74 MHz Discrete HII Absorption Regions Towards The Inner Galaxy

At low radio frequencies (< 100 MHz), classical HII regions may become optically thick (optical depth > 1) and can be observed as discrete absorption regions against the Galactic nonthermal background emission created by Galactic cosmic ray electrons spiraling around magnetic fields. In this w...

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Bibliographic Details
Published inarXiv.org
Main Authors Nord, Michael E, Henning, P A, Rand, R J, Lazio, T Joseph W, Kassim, Namir E
Format Paper
LanguageEnglish
Published Ithaca Cornell University Library, arXiv.org 02.03.2006
Subjects
Absorption
Background radiation
Brightness temperature
Cosmic rays
Emission
Emissivity
Galactic cosmic rays
Galaxies
Radio frequency
Three dimensional models
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Summary:At low radio frequencies (< 100 MHz), classical HII regions may become optically thick (optical depth > 1) and can be observed as discrete absorption regions against the Galactic nonthermal background emission created by Galactic cosmic ray electrons spiraling around magnetic fields. In this work we present 74 MHz observations in the region 26>l>-15, -5<b<5, and report the detection of 92 absorption features associated with known HII regions, and derive the brightness temperature of the Galactic cosmic ray electron synchrotron emission emanating from the column behind these regions. For the 42 HII regions with known distances, the average emissivity of the column behind the HII region is derived. 74 MHz emissivity values range between 0.3 and 1.0 Kelvin per parsec for a model assuming uniform distribution of emissivity. Methods for utilizing this type of data to model the 3-dimensional distribution of cosmic ray emissivity and the possibility of using this method to break the HII region kinematic distance degeneracy are discussed.
ISSN:2331-8422
DOI:10.48550/arxiv.0603057