THE SPECTRUM OF THE DIFFUSE GALACTIC LIGHT: 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 mu m intensity independently measured by the Cosmic Background Explorer and Infrared Astronomy 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. The strength of [N II] and [S II], combined with upper limits on [O III] and He I, indicates 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%.