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

• Published in: Monthly notices of the Royal Astronomical Society 2019, Vol.488 (3), p.4271-4287
• Main 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
• Format: Article
• Language: English
• Subjects: dark ages; diffuse radiation; first stars; methods: data analysis; methods: statistical; radio lines: general; reionization; techniques: interferometric
• ISSN: 0035-8711; 1365-2966; 1365-2966
• DOI: 10.1093/mnras/stz1937

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.