The Origins of the Circumgalactic Medium in the FIRE Simulations

Hafen et al., available on arXiv

Abstract: We use a particle tracking analysis to study the origins of the circumgalactic medium (CGM), separating it into (1) accretion from the intergalactic medium (IGM), (2) wind from the central galaxy, and (3) gas ejected from other galaxies. Our sample consists of 21 FIRE-2 simulations, spanning the halo mass range log(Mh/Msun) ~ 10-12 , and we focus on z=0.25 and z=2. Owing to strong stellar feedback, only ~L* halos retain a baryon mass >~50% of their cosmic budget. Metals are more efficiently retained by halos, with a retention fraction >~50%. Of the CGM mass, >~60% originates as IGM accretion at all masses and redshifts analyzed. Overall, the second most important contribution is wind from the central galaxy, though gas ejected from satellites can contribute a comparable mass in ~L* halos. Gas can persist in the CGM for billions of years, resulting in well mixed halo gas. Sight lines through the CGM are therefore likely to intersect gas of multiple different origins. For low-redshift ~L* halos we do not find significant evidence for a dependence of CGM origin on angle relative to the galaxy stellar disk, except for possibly a disk-like cool component at R <~0.3 Rvir. The metallicity of IGM accretion is systematically lower than the metallicity of winds (typically by >~1 dex), although metallicities depend significantly on the treatment of subgrid metal diffusion. Our results highlight the multiple physical mechanisms that contribute to the CGM and will inform observational efforts to develop a cohesive picture.