\rm deg^2$ subset of Stripe 82 that has the lowest level of Galactic dust emission (or cirrus) foregrounds (i.e., with $N_{\rm H}\lesssim 2\times 10^{21}\, \rm \, cm^{-2}$).

The SPIRE data, obtained from the *Herschel* Science Archive, were reduced using the standard ESA software and the custom-made software package, [SMAP]{} (Levenson et al. 2010, Viero et al. 2013b). Maps were made using an updated version of SMAP/SHIM, which is an iterative map-maker designed to optimally separate large-scale noise from signal. For more details, please refer to Viero et al. (2013b). The maps have a tangent plane (TAN) projection with pixel sizes of 6$\arcsec$, 8.33$\arcsec$ and 12$\arcsec$ at 250, 350, and 500 $\micron$, respectively. In comparison, the full width at half maximum (FWHM) of the SPIRE beams are 18.1$\arcsec$, 25.2$\arcsec$ and 36.6$\arcsec$ at 250, 350, and 500 $\micron$, respectively (Swinyard et al. 2010). The SPIRE maps are converted from their native unit of $\rm Jy\, beam^{-1}$ to $\rm Jy\, sr^{-1}$ by dividing them by the effective beam areas which are 0.9942, 1.765, and 3.730 $\times 10^{-8}$steradians at 250, 350, and 500 $\micron$, respectively (SPIRE Observers’ Manual[^5]). We note that no colour corrections from a flat-spectrum point-source calibration are applied, as they were 