The origins of the circumgalactic medium in the FIRE simulations

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 M-h similar to 10(10)-10(12) M-circle dot, and we focus on z = 0.25 and z = 2. Owing to strong stellar feedback, only similar to L* haloes retain a baryon mass greater than or similar to 50 per cent of their cosmic budget. Metals are more efficiently retained by haloes, with a retention fraction greater than or similar to 50 per cent. Across all masses and redshifts analysed greater than or similar to 60 per cent of the CGM mass originates as IGM accretion (some of which is associated with infalling haloes). Overall, the second most important contribution is wind from the central galaxy, though gas ejected or stripped from satellites can contribute a comparable mass in similar to L* haloes. Gas can persist in the CGM for billions of years, resulting in well mixed-halo gas. Sightlines through the CGM are therefore likely to intersect gas of multiple origins. For low-redshift similar to L* haloes, cool gas (T < 10(4.7) K) is distributed on average preferentially along the galaxy plane, however with strong halo-to-halo variability. The metallicity of IGM accretion is systematically lower than the metallicity of winds (typically by greater than or similar to 1 dex), although CGM and IGM 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.