Anomalous H₂CO Absorption Towards the Galactic Anticenter: A Blind Search for Dense Molecular Clouds

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 (H_2CO). 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 H_2CO 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 H_2CO 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 H_2CO 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 > 10K, dense, 10^3 < n < 10^5 cm^{-3} condensations in molecular clouds, the CO(1-0) line is expected to be optically thick, whereas the H_2CO line is not. This latter difference is likely to be responsible for a significant part of the scatter in the correlation we have found.