Searching for Extremely Blue UV Continuum Slopes at z = 7-11 in JWST/NIRCam Imaging: Implications for Stellar Metallicity and Ionizing Photon Escape in Early Galaxies

The ultraviolet (UV) continuum slope (β, where f _λ ∝ λ ^β) of galaxies is sensitive to a variety of properties, from the metallicity and age of the stellar population to dust attenuation throughout the galaxy. Considerable attention has focused on identifying reionization-era galaxies with very blue UV slopes (β < −3). Not only do such systems provide a signpost of low-metallicity stars, but they also identify galaxies likely to leak ionizing photons from their H ii regions as such blue UV slopes require the reddening effect of nebular continuum to be diminished. In this paper we present a search for reionization-era galaxies with very blue UV colors in recent JWST/NIRCam imaging of the Extended Groth Strip field. We characterize UV slopes for a large sample of z ≃ 7-11 galaxies, finding a median of β = −2.0. Two lower luminosity (M _UV ≃ −19.5) and lower stellar mass (6-10 × 10⁷ M _⊙) systems exhibit extremely blue UV slopes (β = −2.9 to −3.1) and rest-optical photometry indicating weak nebular line emission. Each system is very compact (r _e ≲ 260 pc) with very high star formation-rate surface densities. We model the spectral energy distributions (SEDs) with a suite of BEAGLE models with varying levels of ionizing photon escape. The SEDs cannot be reproduced with our fiducial (f _{esc,H II} = 0) or alpha-enhanced (Z _⋆ < Z _ISM) models. The combined blue UV slopes and weak nebular emission are best-fit by models with significant ionizing photon escape from H ii regions (f _{esc,H II} = 0.5-0.8) and extremely low-metallicity massive stars (Z _⋆ = 0.01-0.06 Z _⊙). The discovery of these galaxies highlights the potential for JWST to identify large numbers of candidate Lyman continuum leaking galaxies in the reionization era and suggests low-metallicity stellar populations may be common in dwarf galaxies at z > 7.