Dust-correlated centimetre-wave radiation from the M78 reflection nebula

Abstract

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 re ection 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 5:80, and CBI2 data at an angular resolution of 4:20. A morphological analysis was undertaken to search for possible correlations with templates that trace di erent emission mechanisms. Using data from WMAP and the Rhodes/HartRAO 2326 MHz survey we constructed the spectral en- ergy distribution (SED) of M 78 in a 450 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 ux density ( = 1:89 0:15) is signi cantly di erent from optically thin-free-free values. We also nd 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 signi cant at small scales (CBI resolutions). However, a free-free background dominates at cm-wavelengths on large scales ( 1 deg). We correct for this uniform background by di erencing against a set of reference elds. The di er- enced SED of M 78 shows excess emission at 10-70 GHz over free-free and a modi ed blackbody, at 3.4 . The excess is matched by the spinning dust model from Draine & Lazarian (1998).