Measurement of the Cosmic Microwave Background by Optical Observations of Interstellar Molecules

The central purpose of this dissertation is the extract the largest feasible amount of information on the short wavelength spectrum of the microwave background radiation from a number of spectra of interstellar molecules that were available in the summer of 1966. The work may be divided into roughly two parts: an observational part and a theoretical part. In the observational part, we first show how one calculates the background intensity from the spectra of the interstellar molecules (Chapters 2 and 3). Second, in order to utilize the existing spectrograms, we develop new techniques of spectrophotometry (Chapter 4, Appendix 4). Third, we present the results of the application of these techniques to the problem at hand, and the resulting upper limits to and measurements of the intensity of the background radiation (Chapter 5).

On the theoretical side, we first consider what assumptions concerning the location of the molecules will be necessary, such that (1) the molecules will set upper limits to the background intensity, and (2) the molecules will yield reliable measurements of the background intensity. It will be seen that the use of the molecules to set intensity upper limits places only minor restrictions on the molecular environment: however, their use to make intensity measurements requires the absence of alternate molecular excitation mechanisms (Chapter 6).

We therefore provide an analysis which considers possible excitation schemes that are consistent with our present understanding of the conditions found in the interstellar medium. We find that a necessary assumption for the molecules to yield reliable intensity measurements is that they reside in a normal HI region (Chapters 6, 7, and 8).

After a review of the existing evidence that the molecules to reside in an HI region, we proceed to present new evidence to further substantiate this contention. This will consist of (1) the observed invariance of the excitation of the interstellar molecules, and (2) the results of the new 21cm HI observations in the direction of these molecular clouds (Chapter 9).