2006/07/27 by Monica Ivette Rodriguez, Ronald J. Allen, Laurent Loinard +1
Physics and Astronomy · #Absorption (acoustics) #Absorption spectroscopy #Astrophysics and Star Formation Studies #Cosmic microwave background #Cosmology and Gravitation Theories #Emission spectrum #Galaxies: Formation, Evolution, Phenomena #Line (geometry) #Microwave #Molecular cloud #Radio telescope #Spectral line #Telescope #astro-ph
paper · pdf · doi:10.1086/507984
published as Astrophys.J.652:1230-1239,2006 · 29 pages (in one-column aastex format), 9 figures, ApJ: Accepted
arxiv created 2006/07/27 · openalex publication_date 2006/11/27 · arxiv updated 2009/12/01 · openalex created_date 2019/06/27 · openalex updated_date 2026/08/05
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 2 CO). 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 2 CO 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 2 CO 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 2 CO 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 (10 3 cm -3 < n < 10 5 cm -3 ) condensations in molecular clouds, the CO(1-0) line is expected to be optically thick, whereas the H 2 CO line is not. This latter difference is likely to be responsible for a significant part of the scatter in the correlation we have found.