Implications of the New Horizons Detection of the Cosmic Optical Background

We used existing data from the New Horizons LORRI camera to measure the cosmic optical background (COB). After removing foregrounds and scattered light, we detect the COB signal at ≥3.8-sigma significance. When we subtract off the additional contribution expected for galaxies below the LORRI detection limit and down to V=30, we find a residual “diffuse COB” (dCOB) signal that is in the range 8.8 ± 4.9 (1.8 stat., 4.5 sys.) nW m^-2 sr^-1 to 11.9 ± 4.6 (1.8 stat., 4.2 sys.) nW m^-2 sr^-1. The surface brightness values corresponding to these dCOB intensities are 27.3 (+0.9, -0.5) mag arcsec^-2 and 27.0 (+0.5, -0.4) mag arcsec^-2, respectively, in the AB system computed at the LORRI pivot wavelength of 0.608 micron. The total error in our COB and dCOB measurements include the uncertainties in the determination of the faint-end slope of the galaxy counts and the effects of cosmic variance. To attribute this diffuse COB signal to undetected faint galaxies requires either that the faint-end slope of the galaxy count - magnitude relation would need to steepen markedly at V ≥ 24 or that the corrected galaxy counts from a variety of existing deep surveys are missing half the galaxies with V ≤ 30. Our diffuse COB signal in the optical passband of LORRI (0.4 - 0.9 micron) is consistent with constraints measured in the range 0.8 - 1.8 microns using NIR spectra of the sky obtained by the CIBER rocket instrument. One origin for such a diffuse COB could be faint compact objects at z ≤ 0.1. A key argument for sources at low redshift is accommodating the very high energy gamma-ray opacity limits on COB components at high redshift.