Dennis Zaritsky

The majority of the baryons in galaxies are inferred to comprise the circumgalactic medium (CGM) – gas that is mostly in either a cool (~10^4K) or a hot (10^5-10^6 K) phase. The former has been studied with QSO absorption line studies and the latter using X-ray observations. The observational picture, however, is unsatisfactory because of the scarcity of sightlines with sufficiently bright QSOs and because the hot phase in most normal galaxies is too cool for X-ray imaging. This deficiency is particularly critical given the central role of the CGM in providing the fuel for subsequent star formation and in accepting the mass and energy from galactic feedback. As such, galaxy evolution models remain relatively unconstrained in their treatment of the CGM and their implementation of baryon cycling. I will present a new way to map the cool CGM using optical emission lines and discuss how some of our basic conceptual understanding of galaxy evolution is reflected in our findings. Finally, I will introduce Aspera, which will map the hot phase in nearby normal galaxies.