Ultra-Faint Dwarfs Defy Simple Scaling Laws, Tightening Constraints on Dark Matter and Early Galaxy Formation

The Pristine Dwarf Galaxy Survey's latest preprint maps metallicity distributions across 12 faint Milky Way satellites using narrow-band CaHK photometry combined with broad-band g,r data from Muñoz et al. (2018) and Pan-STARRS1. Covering out to 5-8 half-light radii and reaching g23, the approach yields 3917 probable members—more than doubling spectroscopic samples—via spatial, photometric, astrometric, and refined metallicity-based membership probabilities. This preprint (not yet peer-reviewed) reveals a clear break from the linear luminosity-metallicity relation below M_V-6, with ultra-faints scattering around [Fe/H]~-2.3, plus 170 extremely metal-poor candidates. Unlike more massive dwarfs, no radial metallicity gradients appear within 2.5 R_h, consistent with inefficient star formation in the smallest halos. The missing-satellites tension (Bullock & Boylan-Kolchin 2017) is sharpened: these systems probe the lowest-mass dark-matter subhalos where reionization and supernova feedback suppress star formation, yet the flat metallicity floor challenges models assuming continuous enrichment. Earlier spectroscopic work (Simon 2019) was limited by small-N statistics and bright-star bias; this photometric strategy exposes contaminants and extends reach but trades precision for volume. Future spectroscopic follow-up remains essential to confirm the 170 EMP candidates and test whether the observed scatter signals stochastic IMF sampling or variable feedback efficiency.