Planck 2015 results. X. Diffuse component separation: Foreground maps
  Planck has mapped the microwave sky in nine frequency bands between 30 and
857 GHz in temperature and seven bands between 30 and 353 GHz in polarization.
In this paper we consider the problem of diffuse astrophysical component
separation, and process these maps within a Bayesian framework to derive a
consistent set of full-sky astrophysical component maps. For the temperature
analysis, we combine the Planck observations with the 9-year WMAP sky maps and
the Haslam et al. 408 MHz map to derive a joint model of CMB, synchrotron,
free-free, spinning dust, CO, line emission in the 94 and 100 GHz channels, and
thermal dust emission. Full-sky maps are provided with angular resolutions
varying between 7.5 arcmin and 1 deg. Global parameters (monopoles, dipoles,
relative calibration, and bandpass errors) are fitted jointly with the sky
model, and best-fit values are tabulated. For polarization, the model includes
CMB, synchrotron, and thermal dust emission. These models provide excellent
fits to the observed data, with rms temperature residuals smaller than 4 uK
over 93% of the sky for all Planck frequencies up to 353 GHz, and fractional
errors smaller than 1% in the remaining 7% of the sky. The main limitations of
the temperature model at the lower frequencies are degeneracies among the
spinning dust, free-free, and synchrotron components; additional observations
from external low-frequency experiments will be essential to break these. The
main limitations of the temperature model at the higher frequencies are
uncertainties in the 545 and 857 GHz calibration and zero-points. For
polarization, the main outstanding issues are instrumental systematics in the
100-353 GHz bands on large angular scales in the form of
temperature-to-polarization leakage, uncertainties in the analog-to-digital
conversion, and very long time constant corrections, all of which are expected
to improve in the near future.
