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The baryonic and chemical content of disc galaxies: A key to galaxy formation


The presently favoured Cold Dark Matter cosmology still faces serious discrepancies with observations on galactic scales. Baryon physics can give a crucial contribution to improve the predictions of cosmological simulations. A key to understand galaxy form ation and evolution is to measure the baryonic content of galaxies: compared to the universal baryon fraction, it reveals if galaxies expel large amounts of the available baryons via stellar energy feedback. Disc galaxies are especially interesting because feedback is fundamental to explain their sizes and angular momenta. Also, the mass of baryons in discs is crucial to derive the density profiles of dark matter haloes, a puzzle for current cosmology. To know the mass of baryons in galaxies we must know th e mass-to-light (M/L) ratio of their luminous stellar component. Accurate data are available for the stellar populations in the Solar Neighbourhood, and we will interpret them with chemo-photometric models to derive the stellar M/L ratio in the local Galac tic Disc. We will then apply the models to external disc galaxies, and by combining chemo-photometric and dynamical analysis we will investigate the stellar M/L ratio as a function of Hubble type, of galaxy mass and in high vs. low surface brightness discs . We will infer how star formation and feedback work in different objects, so that we can include realistic baryon physics in cosmological simulations and improve our understanding of galaxy formation. Our chemical models will also predict the helium conte nt of nearby stars, and the relative helium-to-metal enrichment rate DY/DZ. We can also probe this quantity observationally, from the colour-magnitude diagram of a large sample of nearby low Main Sequence stars. From DY/DZ we can then infer the primordial helium abundance Yp, an important probe of cosmology and Big Bang nucleosynthesis.

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