Dark Halos of Spiral Galaxies: ISO photometry

Gerard Gilmore, M. Unavane

Published 1998-08-13Version 1

We exclude hydrogen burning stars, of any mass above the hydrogen-burning limit and any metallicity, as significant contributors to the massive halos deduced from rotation curves to dominate the outer parts of spiral galaxies. We present and analyse images of 4 nearly edge-on bulgeless spiral galaxies (UGC711, NGC2915, UGC12426, UGC1459) obtained with ISOCAM (The CAMera instrument on board the Infrared Space Observatory) at 14.5micron and 6.75micron. Our sensitivity limit for detection of any diffuse infrared emission associated with the dark halos in these galaxies is a few tens of microJy per 6arcsec x 6arcsec 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-disk emission, consistent with zero halo-like luminosity density distribution. The 95percent upper limit on any emission, for NGC2915 in particular, allows us to exclude very low mass main sequence stars (M > 0.08 Msun), and young brown dwarfs (< 1 Gyr) as significant contributors to dark matter in galactic halos. Combining our results with those of the Galactic microlensing surveys, which exclude objects with M < 0.01 Msun, excludes almost the entire possible mass range of compact baryonic objects from contributing to galactic dark matter.