The hydrodynamical response of cold circumgalactic clouds to quasar radiation

Numerical efforts increasingly resolve the small-scale structure of the circumgalactic medium (CGM), but the dynamical impact of ionising radiation on its cold 10^4, ̊m K component remains poorly understood. We investigated the evolution of static cold gas structures exposed to the extreme-ultraviolet (EUV) radiation of quasars. We developed an analytical framework to describe the evolution of such clouds, introducing a new threshold that defines when a cloud becomes radiation-shielded. The framework was validated using radiation-hydrodynamic simulations of single static clouds. The framework predicts three evolutionary regimes: (i) an optically thin regime, in which radiation uniformly ionises the cloud; (ii) a radiation-shielded regime, where the cloud remains largely unaffected; and (iii) a "rocket-effect" regime, in which an ionisation front propagates through the cloud, ionising the illuminated side while compressing the opposite side and accelerating the surviving cold clump. In this latter regime, the cloud's Lyman-α (Lyα) luminosity can increase by up to one order of magnitude compared to the optically thin case. Such luminosities are as high as 70% of the values obtained from a fluorescent regime without considering hydrodynamical response. Unless the cloud is self-shielded, at least ∼ 50-60,% of Lyα emission arises from recombination. Applying this framework to both a population of clouds along a line of sight and a ray propagating inside a single cold stream, we find that the cold inter-galactic medium (IGM) and CGM around bright quasars ν,LL ∼ 10^ 31.6 , ̊m erg, s^ -1 , Hz^ -1 ) is likely fully ionised, whereas the one around faint quasars (L_ ν,LL ∼ 10^ 28.6 , ̊m erg, s^ -1 , Hz^ -1 ) predominantly experiences a rocket-effect regime. These results imply that the hydrodynamical response of cold CGM structures to quasar radiation must be considered when deriving CGM and IGM physical properties from observations, particularly for faint quasars.