The origin of the first solid particles (dust grains) in the early Universe is an extremely hot topic. Indeed, while in the local Universe most of the dust is produced in the atmospheres of evolved stars, which require about one billion years to evolve, the origin of dust in the early universe (z>6) when the age of the universe was comparable or shorter than this time, has been puzzling. The investigation of the dust extinction curves at of high redshift quasars and Gamma Ray Bursts has revealed that most of the dust in the early universe has likely been produced promptly in the ejecta of core-collapse Supernovae (Maiolino et al. 2004; Gallerani, Maiolino et al. 2010; Stratta, Maiolino, et al. 2007; Stratta, Gallerani, Maiolino 2011). This discovery has had implications on models of early galaxy evolution, early star formation (dust in primeval galaxies is an important coolant enabling the fragmentation of gas clouds and therefore the formation of the first low mass stars), and has had implications on the interpretation of the observational properties of distant galaxies.

The content of dust in galaxies provides an alternative method to investigate the evolution of metals in galaxies (typically about half of the metals are condensed into dust grains) but can potentially also be used as a powerful tool to investigate the evolution of the gas content in large samples of galaxies (by exploiting the fact that the dust-to-gas ratio follows known scaling relations). We have used far-infrared and submillimeter observations with the Herschel satellite to measure the content of dust in thousands of galaxies in the redshift range 0<z<2.5 (Santini, Maiolino et al. 2014). It was found that both the dust content and the inferred gas content in galaxies follow simple, constant scaling relations with the stellar mass and with the star formation rate, at any epoch, out to z~2.5. However, galaxies populate these scaling relation in a different way at different epochs. The result of this effect is a net strong evolution of the gas content in galaxies at high redshift, in a differential way for galaxies with different masses.

The same technique has been applied to AGN host galaxies, enabling us to investigate the gas content in a large sample, as well as an extensive control sample, vastly expanding both in size and redshift distribution relative to our previous study based on CO millimeter observations (Maiolino et al. 1997). The new data reveal that AGN host galaxies are systematically more gas rich than the galaxies not hosting AGNs (Vito, Maiolino et al. 2014). Such correlation can be interpreted simply in terms of gas rich galaxies having a higher probability for a gas cloud to fall within the sphere of influence of the supermassive black hole.