Fourier-space combination of Planck and Herschel images
  Herschel has revolutionized our ability to measure column densities (N$_{\rm
H}$) and temperatures (T) of molecular clouds thanks to its far infrared
multiwavelength coverage. However, the lack of a well defined background
intensity level in the Herschel data limits the accuracy of the N$_{\rm H}$ and
T maps. We provide a method that corrects the missing Herschel background
intensity levels using the Planck model for foreground Galactic thermal dust
emission. We present a Fourier method that combines the publicly available
Planck model on large angular scales with the Herschel images on smaller
angular scales. We apply our method to two regions spanning a range of Galactic
environments: Perseus and the Galactic plane region around $l = 11\deg$
(HiGal--11). We post-process the combined dust continuum emission images to
generate column density and temperature maps. We compare these to previously
adopted constant--offset corrections. We find significant differences
($\gtrsim$20\%) over significant ($\sim$15\%) areas of the maps, at low column
densities ($N_{\rm H}\lesssim10^{22}$\,cm$^{-2}$) and relatively high
temperatures ($T\gtrsim20$\,K). We also apply our method to synthetic
observations of a simulated molecular cloud to validate our method. Our method
successfully corrects the Herschel images, including both the constant--offset
intensity level and the scale-dependent background variations measured by
Planck. Our method improves the previous constant--offset corrections, which
did not account for variations in the background emission levels.
