Interstellar nanoparticles are a major physical component of galaxies, reprocessing starlight, controlling the heating and cooling of the gas, catalyzing chemical reactions and regulating star formation. The abundance, composition, structure and size distribution of these small solid particles, mixed with the interstellar gas, are however poorly-known. They indeed evolve within the interstellar medium and present systematic differences among galaxies. It is thus crucial to obtain detailed, carefully analyzed, empirical constraints of these properties, in a wide diversity of environments. Progress in this field are absolutely necessary to properly interpret observations of nearby star-forming regions and distant galaxies, as well as for precisely modeling interstellar physics.

Of particular interest are the long-wavelength optical properties of the nanoparticle mixture in the millimeter range. This spectral window is currently the least known. Yet, the millimeter opacity of the grain mixture has a central importance, since mass estimates based on spectral energy distribution fitting primarily rely on this quantity. A bias or systematic evolution of the millimeter opacity will directly translate in an inaccuracy in the nanoparticle mass, which is often used as a proxy to infer the gas mass of a region or galaxy.

Our guaranteed time program, IMEGIN (Interpreting the Millimeter Emission of Galaxies at IRAM with NIKA2; PI Madden; 200 hours), with the NIKA2 camera at the 30-m IRAM radiotelescope, has fully mapped 20 nearby galaxies at 1.2 mm and 2 mm. In addition, our open time program, SEINFELD (Submillimeter Excess In Nearby Fairly-Extended Low-metallicity Dwarfs; PI Galliano; 36 hours), is completing the survey down to low-metallicities (the metallicity is the relative abundance of elements heavier than Helium). These new and exceptional data are the first good quality maps of resolved galaxies at millimeter wavelengths, allowing us to study how the grain properties vary with the physical conditions.

The goal of the present PhD project is to combine these observations with other, already existing, multi-wavelength data (in particular, WISE, Spitzer and Herschel), in order to demonstrate how the millimeter opacity depends on the local physical conditions. The first step will consist in processing and homogenizing the data. The student will also have the opportunity to participate in our observing campaigns at Pico Veleta. In a second time, the student will model the spatially-resolved emission, using our in-house, state-of-the-art hierarchical Bayesian code, HerBIE. This will allow the student to produce maps of the nanoparticle properties and compare them to maps of the physical conditions. Finally, these results will be used to model the evolution timescales of the grain properties under the effects of radiation field and gas accretion. The laboratory measurements recently produced by the Toulouse group will be put to profit. This work will be performed within the IMEGIN international collaboration.