Journal Article

The end of the rainbow: what can we say about the extragalactic sub-megahertz radio sky?

Brian C. Lacki

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 406, issue 2, pages 863-880
Published in print August 2010 | ISSN: 0035-8711
Published online July 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.16781.x
The end of the rainbow: what can we say about the extragalactic sub-megahertz radio sky?

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The Galactic disc is opaque to radio waves from extragalactic sources with frequencies ν less than ∼ 3 MHz. However, radio waves with kHz, Hz and even lower frequencies may propagate through the intergalactic medium (IGM). I argue that the presence of these waves can be inferred by using the Universe as our detector. I discuss possible sub-MHz sources and set new non-trivial upper limits on the energy density of sub-MHz radio waves in galaxy clusters and the average cosmic background. Limits based on five effects are considered: (1) changes in the expansion of the Universe from the radiation energy density, (2) heating of the IGM by free–free absorption, (3) radiation pressure squeezing of IGM clouds by external radio waves, (4) synchrotron heating of electrons in clusters and (5) inverse Compton upscattering of sub-MHz radio photons. Any sub-MHz background must have an energy density much smaller than the cosmic microwave background at frequencies below 1 MHz. The free–free absorption bounds from the Lyman α (Lyα) forest are potentially the strongest, but are highly dependent on the properties of sub-MHz radio scattering in the IGM. I estimate an upper limit of 6 × 104 L Mpc−3 for the emissivity within Lyα forest clouds in the frequency range of 5–200 Hz. The sub-MHz energy density in the Coma cluster is constrained to be less than ∼ 10−15 erg cm −3. At present, none of the limits is strong enough to rule out a maximal Tb= 1012 K sub-MHz synchrotron background, but other sources may be constrained with better knowledge of sub-MHz radio propagation in the IGM.

Keywords: miscellaneous; radiation mechanisms: non-thermal; intergalactic medium; radio continuum: general

Journal Article.  14738 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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