Platelet mimicry

Abstract

stars and an older, chemically evolved population, respectively emitting most of the UV and optical light. Sobral et al. corroborated these model-based predictions using images obtained by the Hubble Space Telescope which show that CR7 consists of three spatially sepa-rated clumps. This structure is consistent with a theoreti-cal study 10 that proposed the 'inside-out' for-mation of population III stars. The authors' observations suggest that a wave of star for-mation swept over CR7, progressively moving outwards from the old red clumps towards the young, UV-bright clump 5 kiloparsecs away, where population III stars are inferred to exist. This distance is large enough to prevent con-tamination of the UV-bright clump by metals created in the structure's interior. According to this picture, photons from the old stars not only helped to ionize a large bubble around them, but also prevented any surrounding gas from forming stars for some time by imped-ing the gas's gravitational collapse. This created an ideal environment for population III stars to eventually form in the chemically pristine clump, while allowing the Lyman-α photons to escape because there was no neutral hydrogen to absorb them. This formation mechanism implies that, in Lyman-α emitters, popula-tion III stars would most probably be detected in hybrid stellar populations. Sobral and colleagues' discovery of this population III system is not without caveats. For example, to match the ratio of helium-to-Lyman-α line emission predicted by models, the authors deduce that 75% of the Lyman-α emission must have been lost through scatter-ing or absorption. In addition, the proposed scenario in which evolved, 'second-generation' stars can ionize a region without chemically polluting it, while also conveniently hold-ing back star formation in the vicinity until population III stars arise, seems somewhat ad hoc, although it is not impossible. Finally, the authors state that the properties of CR7 can also be explained by gas falling into a black hole formed from the direct collapse of primordial gas, another theoretical construct. However, material around black holes pro-duces broad emission-line profiles and X-ray radiation, neither of which are detected in the current observations. Deep observations with next-generation X-ray telescopes might be able to distinguish between the population III and black-hole scenarios 11