2006ApJ...652.1230R

We have carried out a blind search in the general direction of the Galactic anticenter for absorption of the cosmic microwave background (CMB) radiation near 4.83 GHz by molecular clouds containing gaseous ortho-formaldehyde (H₂CO). The observations were done using the 25 m radio telescope at Onsala in Sweden and covered strips in Galactic latitude -1°≤b≤+1° at several longitudes in the region 170°≤l≤190°. Spectra were obtained in these strips with a grid spacing corresponding to the telescope resolution of 10'. We have detected H₂CO CMB absorption at ~10% of the survey pointings. This detection rate is likely to increase with further improvements in sensitivity and may become comparable to the detection rate expected from a blind CO survey with a corresponding sensitivity limit. We have mapped some of these detections in more detail and compared the H₂CO absorption to existing maps of CO(1-0) emission in the same regions. There appears to be a rough correlation between the velocity-integrated line strength of the CO(1-0) emission and that of the H₂CO absorption. However, the scatter in this correlation is significantly larger than the measurement errors, indicating differences of detail at and below the linear resolution of our observations (~4-9 pc). Although these two tracers are expected to have similar excitation requirements on the microscopic level characteristic of warm (T_K>10 K) dense (10³/cm3<n<10⁵/cm3) condensations in molecular clouds, the CO(1-0) line is expected to be optically thick, whereas the H₂CO line is not. This latter difference is likely to be responsible for a significant part of the scatter in the correlation we have found.