A direct detection of dust in the outer disks of nearby galaxies

We measure the extent of 100 μm galactic emission in two independent galaxy samples using the IRAS 100 μm Sky Survey images and constrain the distribution of dust at large (≲30 kpc) radii. The first sample consists of 90 nearby (v < 6000 km s^-1) galaxies from the Third Reference Catalogue of Bright Galaxies with similar angular sizes and absolute luminosities (5′ ≤ D₂₅ ≤ 10′ and -22.5 ≤ M_B ≤ -18) that are isolated in the 100 μm images. The second sample consists of 24 local galaxies (v < 1500 km s^-1, 10′ ≤ D₂₅ ≤ 30′). We rescale the 100 μm images of these galaxies using their optical diameters, D₂₅, rotate the images using their optical major-axis position angle, construct the mean and median images, and rebin the final images into polar coordinates to study the 100 μm emission as a function of radius and azimuthal angle. We find that the 100 μm emission extends at least to radii of 33 kpc (2 σ detection) for the typical galaxy in the 5′-10′ sample and to 21 kpc (2 σ detection) in the 10′-30′ sample (H₀ = 75 km s^-1 Mpc^-1). In both samples, the emission is preferentially elongated along the optical major axis. We fit an exponential to the 100 μm emission along the major axis and measure a scale length of 2.5 ± 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.