Planck intermediate results. XV. A study of anomalous microwave emission
  in Galactic clouds
  Anomalous microwave emission (AME) is believed to be due to electric dipole
radiation from small spinning dust grains. The aim of this paper is a
statistical study of the basic properties of AME regions and the environment in
which they emit. We used WMAP and Planck maps, combined with ancillary radio
and IR data, to construct a sample of 98 candidate AME sources, assembling SEDs
for each source using aperture photometry on 1deg-smoothed maps from 0.408 GHz
up to 3000 GHz. Each spectrum is fitted with a simple model of free-free,
synchrotron (where necessary), cosmic microwave background (CMB), thermal dust,
and spinning dust components. We find that 42 of the 98 sources have
significant (>5sigma) excess emission at frequencies between 20 and 60 GHz. An
analysis of the potential contribution of optically thick free-free emission
from ultra-compact HII regions, using IR colour criteria, reduces the
significant AME sample to 27 regions. The spectrum of the AME is consistent
with model spectra of spinning dust. The AME regions tend to be more spatially
extended than regions with little or no AME. The AME intensity is strongly
correlated with the submillimetre/IR flux densities and comparable to previous
AME detections in the literature. AME emissivity, defined as the ratio of AME
to dust optical depth, varies by an order of magnitude for the AME regions. The
AME regions tend to be associated with cooler dust in the range 14-20 K and an
average emissivity index of +1.8, while the non-AME regions are typically
warmer, at 20-27 K. In agreement with previous studies, the AME emissivity
appears to decrease with increasing column density. The emerging picture is
that the bulk of the AME is coming from the polycyclic aromatic hydrocarbons
and small dust grains from the colder neutral interstellar medium phase
(Abridged).
