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 7$.