Far-reaching Dust Distribution in Galaxy Disks
  In most studies of dust in galaxies, dust is only detected from its emission
to approximately the optical radius of the galaxy. By combining the signal of
110 spiral galaxies observed as part of the Herschel Reference Survey, we are
able to improve our sensitivity by an order-of-magnitude over that for a single
object. Here we report the direct detection of dust from its emission that
extends out to at least twice the optical radius. We find that the distribution
of dust is consistent with an exponential at all radii with a gradient of ~-1.7
dex R$_{25}^{-1}$. Our dust temperature declines linearly from ~25 K in the
centre to 15 K at R$_{25}$ from where it remains constant out to ~2.0 R$_{25}$.
The surface-density of dust declines with radius at a similar rate to the
surface-density of stars but more slowly than the surface-density of the
star-formation rate. Studies based on dust extinction and reddening of
high-redshift quasars have concluded that there are substantial amounts of dust
in intergalactic space. By combining our results with the number counts and
angular correlation function from the SDSS, we show that with Milky Way type
dust we can explain the reddening of the quasars by the dust within galactic
disks alone. Given the uncertainties in the properties of any intergalactic
dust, we cannot rule out its existence, but our results show that statistical
investigations of the dust in galactic halos that use the reddening of
high-redshift objects must take account of the dust in galactic disks.
