Anomalous H2CO Absorption toward 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 (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 (TK > 10 K) dense (103 cm-3 < n < 105 cm-3) 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.