Anomalous Flux in the Cosmic Optical Background Detected with New Horizons Observations

We used New Horizons LORRI images to measure the optical-band (0.4 less than or similar to lambda less than or similar to 0.9 mu m) sky brightness within a high-galactic-latitude field selected to have reduced diffuse scattered light from the Milky Way galaxy (DGL), as inferred from the IRIS all-sky 100 mu m map. We also selected the field to significantly reduce the scattered light from bright stars (SSL) outside the LORRI field. Suppression of DGL and SSL reduced the large uncertainties in the background flux levels present in our earlier New Horizons cosmic optical background (COB) results. The raw total sky level, measured when New Horizons was 51.3 au from the Sun, is 24.22 +/- 0.80 nW m(-2) sr(-1). Isolating the COB contribution to the raw total required subtracting scattered light from bright stars and galaxies, faint stars below the photometric detection limit within the field, and the hydrogen plus ionized-helium two-photon continua. This yielded a highly significant detection of the COB at 16.37 +/- 1.47 nW m(-2) sr(-1) at the LORRI pivot wavelength of 0.608 mu m. This result is in strong tension with the hypothesis that the COB only comprises the integrated light of external galaxies (IGL) presently known from deep HST counts. Subtraction of the estimated IGL flux from the total COB level leaves a flux component of unknown origin at 8.06 +/- 1.92 nW m(-2) sr(-1). Its amplitude is equal to the IGL.