Hidden Population III Descendants in Ultrafaint Dwarf Galaxies

The elusive properties of the first (Population III or Pop III) stars can be indirectly unveiled by uncovering their true descendants. To this end, we exploit our data-calibrated model for the best-studied ultrafaint dwarf galaxy (UFD), Boötes I, which tracks the chemical evolution (from carbon to zinc) of individual stars from their formation to the present day. We explore the chemical imprint of Pop III supernovae (SNe), with different explosion energies and masses, showing that they leave distinct chemical signatures in their descendants. We find that UFDs are strongly affected by SN-driven feedback, resulting in a very low fraction of metals being retained by their gravitational potential well (<2.5%). Furthermore, the higher the Pop III SN explosion energy, the lower the fraction of metals retained. Thus, the probability of finding descendants of energetic pair-instability SNe is extremely low in these systems. Conversely, UFDs are ideal cosmic laboratories for identifying the fingerprints of less massive and energetic Pop III SNe, through their [X/Fe] abundance ratios. Digging into the literature data for Boötes I, we uncover three hidden candidates for Pop III descendants: one monoenriched and two multienriched. These stars show the chemical signature of Pop III SNe in the mass range [20–65]M⊙, spanning a wide range of explosion energies [0.3–10]1051 erg. In conclusion, candidates for Pop III descendants are hidden in ancient UFDs, but those monoenriched by a single Pop III SN are extremely rare. Thus, self-consistent models such as the one presented here are required to uncover these precious fossils and probe the properties of the first Pop III SNe.