W30 Revealed: Separation and Analysis of Thermal and Nonthermal Emission in a Galactic Complex
NAVAL RESEARCH LAB WASHINGTON DC
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We present new VLA observations of the W30 Galactic complex at 90 and 20 cm. By combining these data with existing radio continuum and recombination line observations extending from wavelengths of 10 m to 3 cm, we are able to do the following 1 We can improve estimates of the spectra and physical properties of H II regions in the complex. For thermal sources which are optically thick at 90 cm, we derive improved emission measures and filling factors, and for those which are optically thin, we set new upper limits to their emission measures. 2 We can derive improved estimates of the spectrum and physical properties of the supernova remnant SNR G8.7-0.1 associated with the W30 complex. Our 90 cm observations show that the nonthermal emission from the SNR is much larger than previously realized. The newly determined spectral index alpha -0.5 is typical of shell-type SNRs. Relating G8.74-0.1 to H II regions in the W30 complex yields an improved distance estimate for the SNR of 6 plus or minus 1 kpc. At this distance G8.7 - 0.1 is one of the largest known Galactic SNRs, with a physical size of 80 pc, and certainly one of the most distant, low-surface brightness SNRs known in the Milky Way. 3 We can show that the low-frequency turnover in the spectrum of G8.7 - 0.1 is due to absorption by localized thermal gas associated with one or more of the H II regions near the W30 complex and does not require absorption by a distributed ionized component of the ISM. 4 We can show that, with this new distance and larger extent, G8.7 - 0.1 may be associated with the very young pulsar, PSR 1800 - 21.