proximity of the LMC enables us to resolve structures of a few parsecs at the LABOCA resolution to model the Spectral Energy Distributions of individual and isolated regions across the complex and get a handle on the temperature distribution of the dust (hot, warm, cold). I am also comparing the dust distribution with the HI, CO and Hα observations to spatially quantify the variations of the dust-to-gas mass ratio. I finally present the first Herschel observations of the galaxies NGC 6822 and NGC 1705, two low- metallicity galaxies observed as part of the Science Demonstration phase of the telescope launched in May 2009. In NGC 6822, we find that the Spectral Energy Distributions of HII regions show warmer ranges of dust temperatures. We derive very high dust masses when graphite is used in our model to describe carbon dust. Using amorphous carbon, instead, requires less dust mass to account for the same submm emission due to its lower emissivity properties. This indicates that Spectral Energy Distribution models including Herschel constraints may require different dust properties than commonly used.
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