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.
