The first power spectrum limit on the 21-cm signal of neutral hydrogen during the Cosmic Dawn at z = 20-25 from LOFAR
Abstract
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 LOFAR-Low 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 Δ _{21}^2 < (14561 mK)^2 at k∼ 0.038 h cMpc^{-1} and Δ _{21}^2 < (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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- September 2019
- DOI:
- 10.1093/mnras/stz1937
- arXiv:
- arXiv:1809.06661
- Bibcode:
- 2019MNRAS.488.4271G
- Keywords:
-
- methods: data analysis;
- methods: statistical;
- techniques: interferometric;
- dark ages;
- reionization;
- first stars;
- diffuse radiation;
- radio lines: general;
- Astrophysics - Instrumentation and Methods for Astrophysics
- E-Print:
- 19 pages, 16 figures, accepted for publication in MNRAS