We infer the UV luminosities of Local Group galaxies at early cosmic times ( z \sim 2 and z \sim 7 ) by combining stellar population synthesis modeling with star formation histories derived from deep color-magnitude diagrams constructed from Hubble Space Telescope ( HST ) observations .
Our analysis provides a basis for understanding high- z galaxies – including those that may be unobservable even with the James Webb Space Telescope ( JWST ) – in the context of familiar , well-studied objects in the very low- z Universe .
We find that , at the epoch of reionization , all Local Group dwarfs were less luminous than the faintest galaxies detectable in deep HST observations of blank fields .
We predict that JWST will observe z \sim 7 progenitors of galaxies similar to the Large Magellanic Cloud today ; however , the HST Frontier Fields initiative may already be observing such galaxies , highlighting the power of gravitational lensing .
Consensus reionization models require an extrapolation of the observed blank-field luminosity function at z \approx 7 by at least two orders of magnitude in order to maintain reionization .
This scenario requires the progenitors of the Fornax and Sagittarius dwarf spheroidal galaxies to be contributors to the ionizing background at z \sim 7 .
Combined with numerical simulations , our results argue for a break in the UV luminosity function from a faint-end slope of \alpha \sim - 2 at { M _ { UV } } \lesssim - 13 to \alpha \sim - 1.2 at lower luminosities .
Applied to photometric samples at lower redshifts , our analysis suggests that HST observations in lensing fields at z \sim 2 are capable of probing galaxies with luminosities comparable to the expected progenitor of Fornax .