PRELIMINARY EVIDENCE FOR DUST IN GALACTIC HALOS

I use B and I photometry of distant galaxies observed through the halo s of two nearby spiral galaxies, NGC 2835 and NGC 3521, to test for th e existence of dust in galactic halos out to at least a radius of 60 k pc. B-I colors of background galaxies in fields centered along the nea rby galaxy's major axis at a projected separation of 60 kpc are compar ed to the colors of background galaxies in fields centered at a projec ted separation of about 220 kpc. Background galaxies in the fields at smaller projected separations have average B-I colors that are 0.067+/ -0.033 mag redder than those in the outer fields. Such a result will o ccur by chance less than 2.2% of the time and so is taken as prelimina ry evidence that galaxy halos contain more dust at galactic radii of 6 0 than 220 kpc. A model which assumes that the radial dust profile in galactic disks and halos is a single exponential has a corresponding s cale length of 31+/-8 kpc. I argue on the basis of the IRAS 100 mu m f lux maps that absorption by Galactic dust is not responsible for this result and briefly discuss connections between this result and other o bservations of the baryonic component of galaxy halos.