The 2326 MHz radio continuum emission of the Milky Way

by Jonas, Justin Leonard

Abstract (Summary)
The Rhodes/HartRAO SKYMAP survey is the highest frequency (2326 MHz) and highest resolution (20 arcmin) large-area pencil-beam survey of the celestial radio continuum emission. The survey covers 67 % of the entire sky and is one of only two radio continuum surveys that cover the southern sky. These attributes make the survey eminently suitable for studying the extended radio continuum emission from the Milky Way. This thesis describes the methodology used to produce the SKYMAP survey map and presents an analysis of the galactic radio continuum emission.

Mountfort (1989, PhD thesis, Rhodes University) designed and implemented the original SKYMAP observing and data reduction procedures. These original procedures have been modified and extended to accommodate instrumentation upgrades at HartRAO and take advantage of new computer technologies that have become available. A new procedure was developed to perform the merging of the ten individual target-area maps into a self-consistent combined map without discontinuities at the component map boundaries. Calibration and data quality procedures were devised and implemented in order to validate the temperature scale and pointing accuracy of the map data. The uncertainty in the relative full-beam temperature scale is estimated to be 2 % and the RMS pixel noise is less than 30 mK. The uncertainty in the zero-level of the survey map is conservatively estimated to be 80 mK. The RMS pointing accuracy is better than 1.9 arcmin.

The diffuse galactic background (DGB) emission is shown to be consistent with four-arm spiral models for the Milky Way. The Gould Belt system and the galactic warp beyond the solar-circle are seen as non-symmetrical distortions in the DGB. An empirical model of the DGB was subtracted from the 2326 MHz map data in order to accentuate faint, extended radio sources.

A new technique was devised to discriminate between thermal and non-thermal radio continuum emission on the basis of FIR/radio flux ratios. This procedure was used to reduce the source confusion near the galactic plane.

42 new shell-like radio sources with angular diameters ranging from 1.6 deg to 26 deg are identified in the SKYMAP 2326 MHz map. These sources are probably large-diameter supernova remnants (SNRs) that partially redress the incompleteness in existing SNR catalogues caused by limited surface brightness selection effects. The shells of some of these faint, extended sources are incomplete towards high-latitudes. This morphology suggests that these objects are breaking out of the galactic disk and are releasing hot gas into the halo. There is no evidence for linear worm- and chimney-like features in the non-thermal galactic emission.

The spectral index of the diffuse galactic emission is calculated to be beta = 2.72 +/- 0.18 between 408 MHz and 2326 MHz and beta = 2.95 +/- 0.08 between 2326 MHz and 31 GHz, confirming previous work showing that the synchrotron spectrum steepens with increasing frequency. The high frequency spectral index is used to estimate upper limits on the galactic synchrotron foreground contamination of cosmic microwave background radiation measurements.

The high-latitude 2326 MHz galactic emission is shown to correlate weakly with the far-infrared galactic cirrus clouds. This correlation is due to thermal free-free radio emission from extended, low-density HII regions associated with the cirrus dust. The RMS level of this thermal signal is estimated to be 1 mK at 2326 MHz.

Two further experiments that need to be performed in order to complete the SKYMAP project are discussed:

(a) the region of sky near the southern equatorial pole (SEP) needs to be mapped at HartRAO to complete the coverage of the entire southern sky, and (b) independent horn telescope (broad-beam) measurements of the sky temperature are required in order to reduce the baseline uncertainties in the current SKYMAP data.

Bibliographical Information:


School:Rhodes University

School Location:South Africa

Source Type:Master's Thesis

Keywords:physics electronics


Date of Publication:01/01/1999

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