Survival is not Enough: Dust Sputtering, Growth, and H$_2$ Formation in Galactic Winds

Abstract

A substantial amount of dust is found in galactic halos extending far beyond the disks, the origin of which remains an open question. Closely linked and equally puzzling is the detection of molecular gas in high-velocity galactic winds. To address this, we present the first cloud-crushing simulations that self-consistently include non-equilibrium cooling and chemistry with dust growth and sputtering. We find that surviving clouds naturally develop a two-phase structure, with a cold ($\sim 30$ K), dense core embedded in a warm ($\sim 10^4$ K), diffuse envelope. However, the presence of a cold phase does not always lead to molecular winds. While dust initially in the cloud largely survives in $10^6$ K winds, it is severely depleted by sputtering in hotter winds ($\gtrsim 10^7$ K). Importantly, without dust growth, the dust-to-gas ratio (DGR) of the cloud declines rapidly, suppressing the formation of molecular hydrogen (H$_2$) and keeping the entrained cloud atomic, even in cases where the majority of the initial dust survives. Nonthermal sputtering plays a subdominant role in all cases. The entrained clouds develop high molecular fractions only when dust growth is enabled, provided the cloud densities are sufficiently high ($\gtrsim$ 10 - 30 times the critical density for cloud survival). Our results suggest that "in situ" dust growth is essential to explain both the observed abundance of halo dust and the molecular gas in galactic winds.