A Direct Detection of Dust in the Outer Disks of Nearby Galaxies

We measure the extent of 100 micron galactic emission in two independent galaxy samples using the IRAS 100 micron Sky Survey images and constrain the distribution of dust at large ($\ltsim$30 kpc) radii. The first sample consists of 90 nearby (v < 6000 km/s) galaxies from the RC3 catalog with similar angular sizes and absolute luminosities (5 arcmin $\leq $D$_{25} \leq$ 10 arcmin and $-22.5 \leq $M$_{B} \leq -18$) that are isolated in the 100 micron images. The second sample consists of 24 local galaxies (v < 1500 km/s, 10 arcmin $\leq $D$_{25} \leq$ 30 arcmin). We rescale the 100 micron images of these galaxies using their optical diameters, D$_{25}$, rotate the images using their optical major axis position angle, construct the mean and median image, and rebin the final images into polar coordinates to study the 100 micron emission as a function of radius and azimuthal angle. We find that the 100 micron emission extends at least to radii of 27 kpc (2$\sigma$ detection) for the typical galaxy in the 5 arcmin - 10 arcmin sample and to 21 kpc (2$\sigma$ detection) in the 10 arcmin - 30 arcmin sample (H$_{0} = $75 km/s/Mpc). In both samples, the emission is preferentially elongated along the optical major axis. We fit an exponential to the 100 micron emission along the major axis and measure a scale length of $2.5 \pm$ 0.8 kpc (90% confidence interval). Using a simple model that relates the far-IR emission to the stellar distribution, we examine the range of acceptable dust mass distributions allowed by our data and conclude that the dust is more extended than the starlight.