Why is there cold gas inside the Local Bubble?

Abstract

The Local Bubble (LB) extends approximately 100--200~pc from the Sun and contains $10^6$~K plasma and warm photoionised gas. Yet the Local Leo Cold Cloud (LLCC) and the Local Ribbon of Cold Clouds lie only 11--24~pc away, well inside the LB, at temperatures near 20~K. How such cold material forms and survives in this hotter environment remains uncertain, and explanations based on colliding warm clouds are difficult to test observationally. We investigate whether LLCC-like gas can assemble locally in a feedback-driven environment or must be imported as pre-existing cold material, and whether it can survive for several Myr. We use magnetohydrodynamic simulations from the \textsc{SILCC} Project including self-consistent star formation, stellar winds and supernovae, ionising and far-ultraviolet radiation, non-equilibrium chemistry, and cosmic-ray transport. We identify cold diffuse gas with $T<100$~K and $0<n_\mathrm{H_2}\leq2$~cm$^{-3}$ and use Lagrangian tracers to reconstruct its assembly, cooling, survival, and dispersal. The simulations produce cold diffuse gas in warm--hot, feedback-driven surroundings, where non-thermal pressure support is crucial. Cosmic-ray pressure remains nearly continuous across cold-gas interfaces and establishes a pressure floor, while magnetic pressure partly offsets the external thermal-pressure excess. The structures persist in the cold diffuse phase for a median of 2.5~Myr, and most of their material remains cold after they lose spatial coherence. The gas can assemble locally from nearby cold and thermally unstable warm material, developing filamentary or sheet-like morphologies. Cooling to $\sim20$~K is driven not by adiabatic expansion but by non-adiabatic cooling. The LLCC therefore need not have entered the LB as an already-formed cold cloud. LLCC-like gas can instead assemble and cool non-adiabatically in situ.