Fluorescent rings in star-free dark matter haloes

Photoheating of the gas in low-mass dark matter (DM) haloes prevents baryons from cooling, leaving the haloes free of stars. Gas in these `dark' haloes remains exposed to the ultraviolet background (UVB), and so is expected to emit via fluorescent recombination lines. We present a set of radiative transfer simulations, which model dark haloes as spherical gas clouds in hydrostatic equilibrium with a DM halo potential, and in thermal equilibrium with the UVB at redshift z = 0. We use these simulations to predict surface brightnesses in H α, which we show to have a characteristic ring-shaped morphology for haloes in a narrow mass range between ≃10^9.5 and 10^{9.6} {M_⊙}. We explore how this emission depends on physical parameters such as the DM density profile and the UVB spectrum. We predict the abundance of fluorescent haloes on the sky, and discuss possible strategies for their detection. We demonstrate how detailed observations of fluorescent rings can be used to infer the properties of the haloes which host them, such as their density profiles and the mass-concentration relation, as well as to directly measure the UVB amplitude.