The Spectrum of the Diffuse Galactic Light I: The Milky Way in Scattered Light

We measure the optical spectrum of the Diffuse Galactic Light (DGL) -- the local Milky Way in reflection -- using 92,000 blank-sky spectra from the Sloan Digital Sky Survey (SDSS). We correlate the SDSS optical intensity in regions of blank sky against 100 micron intensity independently measured by the COsmic Background Explorer (COBE) and InfraRed Astronomy Satellite (IRAS) satellites, which provides a measure of the dust column density times the intensity of illuminating starlight. The spectrum of scattered light is very blue and shows a clear 4000 Angstrom break and broad Mg b absorption. This is consistent with scattered starlight, and the continuum of the DGL is well-reproduced by a simple radiative transfer model of the Galaxy. We also detect line emission in H\alpha, H\beta, [N II], and [S II], consistent with scattered light from the local interstellar medium (ISM). The strength of [N II] and [S II], combined with upper limits on [O III] and [He I], indicate a relatively soft ionizing spectrum. We find that our measurements of the DGL can constrain dust models, favoring a grain size distribution with relatively few large grains. We also estimate the fraction of high-latitude H\alpha which is scattered to be 19+/-4%.