THE PECULIAR COLORS OF THE HALO LIGHT IN THE EDGE-ON SPIRAL GALAXY NGC5907

The presence of substantial haloes of 'dark matter' around galaxies ha s been inferred from their gravitational influence on the gas and star s in their disks', but the nature of the dark matter remains very unce rtain. The recent detection of faint optical emission from the halo ar ound the edge-on galaxy NGC5907 (refs 2-5), distributed in a manner th at follows the expected distribution of the gravitational mass, provid ed the first direct indication that faint stars might be the repositor y of some dark matter. But it was not clear how much of the mass was p rovided by these intrinsically faint stars. Here we report near-infrar ed observations of the halo emission from this galaxy. Taken together with the optical data, the results produce a very peculiar spectral en ergy distribution, which cannot be explained by any current models of stellar populations. The best approximation is a collection of stars w ith near-solar abundances of heavy elements, along with many low-mass stars. Such a population would be very unexpected for a galactic halo, where the stars should be old and have rather low fractions of heavy elements, but could account for much of the gravitational mass.