CEMPlifying reionization

Abstract

The massive stars that ionized the Universe have short lifetimes and can only be studied near the time of formation, but any low-mass stars that formed contemporaneously might be observable in the local Universe today. We study the abundance pattern and spatial distribution of these ‘siblings of reionizers’ (SoRs) in the EAGLE cosmological hydrodynamical simulation. SoRs tend to be enriched to supersolar levels in α-elements compared to iron, and we investigate the resemblance between SoRs and obthe served stars that are enhanced in carbon. In EAGLE galaxies resembling the Milky Way, ∼40 per cent of carbon-enhanced metal poor (CEMP) stars are SoRs. Conversely, ∼10 per cent of all SoRs are CEMP stars. This fraction increases to ≳50 per cent for SoRs of metallicity [Fe/H] < −4, and at such low metallicities, most of the CEMP stars are of CEMP-no subtype that are lacking neutron capture elements. Although these numbers may well depend on the details of the physical models implemented in EAGLE, the trends we describe are robust as they result from the strong feedback from star formation in early galaxies, itself a key ingredient of most current models of galaxy formation. We further find that most SoRs today reside in haloes with mass Mh ≈> 1012 M⊙, and 50 per cent of them are in the halo of their central galaxy (distance >10 kpc), mainly because they were accreted onto their current host rather than formed in situ. To a good approximation, the SoRs are CEMP-no stars that reside in the stellar haloes of massive galaxies, with nearly half of them contributing to the intracluster light in groups and clusters.