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

Abstract: The ultraviolet (UV) continuum slope ($\beta~$where$~f_\lambda\propto\lambda^\beta$) 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 ($\beta<-3$). Not only do such systems provide a signpost of low-metallicity stars, but they also identify galaxies likely to leak ionizing photons from their HII 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 EGS field. We characterize UV slopes for a large sample of$~z\simeq7-11~$galaxies, finding a median of$~\beta=-2.0$. Two lower luminosity (M${\rm{UV}}\simeq-19.5$) and lower stellar mass (6-10$\times10^7$M$\odot$) systems exhibit extremely blue UV slopes ($\beta=-2.9~$to$~-3.1$) and rest-optical photometry indicating weak nebular line emission. Each system is very compact (r$e\lesssim$260pc) with very high star-formation-rate surface densities. We model the SEDs with a suite of BEAGLE models with varying levels of ionizing photon escape. The SEDs cannot be reproduced with our fiducial (f${\rm{esc,HII}}$=0) or alpha-enhanced (Z$\star<Z{\rm{ISM}}$) models. The combined blue UV slopes and weak nebular emission are best-fit by models with significant ionizing photon escape from HII regions (f${\rm{esc,HII}}$=0.5-0.8) and extremely low-metallicity massive stars (Z$\star$=0.01-0.06Z$_\odot$). 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 $z>7$.