A new method to probe the thermal electron content of the Galaxy through spectral analysis of background sources

作者：Jones D I; Igoshev A P; Haverkorn M^*

来源：Monthly Notices of the Royal Astronomical Society, 2016, 460(3): 3298-3304.

DOI：10.1093/mnras/stw1192

• radiation mechanisms: thermal
• radiative transfer
• ISM: general
• ISM: structure
• Galaxy: structure
• radio continuum: ISM

We present a new method for probing the thermal electron content of the Galaxy by spectral analysis of background point sources in the absorption-only limit to the radiative transfer equation. In this limit, calculating the spectral index, alpha, of these sources using a natural logarithm results in an additive factor, which we denote , resulting from the absorption of radiation due to the Galactic thermal electron population. We find that this effect is important at very low frequencies (nu a parts per thousand(2) 200 MHz), and that the frequency spacing is critical. We model this effect by calculating the emission measure across the sky. A (smooth) thermal electron model for the Galaxy does not fit the observed emission measure distribution, but a simple, cloud-based model to represent the clumpy nature of the warm interstellar medium does. This model statistically reproduces the Galactic emission measure distribution as obtained independently from H alpha data well. We find that at the lowest frequencies (similar to 10-50 MHz), the observed spectral index for a large segment of the Galaxy below Galactic latitudes of a parts per thousand(2)15A degrees could be changed significantly (i.e.). This method therefore provides a correction to low-frequency spectral index measurements of extragalactic sources, and provides a sensitive probe of the thermal electron distribution of the Galaxy using current and next-generation low-frequency radio telescopes. We show that this effect should be robustly detectable individually in the strongest sources, and statistically in source samples at a level of , and 0.02 for source densities of 10, 100, and 1000 sources per square degree.

• 出版日期2016-8-11