Dust-correlated centimetre-wave radiation from the M 78 reflection
  nebula
  An anomalous radio continuum component at cm-wavelengths has been observed in
various sources, including dark clouds. This continuum component represents a
new property of the ISM. In this work we focus on one particular dark cloud,
the bright reflection nebula M 78. The main goal of this work is to invetigate
cm-wave continuum emission in a prominent molecular cloud, nearby and with
complementary observational data. We acquired Cosmic Background Imager (CBI)
visibility data of M 78 at 31 GHz with an angular resolution of $\sim
5.8\arcmin$ and CBI2 data at an angular resolution of $\sim 4.2\arcmin$. A
morphological analysis was undertaken to search for possible correlations with
templates that trace different emission mechanisms. Using data from WMAP and
the Rhodes/HartRAO 2326 MHz survey we constructed the spectral energy
distribution (SED) of M 78 in a $45\arcmin$ circular aperture. We used results
from the literature to constrain the physical conditions and the stellar
content. The 5 GHz -- 31 GHz spectral index in flux density ($\alpha = 1.89\pm
0.15$) is significantly different from optically thin free-free values. We also
find closer morphological agreement with IR dust tracers than with free-free
sources. Dust-correlated cm-wave emission that is not due to free-free is
significant at small scales ($\sim 8\arcmin$). However, a free-free background
dominates at cm-wavelengths on large scales ($\sim 1$ deg). We correct for this
uniform background by differencing against a set of reference fields. The
differenced SED of M 78 shows excess emission at 10-70 GHz over free-free and a
modified blackbody, at $3.4\sigma$. The excess is matched by the spinning dust
model from Draine and Lazarian (1998).
