The origin of cold gas in the circumgalactic medium

Context. The presence of cold gas (T ≲ 104 K) in the circumgalactic medium (CGM) of galaxies has been confirmed in observations and in high-resolution simulations, but its origin is still a puzzle. Possible mechanisms are cold accretion from the intergalactic medium (IGM), clumps embedded in outflows and transported from the disk, and gas detaching from the hot CGM phase via thermal instabilities. Aims. In this work we characterize the history of cold CGM gas in order to identify the dominant origin channels at different evolutionary stages of the main galaxy. Methods. To this end, we tracked gas particles in different snapshots of the smoothed particle hydrodynamics (SPH) cosmological zoom-in simulation Eris2k. We performed a backward tracking of cold gas, starting from different redshifts until we could identify one of the followings origins for the particle: cold inflow, ejection from the disk, cooling down in situ, or stripping from a satellite. We also performed a forward tracking of gas in different components of the galaxy (such as the disk and outflows). Results. We find a clear transition between two epochs. For z > 2, most cold gas (up to 80%) in the CGM comes from cold accretion streams as the galaxy is accreting in the cold mode from the IGM. At lower z, gas either cools down in situ after several recycles (with 10–20% of the gas cooling in outflow), or it is ejected directly from the disk (up to 30%). Outflows have a major contribution to the cold CGM gas budget at z < 1, with almost 50% of the hot gas cooling in outflow. Finally, we discuss possible mechanisms for CGM cooling, showing that the thermally unstable gas with tcool/tff < 1 (precipitation-regulated feedback) is abundant up to r ∼ 100 kpc and cooling times are shorter than 50 Myr for densities n > 10−2 cm−3.