CEMPlifying reionization

The massive stars that ionised 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 super-solar levels in $\alpha$-elements compared to iron. In {\sc eagle} galaxies resembling the Milky Way, $\sim 40$ percent of carbon-enhanced metal poor (CEMP) stars are SoRs. Conversely, $\sim 10$ percent of all SoRs are CEMP stars. This fraction increases to $\gtrsim 50$ percent 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 halos with mass $M_h\gtrapprox 10^{12}$ M$_\odot$, and 50 percent 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 halos of massive galaxies, with nearly half of them contributing to the intracluster light in groups and clusters.