Extragalactic background Light: a measurement at 400 nm using dark cloud
  shadow II. Spectroscopic separation of 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 (Mattila et al. 2017a). The
topic of the present 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
semi-transparent positions. The resulting EBL intensity at 400 nm is $I_{\rm
EBL} = 2.9\pm1.1$ $10^{-9}$ erg cm$^{-2}$s$^{-1}$sr$^{-1}$\AA$^{-1}$, or
$11.6\pm4.4$ nW m$^{-2}$sr$^{-1}$, which represents a 2.6$\sigma$ detection;
the scaling uncertainty is +20%/-16%. At 520 nm we have set a 2$\sigma$ upper
limit of $I_{\rm EBL} \le$4.5 $10^{-9}$ erg
cm$^{-2}$s$^{-1}$sr$^{-1}$\AA$^{-1}$ or $\le$23.4 nW m$^{-2}$sr$^{-1}$
+20%/-16%. Our EBL value at 400 nm is $\ge 2$ 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.
