  Far-infrared imaging surveys of Galactic star-forming regions with Herschel
have shown that a substantial part of the cold interstellar medium appears as a
fascinating web of omnipresent filamentary structures. This highly anisotropic
ingredient of the interstellar material further complicates the difficult
problem of the systematic detection and measurement of dense cores in the
strongly variable but (relatively) isotropic backgrounds. Observational
evidence that stars form in dense filaments creates severe problems for
automated source extraction methods that must reliably distinguish sources not
only from fluctuating backgrounds and noise, but also from the filamentary
structures. A previous paper presented the multi-scale, multi-wavelength source
extraction method getsources based on a fine spatial scale decomposition and
filtering of irrelevant scales from images. In this paper, a multi-scale,
multi-wavelength filament extraction method getfilaments is presented that
solves this problem, substantially improving the robustness of source
extraction with getsources in filamentary backgrounds. The main difference is
that the filaments extracted by getfilaments are now subtracted by getsources
from detection images during source extraction, greatly reducing the chances of
contaminating catalogs with spurious sources. The intimate physical
relationship between forming stars and filaments seen in Herschel observations
demands that accurate filament extraction methods must remove the contribution
of sources and that accurate source extraction methods must be able to remove
underlying filamentary structures. Source extraction with getsources now
provides researchers also with clean images of filaments, free of sources,
noise, and isotropic backgrounds.
