Early onset of the hot circumgalactic medium around Milky Way galaxies

Hot coronae are key regulators in Milky Way (MW)-type galaxy formation and are thought to emerge once a galaxy’s dark matter halo crosses a critical mass threshold, Mvir ~ 1012 M⊙, so that accretion shocks can heat infalling gas to virial temperatures, Tvir. Here we investigate the roles of stellar feedback and accretion shocks in establishing the circumgalactic medium (CGM) around MW-mass galaxies using the VINTERGATAN cosmological simulation suite. We find that stellar feedback drives the early formation of a hot CGM, with TCGM ~ Tvir. By contrast, in feedback-free simulations, accretion shocks alone do not produce a CGM with comparably high average temperatures at early epochs (z > 2), independent of gas metallicity. We show that CGM virialisation (defined when halo-gas cooling time exceeds the free-fall time) begins with the emergence of a hot corona (T > 105 K) that can appear as early as z ≈ 3 at virial radius, Rvir, when feedback is included. This is much earlier than predicted by accretion-shock theory or feedback-free models (z ≲ 1). While accretion shocks alone produce an inside-out virialisation pattern, feedback can alter this behaviour, driving a rapid transition (≲ 0.5 Gyr) that is either outside-in or nearly simultaneous throughout the halo. Finally, these early-forming coronae are largely stable from formation through to the present day, but can appear to evolve simply because the definition of Rvir grows with time. This “CGM pseudo-evolution” is analogous to the pseudo-evolution of dark matter haloes and should be taken into account when interpreting the evolution of hot coronae.