Local Group Ultra-Faint Dwarf Galaxies in the Reionization Era

Daniel R. Weisz; Michael Boylan-Kolchin

doi:10.1093/mnrasl/slx043

paper

Local Group Ultra-Faint Dwarf Galaxies in the Reionization Era

arXiv:1702.06129 · doi:10.1093/mnrasl/slx043

Abstract

Motivated by the stellar fossil record of Local Group (LG) dwarf galaxies, we show that the star-forming ancestors of the faintest ultra-faint dwarf galaxies (UFDs; $M_{V}$ $\sim - 2$ or $M_{⋆}$ $\sim 1 0^{2}$ at $z = 0$ ) had ultra-violet (UV) luminosities of $M_{UV}$ $\sim - 3$ to $- 6$ during reionization ( $z \sim 6 - 10$ ). The existence of such faint galaxies has substantial implications for early epochs of galaxy formation and reionization. If the faint-end slopes of the UV luminosity functions (UVLFs) during reionization are steep ( $α ≲ - 2$ ) to $M_{UV}$ $\sim - 3$ , then: (i) the ancestors of UFDs produced $> 50$ % of UV flux from galaxies; (ii) galaxies can maintain reionization with escape fractions that are $>$ 2 times lower than currently-adopted values; (iii) direct HST and JWST observations may detect only $\sim 10 - 50$ % of the UV light from galaxies; (iv) the cosmic star formation history increases by $≳ 4 - 6$ at $z ≳ 6$ . Significant flux from UFDs, and resultant tensions with LG dwarf galaxy counts, are reduced if the high-redshift UVLF turns over. Independent of the UVLF shape, the existence of a large population of UFDs requires a non-zero luminosity function to $M_{UV}$ $\sim - 3$ during reionization.

6 pages, 4 Figures. Submitted to MNRAS. Comments welcome

Local Group Ultra-Faint Dwarf Galaxies in the Reionization Era

Abstract

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