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 × 10 7 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.