Dark haloes of spiral galaxies:: ISO photometry

We exclude hydrogen-burning stars, of any mass above the hydrogen-burning limit and any metallicity, as significant contributors to the massive haloes deduced from rotation curves to dominate the outer parts of spiral galaxies. We present and analyse images of four nearly edge-on bulgeless spiral galaxies I(UGC 711, NGC 2915, UGC 12426, UGC 1459) obtained with ISOCAM (The CAMera instrument on board the Infrared Space Observatory) at 14.5 and 6.75 mu m. Our sensitivity limit for detection of any diffuse infrared emission associated with the dark haloes in these galaxies is a few tens of mu Jy per 6 x 6 arcsec(2) pixel, with this limit currently set by remaining difficulties in modelling the non-linear behaviour of the detectors. All four galaxies show zero detected signal from extended non-disc emission, consistent with zero halo-like luminosity density distribution. The 95 per cent upper limit on any emission, for NCC 2915 in particular, allows us to exclude very low mass main-sequence stars (M > 0.08 M-circle dot) and young brown dwarfs (less than or similar to 1 Gyr) as significant contributors to dark matter in galactic haloes. Combining our results with those of the Galactic microlensing surveys, which exclude objects with M < 0.01 M-circle dot, excludes almost the entire possible mass range of compact baryonic objects from contributing to Galactic dark matter.