THE GALACTIC PLANE AT 30.9 MHZ

Abstract

This thesis consists of a survey of the Galactic plane at a frequency of 30.9 MHz and of the interpretation of these data. Several other radio surveys of the Galaxy have been carried out at comparable frequencies in the past but with resolution 3 to 10 times poorer than have been employed in this thesis. The poor resolution of the instruments used have restricted previous studies mainly to the large-scale structure of the Galactic background radiation. The 15-arc-minute resolution of the Clark Lake TPT telescope at 30.9 MHz has allowed us to obtain a unique perspective of the Galaxy at decametric wavelengths. The survey extends from 350° < € < 250° with b < 3°. Final maps of this survey will be presented in the thesis. The interpretation and analysis of them can be summarized as follows. 1. Supernova Remnants (SNRs). Of 47 well-known SNRs included in the area of the survey, 40 have been detected. The nondetections, along with turnovers in the spectra of many of the detected remnants, are used to constrain the properties of the intervening ionized gas which is responsible for absorption. The results are consistent with Ooty 327 MHz recombination line data which suggest absorption due to a hot (T ~ 3000 K-8000 K), patchy (scale size ~ 50-200 pc), low-density (ne ~ 1-10 cm3) component of the ISM. These ionized regions are probably localized envelopes associated with the higher density, traditional Η π regions and they are confined mainly to the inner (1 < 50°) portion of the Galaxy. The Clark Lake results alone allow us to place an upper limit on any distributed large-scale ionized component of the ISM of ne ^ 0.25 cm3. In additionna number of extended, non thermal sources have been cataloged and probably represent previously unidentified SNRs. 2. Absorption by Ionized Gas. Prominent absorption by Η π regions in the inner Galaxy is seen against the diffuse galactic background. In most cases this absorption is associated with thermal emission observed at higher frequencies. The foreground emission has been used to estimate the synchrotron emissivity of the Galaxy. 3. Spiral-Arm Structure. A relatively featureless interarm gap is observed near € = 26°, corresponding to the line of sight between the Saggitarius and the Scutum spiral arms. Thus the Clark Lake maps allow an independent determination of the location of spiral-arm features. 4. Source Spectra. Over 300 small-diameter sources, mostly in the outer regions of the Galaxy, have been observed and their low-frequency spectra have been derived. Most of these sources are presumably extragalactic although some may be distant SNRs or distant highly-dispersed pulsars. Because the outer regions of the Milky Way are optically thin at 30.9 MHz, the statistical characteristics of these sources are derived. A detailed source list will be presented in conjunction with the final maps.