Connecting the first galaxies with ultra faint dwarfs in the Local Group: chemical signatures of Population~III stars [GA]

We investigate the star formation history and chemical evolution of isolated analogues of Local Group (LG) ultra faint dwarf galaxies (UFDs; stellar mass range of $10^3$ solar mass $<M_{\ast}<10^6$ solar mass), from the era of the first generation of stars down to $z=0$, by performing a suite of cosmological hydrodynamic zoom-in simulations. We confirm that reionization, combined with supernova feedback, is primarily responsible for the truncated star formation in dwarf galaxies. Our work further demonstrates the importance of Population~III stars and the associated external metal-enrichment in producing low-metallicity stars ($\rm [Fe/H]\lesssim-4$), and for the origin of carbon-enhanced metal-poor (CEMP) stars. We find that dwarf galaxies are composite systems, assembled from multiple progenitor haloes, some of which hosted only Population~II stars formed in environments externally enriched by SNe in neighboring haloes. We show that such external enrichment process naturally produces extremely low-metallicity Pop~II stars. Our simulations can also reproduce the population of CEMP stars by including Pop~III SNe with their intrinsically high $\rm [C/Fe]$ yields. We show that $\alpha$-elements are enhanced at all metallicities, in particular for haloes with $\rm M_{vir}\lesssim2\times10^9$ solar mass, implying that the gas in such small dwarfs has preferentially been contaminated by Type~II SNe from Pop~III and massive Pop~II stars. We illustrate how the simulated chemical enrichment can be used to constrain the star formation histories (SFHs) of true observed dwarf galaxies. Our results indicate that observed UFDs lacking stars with $\rm [Fe/H]>-1.04$ are likely to have SFHs truncated prior to $z=3$. Finally, we predict the existence of extremely low-metallicity stars in LG UFD galaxies that preserve the pure chemical signatures of Pop~III nucleosynthesis.

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M. Jeon, G. Besla and V. Bromm
Mon, 27 Feb 17

Comments: 20 pages, 11 figures, Submitted to ApJ