Personal web page : https://irfu.cea.fr/Pisp/frederic.galliano/
Laboratory link : https://irfu.cea.fr/Phocea/Vie_des_labos/Ast/ast_groupe.php?id_groupe=973
The InterStellar Medium (ISM) is the complex intertwining of phases filling the volume of a galaxy between the stars. It is constituted of: (i) gas, principally hydrogen (75%) and helium (23%), but also heavier elements (C, N, O, etc.; 1%) that can be found in molecular forms; and (ii) dust grains (the remaining 1% of the ISM mass), which are small solid particles of sub-micronic sizes. The ISM is a fundamental constituent of the Universe, as stars are born out of the collapse of dense interstellar clouds, and return some of their mass, enriched in freshly synthesized heavy elements, at the end of their lifetime.
Interstellar dust is particularly important component of the ISM, as it absorbs the visible light and reemits it thermally in the infrared. Certain interstellar regions are totally opaque to visible photons and can only be probed by their infrared emission. In addition, dust is an important agent of the gas heating, by photoelectric effect. It is also the catalyst where dihydrogen, the most abundant molecule in the Universe, forms. Yet, interstellar grain properties (composition, abundance, size distribution, etc.) are still poorly known. This uncertainty thus impedes our understanding of the physics of the ISM, and by extension, of galaxy evolution.
This thesis project aims to progress in our understanding of grain properties, focusing on the variation of these properties in the nearby Universe. Dust indeed evolves in the ISM, and this evolution depends on local conditions (gas density, UV field, etc.). This evolution can be studied in an empirical way, modeling multiwavelength observations, spatially-resolved at the scale of a few hundred parsecs, in nearby galaxies.
Our group has a renowned expertise in this area. We are currently leading a large millimeter observation program of these objects, with the instrument NIKA2 (https://irfu.cea.fr/dap/Phocea/Vie_des_labos/Ast/ast_visu.php?id_ast=4644). We have recently been instrumental in modeling spectral energy distributions for the large European collaboration, DustPedia (http://dustpedia.astro.noa.gr/?AspxAutoDetectCookieSupport=1). We have also developed a unique spectral energy distribution fitting code, implementing a hierarchical Bayesian method, HerBIE (Galliano, 2018 ; https://ui.adsabs.harvard.edu/abs/2018MNRAS.476.1445G/abstract). This code allowed us to estimate the characteristic timescales of cosmic dust evolution (https://irfu.cea.fr/dap/Phocea/Vie_des_labos/Ast/ast.php?t=fait_marquant&id_ast=4929).
The thesis work will have two parts. The first task will consist in developing the HerBIE code, implementing the emission from stellar populations. Besides, this modeling will need to be executed self-consistently with the chemical evolution computation. We will also need to model the radiative transfer through the ISM, in a large number of possible topologies. The second part will consist in applying this code to multiwavelength observations of nearby galaxies, in order to demonstrate trends between grain properties and the physical conditions of the ISM, and constrain dust evolution processes.