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

2010/04/12 by Brian C. Lacki
Physics and Astronomy · #Astrophysics and Cosmic Phenomena #Compton scattering #Cosmic microwave background #Galaxies: Formation, Evolution, Phenomena #Galaxy #Galaxy cluster #Intergalactic travel #Opacity #Radio Astronomy Observations and Technology #Radio galaxy #Radio wave #Synchrotron #Synchrotron radiation #astro-ph.CO

paper · pdf · doi:10.1111/j.1365-2966.2010.16781.x

published as Monthly Notices of the Royal Astronomical Society (2010), 406, 863 · Accepted by MNRAS, 4 figures, 20 pages.

arxiv created 2010/04/12 · openalex publication_date 2010/06/15 · arxiv updated 2015/05/18 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05

Abstract

The Galactic disc is opaque to radio waves from extragalactic sources with frequencies 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 (Ly) 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 10 4 L Mpc -3 for the emissivity within Ly 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 T b = 10 12 K sub-MHz synchrotron background, but other sources may be constrained with better knowledge of sub-MHz radio propagation in the IGM.

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