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, $M_{\mathrm{v}\mathrm{i}\mathrm{r}}\sim {10}^{12}{\mathrm{M}}_{\odot }$, so that accretion shocks can heat infalling gas to virial temperatures, $T_{\mathrm{v}\mathrm{i}\mathrm{r}}$. 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 $T_{\mathrm{C}\mathrm{G}\mathrm{M}}\sim T_{\mathrm{v}\mathrm{i}\mathrm{r}}$. 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 ($T>{10}^5$ K) that can appear as early as $z\approx 3$ at virial radius, $R_{\mathrm{v}\mathrm{i}\mathrm{r}}$, when feedback is included. This is much earlier than predicted by accretion-shock theory or feedback-free models ($z\lesssim 1$). While accretion shocks alone produce an inside-out virialisation pattern, feedback can alter this behaviour, driving a rapid transition ($\lesssim 0.5$ 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 $R_{\mathrm{v}\mathrm{i}\mathrm{r}}$ 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.