Strong 21-cm fluctuationsãndãnisotropy due to the line-of-sight effect of radio galaxiesãt cosmic dawn

Sudipta Sikder*, Rennan Barkana, Anastasia Fialkov, Itamar Reis

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

3 Scopus citations


The reported detection of the global 21-cm signal by the EDGES collaboration is significantly stronger than standardãstrophysical predictions. One possible explanation isãn early radio excessãbove the cosmic microwave background. Suchã radio background could have been produced by high-redshift galaxies, if they were especially efficient in producing low-frequency synchrotron radiation. We have previously studied the effects of suchãn inhomogeneous radio background on the 21-cm signal; ho we ver, we madeã simplifyingãssumption of isotropy of the background seen by each hydrogen cloud. Here, we performã complete calculation thatãccounts for the fact that the 21-cmãbsorption occursãlong the line of sight,ãnd is therefore sensitive to radio sources lying behind eachãbsorbing cloud. We find that the complete calculation strongly enhances the 21-cm power spectrum during cosmic dawn, by up to two orders of magnitude; on the other hand, the effect on the global 21-cm signal is onlyãt the 5 per cent level. Inãddition to making the high-redshift 21-cm fluctuations potentially more easily observable, the line-of-sight radio effect inducesã newãnisotropy in the 21-cm power spectrum. While these effectsãre particularly large for the case ofãn extremely enhanced radio efficiency, they make it more feasible to detect evenã moderately enhanced radio efficiency in early galaxies. This is especially rele vãnt since the EDGES signal has been contested by the SARAS experiment.

Original languageEnglish
Pages (from-to)10975-10985
Number of pages11
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - 1 Feb 2024


FundersFunder number
Royal Society
Israel Science Foundation2359/20


    • first stars -cosmology: observations -cosmology: theory
    • methods: numerical -methods: statistical -darkãges
    • reionization


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