New Synoptic Observations of the Cosmic Optical Background with New Horizons (2407.06273v3)

Abstract: We obtained New Horizons LORRI images to measure the cosmic optical background (COB) intensity integrated over $0.4\lesssim\lambda\lesssim0.9{~\rm\mu m}.$ The survey comprises 16 high Galactic-latitude fields selected to minimize scattered diffuse Galactic light (DGL) from the Milky Way galaxy, as well as scattered light from bright stars. This work supersedes an earlier analysis based on observations of one of the present fields. Isolating the COB contribution to the raw total sky levels measured in the fields requires subtracting the remaining scattered light from bright stars and galaxies, intensity from faint stars within the fields fainter than the photometric detection-limit, and the DGL foreground. DGL is estimated from Planck HFI $350 {~\rm\mu m}$ and $550 {~\rm\mu m}$ intensities, using a new self-calibrated indicator based on the 16 fields augmented with eight additional DGL calibration fields obtained as part of the survey. The survey yields a highly significant detection ($6.8\sigma$) of the COB at ${\rm 11.16\pm 1.65~(1.47~sys,~0.75~ran) ~nW ~m^{-2} ~sr^{-1}}$ at the LORRI pivot wavelength of 0.608 $\mu$m. The estimated integrated intensity from background galaxies, ${\rm 8.17\pm 1.18 ~nW ~m^{-2} ~sr^{-1}},$ can account for the great majority of this signal. The rest of the COB signal, ${\rm 2.99\pm2.03~ (1.75~sys,~1.03~ran) ~nW ~m^{-2} ~sr^{-1}},$ is formally classified as anomalous intensity but is not significantly different from zero. The simplest interpretation is that the COB is completely due to galaxies.

• The paper introduces a robust analysis of the cosmic optical background using LORRI data from New Horizons, minimizing zodiacal light and foreground contamination.
• The paper employs a novel self-calibrated DGL indicator with Planck HFI data to significantly reduce errors in background light estimates.
• The paper finds that the COB intensity is primarily due to integrated galaxy light, with only a minor, statistically uncertain excess component.

Synoptic Observations and Measurements of the Cosmic Optical Background with New Horizons

The paper presented by Postman, Lauer, Parker et al. introduces a comprehensive analysis of the cosmic optical background (COB) based on observations from the Long-Range Reconnaissance Imager (LORRI) on NASA's New Horizons spacecraft. Situated beyond the heliocentric distance of 50 AU, New Horizons offers a vantage devoid of zodiacal light interference, providing a unique opportunity to measure the COB with reduced foreground contamination.

Methodological Framework

The paper is founded on a robust observational strategy targeting 16 high Galactic latitude fields, intentionally selected to minimize scattered light contributions from diffuse Galactic light (DGL) and stellar backgrounds. This work extends previous efforts, employing an analytical framework that systematically disentangles various light components contributing to the raw sky brightness measured by LORRI.

The authors improve upon earlier COB estimates by introducing a novel self-calibrated DGL indicator leveraging Planck High Frequency Instrument (HFI) data at 350 and 550 microns. This approach yields an empirical relationship that provides a refined estimate of the DGL contribution, significantly reducing errors compared to previous models.

Key Results

The paper reports a highly significant detection of the COB with an intensity of $11.16 \pm 1.65 \, \text{nW m}^{-2} \text{sr}^{-1}$ at a LORRI pivot wavelength of 0.608 microns. A substantial portion of this signal is attributable to the integrated intensity from background galaxies, estimated at $8.17 \pm 1.18 \, \text{nW m}^{-2} \text{sr}^{-1}$, supporting the hypothesis that the majority of the COB is due to known galactic populations.

Further, the analysis posits an anomalous intensity component of $2.99 \pm 2.03 \, \text{nW m}^{-2} \text{sr}^{-1}$, which, although not statistically significant from zero, indicates the possible presence of other light sources not accounted for by current galaxy models. However, the data align with the simplest interpretation that the COB is primarily due to the known population of galaxies.

Implications and Future Prospects

This research contributes to the ongoing discourse regarding the sources and composition of the COB. The alignment of the measured COB with the integrated galaxy light suggests that extragalactic light contributions from known structures might be sufficient to account for the observed background intensities, challenging previous assertions of substantial unexplained components such as intrahalo light or low surface brightness galaxies.

The findings underscore the necessity for further investigation into both the faint-end of galaxy counts and the role of previously underexplored sources in contributing to the COB. As cosmological simulations and observational datasets advance, incorporating higher resolution data and extended wavelength coverage, they will be instrumental in refining these estimates.

In the broader context of understanding diffuse extragalactic light, this paper exemplifies the efficacy of leveraging strategic observational platforms combined with advanced analytical techniques to unveil the mysteries of the cosmic optical background. Future missions that can replicate or extend the methodologies employed here would further elucidate the contributions to the COB, potentially revisiting the role of large-scale structures and intergalactic medium emissions.