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• W2041467376 abstract "view Abstract Citations (5) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radio discrete sources and galactic absorption at 21 CM wave length. Hagen, John P. ; McClain, E. F. ; Hepburn, Nannielou Abstract Observations have been made of twenty sources of 21 cm continuous radiation, and in several cases there is clear evidence of absorption of the neutral hydrogen radiation by the region identified as the radio source. Our program has been to study several selected regions in the sky and determine the spectrum with the hydrogen line equipment, the angular dimension of the region, and the relative brightness of the region in the continuous background. The 21 cm receiver installed in the 5o-foot radio telescope uses two channels, one approximately 1.9 mc wide which is normally used as a comparison channel and a second one 55 kc wide which is tuned to the hydrogen radiation whose frequency in the absence of doppler shift is 1420.405 mc. These two channels are separated by 3.25 mc. In the experiments reported here a measurement was made of the hydrogen line radiation by observing the difference between the broad and narrow channels, and in addition the background signal was simultaneously measured in the broad channel alone. The broad channel corresponds to a conventional radiometer when used in this way. The table lists the 21 cm sources in the order in which they were observed at this wave length. TABLE I. SOURCES OBSERVED AT 25 CM WAVE LENGTH No. Source R.A. Dec. T~~K Fluxt i Cas 23h25 +58~32' 5 266 2 Tau 5 3' +22 30 46 iofi 3 CygA `9 58 +40 30 63 546 4 Mi7 i8 `7 -i6 oo 35 82 5 NRLNo.5 `7 43 -28 45 73 i70 6 Ori 5 32 - 5 20 i8 42 7 Cen `3 22 -42 40 `7 40 8 Vir 12 27 +12 20 `3 30 9 NRLNo.9 `7 43 -28 45 6 10 M20 57 57 -23 03 i8 42 ii CygX 20 40 +40 40 i6 12* NGC 6604 i8 i6 -12 50 4 9 i3 IC 443 6 `3 +22 40 7 i6 54* Mi6 i8 `7 -`3 40 9 21 15 S78 20 22 +40 20 15 34 i6 NGC 7000 20 53 +44 30 4 57* S79 20 30 +43 45 3 7 18* Ori B 5 39 - 1 30 2 5 19* S45 i8 24 -12 30 II 26 20 Aql i8 59 + 5 4 9 * New source. t Unit = so-'5 w m~' c~' sec~'. Fourteen of these sources were seen for the first time at 21 cm and seven of the sources are being reported for the first time.' The temperatures and fluxes are believed to have a probable error of +20 per cent to -15 per cent, and the positions to be accurate within + 15', depending on the strength of the source. Of special interest are the ten H ii regions, which were located by using the optical positions of known H ii regions as a guide in searching for the radiation at 21 cm. Five of these are new sources and are indicated by an asterisk in the first column of the table. Sources NRL Nos. 5 and 9, listed by Haddock, Mayer, and Sloanaker at 9.4 cm,5 were found to be of surprising interest at 21 cm. At the position of the radio sources the receiver was tuned to a peak in the hydrogen spectrum and the antenna swept in hour angle or declination through the position of the source. At the position of source NRL No. 5 an outstanding case of absorption of the neutral hydrogen radiation by the radio source was discovered.' A similar but less striking case of absorption occurs in the region of the Crab Nebula. The diameter of source No. 5 lies between 210 and 1210, and it is superposed on the much wider source No. 9. The absorption of the hydrogen radiation amounts to 50 per cent, so that the source must be quite opaque and most of the neutral hydrogen must be behind the source. The position of source No. 5, R.A. 17b43m, Dec. 28o45~, is in the bright star cloud in Sagittarius, and it is interesting to speculate that the narrow hot peak in the background emission corresponds to the dense core of the nucleus of our galaxy where there may be little hydrogen. The neutral hydrogen which is absorbed must come from hydrogen clouds beyond the nucleus, but since there is little Doppler shift in the line radiation it is impossible to infer distance. I.Proc. Inst. Radio Eng. (in press). 2.Ap. J. 119, 456, `954. 3.Ap. J. (in press). Naval Research Laboratory, Washington, D. C. Publication: The Astronomical Journal Pub Date: August 1954 DOI: 10.1086/107072 Bibcode: 1954AJ.....59Q.323H full text sources ADS |" @default.
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• W2041467376 date "1954-08-01" @default.
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• W2041467376 title "Radio discrete sources and galactic absorption at 21 CM wave length." @default.
• W2041467376 doi "https://doi.org/10.1086/107072" @default.
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