The in situ formation of molecular and warm ionized gas triggered by hot galactic outflows

Abstract

ABSTRACT Molecular outflows contributing to the matter cycle of star-forming galaxies are now observed in small and large systems at low and high redshift. Their physical origin is still unclear. In most theoretical studies, only warm ionized/neutral and hot gas outflowing from the interstellar medium is generated by star formation. We investigate an in situ H2 formation scenario in the outflow using high-resolution simulations, including non-equilibrium chemistry and self-gravity, of turbulent, warm, and atomic clouds with densities 0.1, 0.5, and $1\, \mathrm{cm}^{-3}$ exposed to a magnetized hot wind. For cloud densities $\gtrsim 0.5\, \mathrm{cm}^{-3}$, a magnetized wind triggers H2 formation before cloud dispersal. Up to 3 per cent of the initial cloud mass can become molecular on $\sim \! 10\, \mathrm{Myr}$ time-scales. The effect is stronger for winds with perpendicular B-fields and intermediate density clouds ($n_\mathrm{c}\sim 0.5\, \mathrm{cm}^{-3}$). Here, H2 formation can be boosted by up to one order of magnitude compared to isolated cooling clouds independent of self-gravity. Self-gravity preserves the densest clouds well past their $\sim \! 15\, \mathrm{Myr}$ cloud crushing time-scales. This model could provide a plausible in situ origin for the observed molecular gas. All simulations form warm ionized gas, which represents an important observable phase. The amount of warm ionized gas is almost independent of the cloud density but solely depends on the magnetic field configuration in the wind. For low-density clouds ($0.1\, \mathrm{cm}^{-3}$), up to 60 per cent of the initially atomic cloud mass can become warm and ionized.