Same galaxy, different CGM: how the metal loading of galactic winds regulates the baryon cycle in Milky Way-mass galaxies
The circumgalactic medium (CGM) is both the reservoir of gas that fuels star formation and galaxy growth, and the repository for the mass, energy, and metals expelled through stellar evolution and feedback. We present a controlled experiment using a suite of five cosmological magnetohydrodynamical simulations of a Milky Way-mass halo from the Auriga suite, where we vary the metal content and energy loading of galactic winds driven by stellar feedback. We chose these parameters in combination such that all five runs obtain very similar (within $\approx$10\%) $z=0$ stellar masses and morphologies and thus produce similar amounts of metals throughout their lifetimes. Our simulations differ substantially in where these metals end up, spatially within the halo and across the different baryonic components (stars, gas disc, CGM, and the intergalactic medium). The metal content affects the cooling efficiency of the halo gas, regulating the CGM cool gas fraction and the accretion rates onto the galaxy. Between our two extreme models, we report an order of magnitude difference in the median gas metallicity at vertical heights $|z| \gtrsim 30$ kpc above the disc plane. Current constraints on the metallicity of the Milky Way halo gas from absorption-line measurements of intermediate- and high-velocity clouds at $|z| \lesssim 15$ kpc are broadly consistent with our simulations, though a direct comparison is limited by differences in the adopted methods. Our findings show that galaxies with nearly identical stellar content and morphology can differ substantially in their CGM, which offers a promising avenue for constraining feedback physics within galaxy formation models.