Stellar metallicities as a tracer of quenching mechanisms

Star-forming galaxies can be transformed into passive systems by a multitude of processes that shut down (i.e. ‘quench’) star formation, such as the halting of cold gas accretion (starvation) or the rapid removal of gas in AGN-driven outflows. However, it remains unclear which processes are the most significant, primary drivers of the star-forming–passive bimodality. In this talk, I will discuss how measurements of the relative level of chemical enrichment in star-forming and passive galaxies can be used to derive valuable new insights into the quenching process, using stellar metallicities as a tracer of quenching mechanisms. Having analysed the stellar metallicities of tens of thousands of local star-forming, green valley and passive galaxies in the Sloan Digital Sky Survey, I will demonstrate that galaxies typically undergo a significant chemical transformation during the quenching process. Passive galaxies are in fact considerably more metal-rich (> 0.2 dex) than star-forming galaxies of the same stellar mass, indicating that the bulk of metals in passive galaxies are formed during the quenching process, rather than during the main sequence evolution of their star-forming progenitors. I will show that this significant difference in stellar metallicity between star-forming and passive galaxies implies that for galaxies at all masses, quenching must have involved an extended phase of starvation (where the accretion of metal-poor gas from the CGM/IGM is halted). I will further highlight how our chemical-enrichment-based technique has been applied to yield new insights into the mass-, environment- and radial-dependence of galaxy quenching.