The first power spectrum limit on the 21-cm signal of neutral hydrogen during the Cosmic Dawn at z = 20–25 from LOFAR. Issue 3 (12th July 2019)

• Record Type: Journal Article
• Authors: Gehlot, B K; Mertens, F G; Koopmans, L V E; Brentjens, M A; Zaroubi, S; Ciardi, B; Ghosh, A; Hatef, M; Iliev, I T; Jelić, V; Kooistra, R; Krause, F; Mellema, G; Mevius, M; Mitra, M; Offringa, A R; Pandey, V N; Sardarabadi, A M; Schaye, J; Silva, M B; Vedantham, H K; Yatawatta, S
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 488:Issue 3(2019)
• Page Start: 4271
• Page End: 4287
• Publication Date: 2019-07-12
• Subjects: methods: data analysis -- methods: statistical -- techniques: interferometric -- dark ages, reionization, first stars -- diffuse radiation -- radio lines: general; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stz1937
• Languages: English
• ISSNs: 0035-8711

Abstract

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 $\sim 30\!-\!50{{\ \rm per\ cent}}$ 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 $\Delta _{21}^2 \lt (14561\, \text{mK})^2$ at $k\sim 0.038\, h\, \text{cMpc}^{-1}$ and $\Delta _{21}^2 \lt (14886\, \text{mK})^2$ at $k\sim 0.038 \, h\, \text{cMpc}^{-1}$ 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.