How Do Supernovae Impact the Circumgalactic Medium? I. Large-Scale Fountains Around a Milky Way-Like Galaxy

Abstract: Feedback is indispensable in galaxy formation. However, lacking resolutions, cosmological simulations often use ad hoc feedback parameters. Conversely, small-box simulations, while better resolving the feedback, cannot capture gas evolution beyond the simulation domain. We aim to bridge the gap by implementing small-box results of supernovae-driven outflows into dark matter halo-scale simulations and studying their impact on large scales. Galactic outflows are multiphase, but small-box simulations show that the hot phase (T$\approx$ 10$^{6-7}$ K) carries the majority of energy and metals. We implement hot outflows in idealized simulations of the Milky Way halo, and examine how they impact the circumgalactic medium (CGM). In this paper, we discuss the case when the star formation surface density is low and therefore the emerging hot outflows are gravitationally bound by the halo. We find that outflows form a large-scale, metal-enriched atmosphere with fountain motions. As hot gas accumulates, the inner atmosphere becomes "saturated". Cool gas condenses, with a rate balancing the injection of the hot outflows. This balance leads to a universal density profile of the hot atmosphere, independent of mass outflow rate. The atmosphere has a radially-decreasing temperature, naturally producing the observed X-ray luminosity and column densities of O VI, O VII, O VIII. The self-regulated atmosphere has a baryon and a metal mass of $(0.5-1.2)\times 10^{10}M_\odot$ and $(0.6-1.4)\times 10^8 M_\odot$, respectively, small compared to the ``missing" baryons and metals from the halo. We conjecture that the missing materials reside at even larger radii, ejected by more powerful outflows in the past.