Place: 4421 Sterling Hall, Coffee and Cookies 3:30 PM, Talk begins at 3:45 PM
Speaker: Nicholas Lehner, University of Notre Dame
Abstract: It has become clear that the level of activity in a galaxy is affected by phenomena occurring in its disk, but also in its halo, known as the circumgalactic medium (CGM) -- the gas at the interface of a galaxy and the intergalactic medium. Gas flows through the CGM directly influence the baryon distribution and metal content of their host galaxies as well as the level of stellar activity in the disk, and hence the evolution of galaxies. The metallicity is a key parameter that allows us to directly determine the level and type of enrichment of the CGM gas and its possible origin(s). I will present an HI-selected absorption-line survey of the gas at low and high redshift where we estimated the metallicity of the gas for several hundreds of CGM absorbers. I will show that there is a a significant fraction of the CGM that is metal rich at any z. I will show that there is surprisingly also a large reservoir of metal-poor gas at any z, some of this gas showing little chemical enrichment over several billions of years. I will argue that this metal-poor gas might be the long-sought cold flow accretion that provides a source of gas for ongoing star-formation over billions of years in cosmological simulations. I will finally discuss these observational results in the context of recent cosmological simulations and future radio 21-cm HI emission surveys.