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

Abstract

We used New Horizons LORRI images to measure the optical-band (0.4 λ 0.9µm) 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 µm 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 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 µm. 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.a The NSF's OIR Lab is operated by AURA, Inc. under cooperative agreement with NSF.

Night Sky Brightness Measurement, Quality Assessment and Monitoring

“…h Lauer et al [55] report "a flux component of unknown origin" of 8.06±1.92 nW m −2 sr −1 in New Horizons Long-range Reconnaissance Imager (LORRI) measurements at λ = 0.608 µm after subtracting the estimated contribution from the integrated light of external galaxies. dark nighttime conditions, which we reproduce here in an abbreviated form in Table 3.…”

The cosmic optical background excess, dark matter, and line-intensity mapping

“…This makes NASA's New Horizons spacecraft an extraordinary platform to measure the COB, as demonstrated by analyses using archival images of the New Horizons' Long Range Reconnaisance Imager (LORRI) [8,9]. The New Horizons' LORRI has recently provided, using targeted observations taken at 51.3 AU from the Sun, the first high signal-to-noise detection of the COB, yielding a flux of photons with wavelengths ∼ 0.4 − 0.9 µm (∼ 1.3 − 3 eV) of 16.37 ± 1.47 nW/m 2 /sr [10]. This measurement, obtained after subtracting contributions from diffuse Galactic light, scattered light from stars and galaxies outside the LORRI field, faint stars below the detection limit, hydrogen and ionized helium twophoton continua, and foregrounds from the spacecraft, exceeds the flux expected from deep Hubble Space Telescope galaxy counts by 8.06 ± 1.92 nW/m 2 /sr [10]; this is roughly a factor-of-two excess, with 4σ significance.…”

“…The New Horizons' LORRI has recently provided, using targeted observations taken at 51.3 AU from the Sun, the first high signal-to-noise detection of the COB, yielding a flux of photons with wavelengths ∼ 0.4 − 0.9 µm (∼ 1.3 − 3 eV) of 16.37 ± 1.47 nW/m 2 /sr [10]. This measurement, obtained after subtracting contributions from diffuse Galactic light, scattered light from stars and galaxies outside the LORRI field, faint stars below the detection limit, hydrogen and ionized helium twophoton continua, and foregrounds from the spacecraft, exceeds the flux expected from deep Hubble Space Telescope galaxy counts by 8.06 ± 1.92 nW/m 2 /sr [10]; this is roughly a factor-of-two excess, with 4σ significance. Possible astrophysical explanations include a faint population of galaxies not accounted for in the prediction from deep counts of HST [11], light from stars tidally removed from galaxies, a population of faint sources within extended halos (intrahalo light) [12][13][14], or direct-collapse black holes at very high redshift [15].…”