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

Abstract

In a project aimed at measuring the optical Extragalactic Background Light\n(EBL) we are using the shadow of a dark cloud.We have performed, with the ESO\nVLT/FORS, spectrophotometry of the surface brightness towards the\nhigh-galactic-latitude dark cloud Lynds 1642. A spectrum representing the\ndifference between the opaque core of the cloud and several unobscured\npositions around the cloud was presented in Paper I (Mattila et al. 2017a). The\ntopic of the present paper is the separation of the scattered starlight from\nthe dark cloud itself which is the only remaining foreground component in this\ndifference. While the scattered starlight spectrum has the characteristic\nFraunhofer lines and the discontinuity at 400 nm, typical of integrated light\nof galaxies, the EBL spectrum is a smooth one without these features. As\ntemplate for the scattered starlight we make use of the spectra at two\nsemi-transparent positions. The resulting EBL intensity at 400 nm is $I_{\\rm\nEBL} = 2.9\\pm1.1$ $10^{-9}$ erg cm$^{-2}$s$^{-1}$sr$^{-1}$\\AA$^{-1}$, or\n$11.6\\pm4.4$ nW m$^{-2}$sr$^{-1}$, which represents a 2.6$\\sigma$ detection;\nthe scaling uncertainty is +20%/-16%. At 520 nm we have set a 2$\\sigma$ upper\nlimit of $I_{\\rm EBL} \\le$4.5 $10^{-9}$ erg\ncm$^{-2}$s$^{-1}$sr$^{-1}$\\AA$^{-1}$ or $\\le$23.4 nW m$^{-2}$sr$^{-1}$\n+20%/-16%. Our EBL value at 400 nm is $\\ge 2$ times as high as the integrated\nlight of galaxies. No known diffuse light sources, such as light from Milky Way\nhalo, intra-cluster or intra-group stars appear capable of explaining the\nobserved EBL excess over the integrated light of galaxies.\n