Planck 2015 results. XXV. Diffuse low-frequency Galactic foregrounds
  (abridged) We discuss the Galactic foreground emission between 20 and 100GHz
based on observations by Planck/WMAP. The Commander component-separation tool
has been used to separate the various astrophysical processes in total
intensity. Comparison with RRL templates verifies the recovery of the free-free
emission along the Galactic plane. Comparison of the high-latitude Halpha
emission with our free-free map shows residuals that correlate with dust
optical depth, consistent with a fraction (~30%) of Halpha having been
scattered by high-latitude dust. We highlight a number of diffuse spinning dust
morphological features at high latitude. There is substantial spatial variation
in the spinning dust spectrum, with the emission peak ranging from below 20GHz
to more than 50GHz. There is a strong tendency for the spinning dust component
near many prominent HII regions to have a higher peak frequency, suggesting
that this increase in peak frequency is associated with dust in the
photodissociation regions around the nebulae. The emissivity of spinning dust
in these diffuse regions is of the same order as previous detections in the
literature. Over the entire sky, the commander solution finds more anomalous
microwave emission than the WMAP component maps, at the expense of synchrotron
and free-free emission. This can be explained by the difficulty in separating
multiple broadband components with a limited number of frequency maps. Future
surveys (5-20GHz), will greatly improve the separation by constraining the
synchrotron spectrum. We combine Planck/WMAP data to make the highest S/N ratio
maps yet of the intensity of the all-sky polarized synchrotron emission at
frequencies above a few GHz. Most of the high-latitude polarized emission is
associated with distinct large-scale loops and spurs, and we re-discuss their
structure...
