New Horizons Observations of the Cosmic Optical Background (2011.03052v2)

Abstract: We used existing data from the New Horizons LORRI camera to measure the optical-band ($0.4\lesssim\lambda\lesssim0.9{\rm\mu m}$) sky brightness within seven high galactic latitude fields. The average raw level measured while New Horizons was 42 to 45 AU from the Sun is $33.2\pm0.5{\rm ~nW ~m^{-2} ~sr^{-1}}.$ This is $\sim10\times$ darker than the darkest sky accessible to the {\it Hubble Space Telescope}, highlighting the utility of New Horizons for detecting the cosmic optical background (COB). Isolating the COB contribution to the raw total requires subtracting scattered light from bright stars and galaxies, faint stars below the photometric detection-limit within the fields, and diffuse Milky Way light scattered by infrared cirrus. We remove newly identified residual zodiacal light from the IRIS $100\mu$m all sky maps to generate two different estimates for the diffuse galactic light (DGL). Using these yields a highly significant detection of the COB in the range ${\rm 15.9\pm 4.2\ (1.8~stat., 3.7~sys.) ~nW ~m^{-2} ~sr^{-1}}$ to ${\rm 18.7\pm 3.8\ (1.8~stat., 3.3 ~sys.)~ nW ~m^{-2} ~sr^{-1}}$ at the LORRI pivot wavelength of 0.608 $\mu$m. Subtraction of the integrated light of galaxies (IGL) fainter than the photometric detection-limit from the total COB level leaves a diffuse flux component of unknown origin in the range ${\rm 8.8\pm4.9\ (1.8 ~stat., 4.5 ~sys.) ~nW ~m^{-2} ~sr^{-1}}$ to ${\rm 11.9\pm4.6\ (1.8 ~stat., 4.2 ~sys.) ~nW ~m^{-2} ~sr^{-1}}$. Explaining it with undetected galaxies requires the galaxy-count faint-end slope to steepen markedly at $V>24$ or that existing surveys are missing half the galaxies with $V< 30.$

• The paper presents novel COB measurements using New Horizons LORRI data from beyond 42 AU, achieving a raw sky brightness of 33.2 nW m⁻² sr⁻¹.
• It employs precise subtraction of known light sources and models for diffuse galactic light, isolating residual signals of 8.8 to 11.9 nW m⁻² sr⁻¹.
• These findings suggest missing faint galaxies or unidentified diffuse sources, challenging current extra-galactic light models and prompting further study.

Observations of the Cosmic Optical Background from Beyond the Kuiper Belt

The paper entitled "New Horizons Observations of the Cosmic Optical Background" by Lauer et al. presents a comprehensive analysis of the cosmic optical background (COB) using data from the New Horizons spacecraft. The spacecraft's Long-Range Reconnaissance Imager (LORRI) was used to capture images from distances beyond 42 AU, providing a unique vantage point free from the intense zodiacal light contamination prevalent within the inner solar system. This paper aims to refine measurements of the COB and investigate any diffuse component not directly linked to discrete sources such as galaxies.

The methodology centers on measuring the average sky brightness in seven high-galactic-latitude fields, achieving a raw average level of 33.2 nW m$^{-2}$ sr$^{-1}$, significantly darker than achievable from within the Earth’s vicinity. A crucial aspect of the analysis involves subtracting contributions from known sources, including resolved and unresolved stars, galaxies, and diffuse galactic light (DGL)—the latter modeled in part by infrared cirrus emission. The researchers also characterized and corrected for scattered starlight, leveraging an extensive star catalog and a known instrument point spread function (PSF).

The results yielded a COB detection in the range of about 15.9 nW m$^{-2}$ sr$^{-1}$ to 18.7 nW m$^{-2}$ sr$^{-1}$, with residual signals—potentially from a diffuse COB component—ranging from 8.8 nW m$^{-2}$ sr$^{-1}$ to 11.9 nW m$^{-2}$ sr$^{-1}$. The analysis intriguingly suggests the possibility of unidentified diffuse light sources or missing galaxy counts beyond current photometric catalog limits.

This research advances the astrophysical understanding of the COB, an integral marker for the cumulative light of the universe’s stars and galactic evolution. It also challenges models of light contributions from extra-galactic sources by indicating potential deficiencies, hypothesizing a significant population of faint galaxies missed by existing surveys or a substantial amount of light from processes or sources unaccounted for. Future COB studies will benefit from extending observations to more fields and exploring modifications to galaxy luminosity function models. Additionally, improving zodiacal light and DGL estimates remains crucial to refining COB measurements.

In conclusion, New Horizons’ out-of-the-solar-system vantage underscores the dynamic nature of COB quantification, challenging existing paradigms with findings that suggest either a more complex cosmic narrative or limitations in current observational capabilities. Continued exploration, both through observational datasets and theoretical advancements, is vital for unraveling these cosmic mysteries.