Interferometric Observations of Galactic Hydroxyl Emission

Measurements of the sizes and spatial distribution of the source of OH emission at 1665 MHz near the HII regions W3, W24, W49, and NGC6334 are reported here.  The initial measurements were made on W3 with an interferometer having a fringe spacing of 3" of arc.  The spectral features were found to be unresolved but spatially separated from one another.  The equipment and data processing technique was developed to make spectra line observations with interferometers having elements widely spaced and not connected to one another in real time.  This was done in conjunction with a joint group from the National Radio Astronomy Observatory and the Arecibo Ionospheric Observatory.  The tests of the phase stability of this system, which was controlled by atomic frequency standards, are reported here.  Successive measurements of W3 with resolutions of 0.04, 0.01 and 0.005 seconds of arc have resolved all the features of this source.  The individual features are typically found to consist of several components separated by about 0.02".  The size of the smallest feature is 0.004" and its brightness temperature is 5x10¹² °K.  A map of the relative position of the features was made which shows that the emission points are scattered over an area of about 3 square arc seconds.  The separation between the two strongest features has not changed noticeably in a year’s time.  Maser models with IR or UV pumping can be adapted to fit the observed geometry.  Typical feature sizes in NGC6334, W49, and W24 are 0.015, 0.05 and 0.09 seconds of arc respectively.  It is suggested that the large angular sizes of W49 and W24 could be caused by interstellar scattering.

The results of an unsuccessful search for radiation from interstellar OH in the ground state are reported.