Extragalactic background light: a measurement at 400 nm using dark cloud shadow - II. Spectroscopic separation of the dark cloud's light, and results

In a project aimed at measuring the optical extragalactic background light (EBL), we are using the shadow of a dark cloud. We have performed, with the ESO VLT/FORS, spectrophotometry of the surface brightness towards the high-galactic-latitude dark cloud Lynds 1642. A spectrum representing the difference between the opaque core of the cloud and several unobscured positions around the cloud was presented in Paper I. The topic of this paper is the separation of the scattered starlight from the dark cloud itself which is the only remaining foreground component in this difference. While the scattered starlight spectrum has the characteristic Fraunhofer lines and the discontinuity at 400 nm, typical of integrated light of galaxies, the EBL spectrum is a smooth one without these features. As template for the scattered starlight, we make use of the spectra at two semitransparent positions. The resulting EBL intensity at 400 nm is I-EBL = 2.9 +/- 1.1 10(-9) erg cm(-2) s(-1) sr(-1) angstrom(-1) or 11.6 +/- 4.4 nW m(-2) sr(-1), which represents a 2.6 sigma detection; the scaling uncertainty is +20 per cent/-16 per cent. At 520 nm, we have set a 2 sigma upper limit of I-EBL <= 4.5 10(-9) erg cm(-2) s(-1) sr(-1) angstrom(-1) or = 23.4 nW m(-2) sr(-1) + 20 per cent/-16 per cent. Our EBL value at 400 nm is greater than or similar to 2 times as high as the integrated light of galaxies. No known diffuse light sources, such as light from Milky Way halo, intra-cluster or intra-group stars appear capable of explaining the observed EBL excess over the integrated light of galaxies.