Relic H ii regions and radiative feedback at high redshifts

Ultraviolet (UV) radiation from early astrophysical sources could have a large impact on subsequent star formation in nearby protogalaxies, and in general on the progress of cosmological reionization. Theoretical arguments based on the absence of metals in the early Universe suggest that the first stars were likely massive, bright, yet short-lived, with lifetimes of a few million years. Here we study the radiative feedback arising from such stars using hydrodynamical simulations with transient UV backgrounds (UVBs) and persistent Lyman–Werner backgrounds (LWBs) of varying intensity. We extend our prior work in Mesinger et al., by studying a more typical region whose protogalaxies form at lower redshifts, z∼ 13–20, in the epoch likely preceding the bulk of reionization. We confirm our previous results that feedback in the relic H ii regions resulting from such transient radiation is itself transient. Feedback effects dwindle away after ∼30 per cent of the Hubble time, and the same critical specific intensity of JUV∼ 0.1 × 10^−21 erg s^−1 cm^−2 Hz^−1 sr^−1 separates positive and negative feedback regimes. This suggests that overall feedback is fairly insensitive to the large-scale environment, overdensity and redshift-dependent halo parameters, and can accurately be modelled in this regime with just the intensity of the impinging UVB. Additionally, we discover a second episode of eventual positive feedback in haloes which have not yet collapsed when their progenitor regions were exposed to the transient UVB. When exposed to the transient UVB, this gas suffers relatively little density depletion but a significant enhancement of the molecular hydrogen abundance, thus resulting in net positive feedback. This eventual positive feedback appears in all runs, regardless of the strength of the UVB. However, this feedback regime is very sensitive to the presence of Lyman–Werner radiation, and notable effects disappear under fairly modest background intensities of JLW≳ 10^−3× 10^−21 erg s^−1 cm^−2 Hz^−1 sr^−1, assuming the region is optically thin for LW photons. Nevertheless, when exposed to the same LWB, haloes inside relic H ii regions always have a higher H2 abundance and shorter cooling times than haloes outside relic H ii regions, allowing gas to cool faster once it finally begins to collapse on to the halo. We conclude that UV radiative feedback in relic H ii regions, although a complicated process, seems unlikely to have a major impact on the progress of cosmological reionization, provided that present estimates of the lifetime and luminosity of a Population III star are accurate. More likely is that the build-up of the LWB ultimately governs the feedback strength until a persistent UV background can be established.