Connecting the First Galaxies with Ultrafaint Dwarfs in the Local Group: Chemical Signatures of Population III Stars

Abstract

We investigate the star formation history (SFH) and chemical evolution of isolated analogs of Local Group (LG) ultrafaint dwarf galaxies (UFDs; stellar mass range of 10(2)M(circle dot) < M-*< 10(5) M-circle dot) and gas-rich, low-mass dwarfs (Leo P analogs; stellar mass range of 10(5)M(circle dot) < M-*< 10(6) M-circle dot). We perform a suite of cosmological hydrodynamic zoom-in simulations to follow their evolution from the era of the first generation of stars down to z=0. We confirm that reionization, combined with supernova (SN) feedback, is primarily responsible for the truncated star formation in UFDs. Specifically, halos with a virial mass of M-vir less than or similar to 2 x 10(9) M-circle dot form greater than or similar to 90% of stars prior to reionization. Our work further demonstrates the importance of Population. III stars, with their intrinsically high [C/Fe] yields and the associated external metal enrichment, in producing low-metallicity stars ([Fe/H] less than or similar to -4) and carbon-enhanced metal-poor (CEMP) stars. We find that UFDs are composite systems, assembled from multiple progenitor halos, some of which hosted only Population. II stars formed in environments externally enriched by SNe in neighboring halos, naturally producing extremely low metallicity Population II stars. We illustrate how the simulated chemical enrichment may be used to constrain the SFHs of true observed UFDs. We find that Leo P analogs can form in halos with M-vir similar to 4 x 10(9) M-circle dot 9 (z = 0). Such systems are less affected byreionization and continue to form stars until z = 0, causing higher-metallicity tails. Finally, we predict the existence of extremely low metallicity stars in LG UFD galaxies that preserve the pure chemical signatures of Population III nucleosynthesis.