A Galaxy with an Extremely Blue Ultraviolet Slope β = −3 at z = 9.25 Identified by JWST Spectroscopy: Evidence for a Weak Nebular Continuum and Efficient Ionizing Photon Escape?

We investigate UV continuum slopes β of 863 galaxies at 4 < z < 14 using archival JWST/NIRSpec PRISM spectra obtained from major JWST Guaranteed Time Observations (GTO), Early Release Science (ERS), and General Observers (GO) programs, including JADES, CEERS, and UNCOVER. Among these galaxies, we identify a remarkable galaxy at z = 9.25, dubbed extremely blue galaxy 1 (EBG-1), with a significantly blue UV slope β = −2.99 ± 0.15, unlike the rest of the galaxies that exhibit red continua or ambiguous blue continua hindered by large uncertainties. We confirm that the β value negligibly changes by the data reduction and fitting wavelength ranges for UV emission/absorption line masking. The extreme blue slope, β = −3.0, rules out significant contributions from dust extinction or AGN activity. Comparing with stellar and nebular emission models, we find that such a blue UV slope cannot be reproduced solely by stellar models even with very young, metal-poor, or top-heavy contiguous star formation associated with strong nebular continua making the UV slopes red, but with a high ionizing photon escape fraction, , for a weak nebular continuum. While the Hβ emission line is not detected, likely due to the limited sensitivity of the spectrum, we find moderately weak [O iii]λλ4959,5007 emission lines for the given star formation rate (3 M⊙ yr−1) and stellar mass (108.0M⊙) that are about 3 times weaker than the average emission lines, again suggestive of the high ionizing photon escape fraction, or more. EBG-1 would provide crucial insights into stellar and nebular continuum emission in high-redshift galaxies, serving as an example of the ionizing photon escaping site at the epoch of reionization.