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1968Apjs...15..131G OH ABSORPTION in the GALAXY* W

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1968Apjs...15..131G OH ABSORPTION in the GALAXY* W OH ABSORPTION IN THE GALAXY* W. Miller Gossf Radio Astronomy Laboratory, University of California, Berkeley 1968ApJS...15..131G Received April 26, 1967 ABSTRACT A survey of northern hemisphere radio sources for 18-cm OH absorption has been completed using the 85-foot Hat Creek telescope of the University of California. The observations were made with the 100-channel receiver with frequency resolutions of 10 kHz (1.8 km/s) and 2 kHz (0.36 km/s). Galactic OH absorption lines have been found in 26 galactic sources and two extragalactic sources (W7 and Cyg A): W1 (NGC 7822), W9 (Tau A), W10 (Orion A), W12 (NGC 2024), W14 (IC 443), W22 (NGC 6357), W28, W29 (M8), W30, W31, W33, W35 (NGC 6604), W37 (M16), W38 (M17), W41, W42, W43, W44, W47, W66, W67, W69, W72, W73, W80 (NGC 7000), and W81 (Cas A). Four of these sources contain OH emission in the main lines (1667 MHz and 1665 MHz) in addition to the absorption: Orion A at 1667 and 1665 MHz, W33 at 1667 and 1665 MHz, W42 at 1667 MHz, and W43 at 1667 and 1665 MHz. The W42, W33, and Orion (1667 MHz) sources differ from the previously discovered emission sources in that the emission is at the high-velocity side of one of the absorption lines. For the sources W10, W12, W22, W28, W41, W43, W44, W51, and W81 all four lines of the OH multi- plet have been observed. Most of the sources show normal intensity ratios in the main lines. Except for W12 and W22 all the sources show emission in the 1612 MHz and/or 1720 MHz satellite lines. Even for W12 and W22 the intensity ratios are incompatible with a unique excitation temperature for the A doublet. The W12 results suggest that the rest frequency of 1720 MHz should be 1720.527 ± 0.003 MHz. This frequency is 6 kHz less than Radford’s value and essentially removes the discrepancy in the frequency sum rule. Two H n regions which lie in the direction of W43 have been identified from their 158a H recombina- tion lines. The 158a He line from the stronger of the two H n regions has been detected. In many cases, it is possible to compare the OH absorption lines observed in this investigation with the H i absorption lines found by other observers. The general agreement in velocity suggests that the OH has a distribution similar to that of thé H i. The solution of the equation of transfer appropriate to the OH absorption case is presented. The im- portance of the isotropic background and the galactic background is stressed. The major processes establishing the excitation of OH in H i regions are considered. The level of excita- tion is controlled by (1) collisions with positive ions and electrons, and (2) the 18-cm isotropic radiation field (assumed to be 3° K). The calculations predict an excitation temperature of about 20° K, while the observational evidence indicates that the excitation temperature is less than about 10° K. Two pos- sible causes for this discrepancy are discussed. For the purpose of the estimation of OH projected densities, it is assumed that the excitation tempera- ture is 3° K (the lower limit). Based on this assumption the ratios of the OH to H i projected densities are in the range 10-8-10-7. I. INTRODUCTION It has long been recognized that neutral hydrogen absorption techniques provide a useful tool for studying conditions in the interstellar medium. The purpose of this in- vestigation is to extend such studies by examination of the spectra of northern hemisphere radio sources for OH 18-cm absorption lines arising from the ground-state A doublet. These observations provide the following information: (1) the galactic distribution of OH; (2) the projected densities of OH (divided by the excitation temperature); and (3) the widths of the individual Doppler features. From such information knowledge can be obtained regarding molecular processes and excitation conditions in the interstellar medium. Often the relation of OH to other constituents of the interstellar medium can be ascertained. Finally, in some cases the OH lines can serve as a tool for the study of galactic structure. * Based on a thesis submitted to the University of California, Berkeley, in partial fulfilment of the requirements for the Ph.D. degree. f Now at C.S.I.R.O. Radiophysics, Sydney, Australia. 131 © American Astronomical Society • Provided by the NASA Astrophysics Data System 132 W. MILLER GOSS Galactic OH absorption lines have been found in the spectra of twenty-six galactic and two extragalactic sources (Cyg A and W7). The absorption lines in the direction of 1968ApJS...15..131G W3 and NGC 6334 are discussed by Weaver, Dieter, and Williams (1968). Five of the twenty-eight sources have been reported elsewhere and are here discussed in more detail. These sources are Gas A (Weinreb, Barrett, Meeks, and Henry 1963), W43 and W51 (Williams 1965), W75 (Weaver, Williams, Dieter, and Lum 1965) and Cyg A (Menon, private communication). In addition, OH emission lines were found in the direction of ten sources. Section II is a description of the instrumentation and numerical parameters used in this survey. Section HI is a summary of the observations. Section IV is a discussion of the equation of transfer appropriate to OH. In § V we discuss the problem of the excita- tion temperature of the OH A doublet. The conclusions follow in § VI. II. INSTRUMENTAL AND NUMERICAL PARAMETERS The observations were made with the 85-foot Hat Creek radio telescope of the Uni- versity of California. A low-noise parametric amplifier was used as a preamplifier fol- lowed by a 100-channel receiver. Band widths of 10 kHz (corresponding to 1.8 km/s) and 2 kHz (corresponding to 0.36 km/s) were used. For the OH observations frequency switching was employed; a sky horn was utilized as a reference load for the 18-cm con- tinuum observations. The position angle of the E-vector was 90° during these observa- tions. TABLE 1 Instrumental Parameters at 1666 MHz 2 Ae 300 ± 10 m 0.57 + 0.02 7]b 0.96 ¿ 0.07 dii2 3l!4 ± CO in E plane dij2 35Í6 ± CO in E plane During each of the observing runs (April-May and October-November, 1966) a gas- discharge tube was used as a secondary standard ; it was calibrated against the standard non-thermal sources Cas A, Cyg A, Tau A, and Virgo A using the fluxes measured by Baars, Mezger, and Wendker (1965). The effective area, Ae, and aperture efficiency, tja, of the antenna were determined by calibrating the noise tube with a hot-cold load. The instrumental parameters are summarized in Table 1. The derived value of A e agrees quite closely with the value of 304 m2 obtained by W. J. Welch (private com- munication) in 1965 at a frequency of 1515 MHz with the same feed system. The half-power beam widths, 0i/2, were derived from scans of the bright OH emission fines in W3 and W49. From the values of 0i/2 and tja the beam efficiency, tjb, was calculated by using the expression given by Mezger and Henderson (1967). The values of A e at 1612 and 1720 MHz were found to be 320 ± 30 and 330 ± 30 m2, respectively. The system temperature was in the range of 130o-200° K during these observations. The deteriora- tion of the system during the latter part of the second observing period was due to a faulty component in the preamplifier. The pointing of the 85-foot telescope has been discussed in detail by Welch, Thornton, and Lohman (1966). Further tests by Welch (private communication) made by observ- ing the Moon at low declinations (^ —28°) have indicated that the pointing errors are less than 1' at these declinations. The linearity of the square-law detectors was checked on the strong OH absorption fines in Cas A—the most unfavorable case because of its large antenna temperature. Deviations from linearity were found to be less than 10 per cent and will be neglected in this discussion. © American Astronomical Society • Provided by the NASA Astrophysics Data System OH ABSORPTION 133 The problem of the rest frequencies for the four OH lines has been discussed by Robin- son and McGee (1967). From the results obtained in this investigation for the source 1968ApJS...15..131G W12 (§ III) there is a strong indication that Radford’s (1964) value for the F = 2 to F = 1 line (1720 MHz) should be decreased by about 6 kHz to 1720.527 ± 0.003 MHz. This revised frequency has been adopted in the present investigation. As Table 2 indi- cates, this revision essentially removes the discrepancy in the frequency sum rule for the four lines, i.e., the sum of the 1667 and 1665 MHz frequencies is 1 kHz larger than the sum of the 1612 and 1720 MHz frequencies. Further evidence favoring this revision is provided by the other sources investigated; use of the derived rest frequency improves the velocity agreement in many cases. The Cas A results (§ III) for the lines at 1667 and 1665 MHz indicate that Radford’s frequencies for the main lines are quite consistent; in the Orion arm lines the agreement in velocities is better than 0.1 km/s, i.e., 0.5 kHz TABLE 2 2 Rest Frequencies of OH n3/2 a Doublet Lines Radford (1964) This Work Transition (kHz) (kHz) F= 1—>2 1612:231 ±2 F= 1—>1 1665:401 ±2 F=2—>2 1667:358 + 2 E=2—>1 1720:533 + 2 1720:527+3 For the observations of the 158a lines of hydrogen and helium in W43 the rest fre- quencies used were 1651.5416 and 1652.214 MHz, respectively.
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