The fates of the circumgalactic medium in the FIRE simulations

We analyse the different fates of the circumgalactic medium (CGM) in FIRE-2 cosmological simulations, focusing on the redshifts z = 0.25 and 2 representative of recent surveys. Our analysis includes 21 zoom-in simulations covering the halo mass range M-h(z = 0) similar to 10(10)-10(12)M(circle dot). We analyse both where the gas ends up after first leaving the CGM (its `proximate' fate) and its location at z = 0 (its 'ultimate' fate). Of the CGM at z = 2, about half is found in the ISM or stars of the central galaxy by z = 0 in M-h(z = 2) similar to 5 x 10(11) M-circle dot haloes, but most of the CGM in lower mass haloes ends up in the intergalactic medium (IGM). This is so even though most of the CGM in M-h(z = 2) similar to 5 x 10(10) M-circle dot haloes first accretes on to the central galaxy before being ejected into the IGM. On the other hand, most of the CGM mass at z = 0.25 remains in the CGM by z = 0 at all halo masses analysed. Of the CGM gas that subsequently accretes on to the central galaxy in the progenitors of M-h(z = 0) similar to 10(12)M(circle dot) haloes, most of it is cool (T similar to 10(4) K) at z = 2 but hot (similar to T-vir) at z similar to 0.25, consistent with the expected transition from cold mode to hot mode accretion. Despite the transition in accretion mode, at both z = 0.25 and 2 greater than or similar to 80 per cent of the cool gas in M-h greater than or similar to 10(11) M-circle dot haloes will accrete on to a galaxy. We find that the metallicity of CGM gas is typically a poor predictor of both its proximate and ultimate fates. This is because there is in general little correlation between the origin of CGM gas and its fate owing to substantial mixing while in the CGM.