Anomalous H_2CO Absorption Towards the Galactic Anticenter: A Blind Search for Dense Molecular Clouds

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dc.creatorRodriguez, Monica Ivette
dc.creatorAllen, Ronald J.
dc.creatorLoinard, Laurent
dc.creatorWiklind, Tommy
dc.date2006-07-27
dc.date.accessioned2026-07-07T10:39:40Z
dc.date.available2026-07-07T10:39:40Z
dc.descriptionWe 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.
dc.description29 pages (in one-column aastex format), 9 figures, ApJ: Accepted
dc.identifierhttps://arxiv.org/abs/astro-ph/0607616
dc.identifierhttp://arxiv.org/abs/astro-ph/0607616
dc.identifierAstrophys.J.652:1230-1239,2006
dc.identifierdoi:10.1086/507984
dc.identifier.urihttp://salesiana.dossiersoluciones.com/handle/123456789/181110
dc.subjectAstrophysics
dc.titleAnomalous H_2CO Absorption Towards the Galactic Anticenter: A Blind Search for Dense Molecular Clouds
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