0 Assigned ID: 706 ----- Attempting Submission 1 (Fri Apr 07 18:22:27 GMT 2017) ----- HST COS observations have unexpectedly discovered ubiquitous OVI-absorbing gas around star forming galaxies, constituting a substantial fraction (10% or more) of the galaxy's baryonic mass. This is extremely surprising: in collisional ionization equilibrium, OVI peaks only in narrow temperature range around ~2e5 K, below the virial temperature. In steady-state, such a large mass of gas at this temperature implies a cooling flow of 30 Msun/year or more! Other unexplained properties include kinematic similarities with low ionization lines and a relative dearth of OVI in quiescent galaxies. We will study the origin of OVI gas using semi-cosmological simulations which 'grow' a galaxy halo with a specified accretion history, including star formation and feedback physics. Unlike cosmological zoom-in simulations, which focus on resolving the disks of galaxies, our simulations are specifically designed to study the CGM and enable us to cover a wide range of parameter space at high resolution. We show a simulation which produces OVI profiles in excellent agreement with observations, with no fine-tuning. Here, OVI arises from a thick, partially-photoionized shell outside the virial radius; this critically requires that gas in galaxy outskirts be metal enriched and low pressure. We will study the physical origin of this result, and how OVI observations probe galaxy formation and CGM physics. We will generate OVI and other metal line profiles which can be directly compared against HST observations. Key predictions to explore are the disappearance of OVI gas in higher mass or redshift halos (where higher pressures prevail), and OVI in emission or resonant scattering. Galaxies and the IGM 0 Regular false false true SUPPORTED