The first power spectrum limit on the 21-cm signal of neutral hydrogen during the Cosmic Dawn at z = 20-25 from LOFAR

B. K. Gehlot, F. G. Mertens, L. V.E. Koopmans, M. A. Brentjens, S. Zaroubi, B. Ciardi, A. Ghosh, M. Hatef, I. T. Iliev, V. Jelic, R. Kooistra, F. Krause, G. Mellema, M. Mevius, M. Mitra, A. R. Offringa, V. N. Pandey, A. M. Sardarabadi, J. Schaye, M. B. SilvaH. K. Vedantham, S. Yatawatta

Research output: Contribution to journalArticlepeer-review


Observations of the redshifted 21-cm hyperfine line of neutral hydrogen from early phases of the Universe such as Cosmic Dawn and the Epoch of Reionization promise to open a new window onto the early formation of stars and galaxies. We present the first upper limits on the power spectrum of redshifted 21-cm brightness temperature fluctuations in the redshift range z = 19.8-25.2 (54-68 MHz frequency range) using 14 h of data obtained with the LOFARLow Band Antenna (LBA) array. We also demonstrate the application of a multiple pointing calibration technique to calibrate the LOFAR-LBA dual-pointing observations centred on the North Celestial Pole and the radio galaxy 3C220.3. We observe an unexplained excess of ∼ 30-50 per cent in Stokes / noise compared to Stokes V for the two observed fields, which decorrelates on ≥12 s and might have a physical origin. We show that enforcing smoothness of gain errors along frequency direction during calibration reduces the additional variance in Stokes I compared Stokes V introduced by the calibration on sub-band level. After subtraction of smooth foregrounds, we achieve a 2σ upper limit on the 21-cm power spectrum of Δ221 < (14561 mK)2 at k ∼ 0.038 h cMpc-1 and Δ221 < (14886 mK)2 at k ∼ 0.038 h cMpc-1 for the 3C220 and NCP fields respectively and both upper limits are consistent with each other. The upper limits for the two fields are still dominated by systematics on most k modes.

Original languageEnglish
Pages (from-to)4271-4287
Number of pages17
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - 2019

Bibliographical note

Funding Information:
BKG and LVEK acknowledge the financial support from a NOVA cross-network grant. FGM acknowledges support from a SKA-NL (Square Kilometer Array - Netherlands) roadmap grant from the Dutch Ministry of OCW (Ministerie van Onderwijs, Cultuur en Wetenschappen). LOFAR, the Low Frequency Array designed and constructed by ASTRON, has facilities in several countries, that are owned by various parties (each with their own funding sources), and that are collectively operated by the International LOFAR Telescope (ILT) foundation under a joint scientific policy.

Publisher Copyright:
© 2019 The Author(s).


  • dark ages, reionization, first stars
  • diffuse radiation
  • methods: data analysis
  • methods: statistical
  • radio lines: general
  • techniques: interferometric


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