Metal flows of the circumgalactic medium, and the metal budget in galactic halos

Muratov et al., available on arXiv.

Abstract: We present an analysis of the flow of metals through the circumgalactic medium in the Feedback in Realistic Environments (FIRE) simulations of galaxy formation, ranging from isolated dwarfs to L*. We find that nearly all metals produced in high-redshift galaxies are carried out in winds that escape the galaxy and reach 0.25Rvir. When measured at 0.25Rvir the metallicity of outflows is greater than, but similar to the ISM metallicity. Many metals thus reside in a reservoir in the CGM. We find that the outflowing flux through Rvir is lower than that at 0.25Rvir by a factor of ~2-5. Cooling and recycling from this reservoir determine the metal budget in the ISM. The inflow metallicity at Rvir is generally very low, but outflow and inflow metallicities are similar in the inner halo. At low redshift, massive galaxies no longer generate outflows that reach the CGM, causing a divergence in CGM and ISM metallicity. Dwarf galaxies continue to generate outflows, which become increasingly dominated by metal-poor gas, while the galaxies themselves preferentially retains metal ejecta. In all but the least massive galaxy considered, a majority of the metals are within the halo at z=0. We measure the fraction of metals in CGM, ISM, stars, and roughly quantify the thermal state of CGM metals in each halo. The total amount of metals in the low-redshift CGM of two simulated L* galaxies is consistent with estimates from the COS halos survey, while for the other two it appears to be lower.