A Measurement of the Cosmic Optical Background and Diffuse Galactic Light Scaling from the R < 50 AU New Horizons-LORRI Data

TL;DR

This paper reports a measurement of the cosmic optical background using New Horizons data, accounting for foregrounds and systematics, revealing more diffuse light than expected from galaxy counts.

Contribution

First measurement of the cosmic optical background at 47 AU using a blind analysis with New Horizons-LORRI data, addressing foregrounds and instrumental systematics.

Findings

COB surface brightness measured at 21.98 ± 1.23 (stat.) ± 1.36 (cal.) nW/m²/sr

Results indicate more diffuse light than galaxy counts predict

Supports the existence of additional diffuse emission sources

Abstract

Direct photometric measurements of the cosmic optical background (COB) provide an important point of comparison to both other measurement methodologies and models of cosmic structure formation, and permit a cosmic consistency test with the potential to reveal additional diffuse sources of emission. The COB has been challenging to measure from Earth due to the difficulty of isolating it from the diffuse light scattered from interplanetary dust in our solar system. We present a measurement of the COB using data taken by the Long-Range Reconnaissance Imager (LORRI) on NASA's New Horizons mission, considering all data acquired to 47 AU. We employ a blind methodology where our analysis choices are developed against a subset of the full data set, which is then unblinded. Dark current and other instrumental systematics are accounted for, including a number of sources of scattered light. We fully characterize and remove structured and diffuse astrophysical foregrounds including bright stars, the integrated starlight from faint unresolved sources, and diffuse galactic light. For the full data set, we find the surface brightness of the COB to be $\lambda I_{\lambda}^{\mathrm{COB}}$ $=$ 21.98 $\pm$ 1.23 (stat.) $\pm$ 1.36 (cal.) nW m$^{-2}$ sr$^{-1}$. This result supports recent determinations that find a factor of 2 ${-}$ 3 $\times$ more light than expected from the integrated light from galaxies and motivate new diffuse intensity measurements with more capable instruments that can support spectral measurements over the optical and near-IR.