Infrared-correlated 31-GHz radio emission from Orion East

Abstract

Lynds dark cloud LDN1622 represents one of the best examples of anomalous dust emission, possibly originating from small spinning dust grains. We present Cosmic Background Imager (CBI) 31-GHz data of LDN1621, a diffuse dark cloud to the north of LDN1622 in a region known as Orion East. A broken ring-like structure with diameter approximate to 20 arcmin of diffuse emission is detected at 31 GHz, at approximate to 20-30 mJy beam-1 with an angular resolution of approximate to 5 arcmin. The ring-like structure is highly correlated with far-infrared (FIR) emission at 12-100 mu m with correlation coefficients of r approximate to 0.7-0.8, significant at similar to 10 Sigma. The FIR-correlated emission at 31 GHz therefore appears to be mostly due to radiation associated with dust. Multifrequency data are used to place constraints on other components of emission that could be contributing to the 31-GHz flux. An analysis of the GB6 survey maps at 4.85 GHz yields a 3 Sigma upper limit on free-free emission of 7.2 mJy beam-1 (less than or similar to 30 per cent of the observed flux) at the CBI resolution. The bulk of the 31-GHz flux therefore appears to be mostly due to dust radiation. Aperture photometry, at an angular resolution of 13 arcmin and with an aperture of diameter 30 arcmin, allowed the use of IRAS maps and the Wilkinson Microwave Anisotropy Probe 5-yr W-band map at 93.5 GHz. A single modified blackbody model was fitted to the data to estimate the contribution from thermal dust, which amounts to similar to 10 per cent at 31 GHz. In this model, an excess of 1.52 +/- 0.66 Jy (2.3 Sigma) is seen at 31 GHz. Correlations with the IRAS 100 mu m gave a coupling coefficient of 18.1 +/- 4.4 mu K (MJy sr-1)-1, consistent with the values found for LDN1622.