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 investigate the 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 ( CBI resolutions ) .
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 & Lazarian ( 21 ) .