SKYSURF IX – the cosmic optical background from integrated galaxy light measurements

ABSTRACT As part of the SKYSURF Hubble Space Telescope (HST) Legacy Archival programme we present galaxy number counts which yield measurements of the extragalactic background light (EBL) at 15 different wavelengths. We have processed 82,752 HST images across 23 filters into 16 686 mosaics using the same software and processing pipeline throughout. Using 17/23 filters that give reliable galaxy counts, we constrain the integrated galaxy light (IGL) with a 1.4–9 per cent error between 0.3 and 1.6 $\mu$m in combination with 8 bands from WAVES (Wide Area VISTA Extragalactic Survey) and DEVILS (Deep Extragalactic Visible Legacy Survey). While HST was never intended to undertake large area surveys, through extensive quality control and filtering, we were able to extract a reliable and representative sample of fields distributed across the sky. Our final catalogues cover a combined $\approx 19.6 \, \mathrm{deg}^2$, with individual filters covering areas ranging from $\approx 0.16\hbox{-}7.0 \, \mathrm{deg}^2$. The combination of numerous independent sight-lines and area coverage allows us to reduce cosmic variance uncertainties in deep number counts to 0.21–1.8 per cent. For the first time we are able to establish a measurement of the IGL, $\mathrm{8.85 \pm 0.30 \, nW\, m^{-2} \, sr^{-1}}$, at 0.59 $\mu$m using HST data. We obtain a cosmic optical background value of $\mathrm{25.36 \pm 0.52 \, nW\, m^{-2} \, sr^{-1}}$. Different techniques used to measure the COB, both directly and indirectly, have recently converged indicating that the COB arises almost exclusively from processes within galaxies. This in combination with the recent values reported from New Horizons and very high energy (VHE) constraints leaves very little room for any diffuse emission coming from outside the Milky Way.