A FAINT LUMINOUS HALO THAT MAY TRACE THE DARK-MATTER AROUND SPIRAL GALAXY NGC5907

THE presence of unseen haloes of 'dark matter' has long been inferred from the high rotation speeds of gas and stars in the outer parts of s piral galaxies(1). The volume density of this dark matter decreases le ss quickly from the galactic centre than does that of the luminous mas s (such as that in stars), meaning that the dark matter dominates the mass far from the centre(1,2). While searching for faint starlight awa y from the plane of the edge-on spiral galaxy NGC5907 (ref. 3), we hav e found that the galaxy is surrounded by a faint luminous halo. The in tensity of light from this halo falls less steeply than any known lumi nous component of spiral galaxies, but is consistent with the distribu tion of dark mass inferred from the galaxy's rotation curve.