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 \sim - 1.7 dex R _ { 25 } ^ { -1 } .
Our dust temperature declines linearly from \sim 25 K in the centre to 15 K at R _ { 25 } from where it remains constant out to \sim 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 can not 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 .