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 hours 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% in Stokes I noise compared to Stokes V for the two observed fields, which decorrelates on 12 seconds 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 ∆ 2 21 < (14561 mK) 2 at k ∼ 0.038 h cMpc −1 and ∆ 2 21 < (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.

Constraining the intergalactic medium at z ≈ 9.1 using LOFAR Epoch of Reionization observations

“…Recently, upper limits were provided for even higher redshifts. Gehlot et al (2019) placed upper limits on the power spectrum in the redshift range z = 19.8 − 25.2 using observations with the LOFAR-Low Band Antenna array and Eastwood et al (2019) placed upper limits at z ≈ 18.4 using observations with the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA) 6 . Mertens et al (2020) have provided the second LOFAR upper limit on the 21-cm power spectrum at redshift ≈ 9.1 using 10 nights of observations.…”

21-cm observations and warm dark matter models

“…(b) Baryonic collapse and star formation in different DM universes In general, the naive expectation that what is known from CDM simulations would also 2 An alternative parametrization is given by the mass of nonresonantly sterile neutrinos [40]. The two models lead to an almost identical shape of the matter power spectrum and therefore their masses are related to one another in a nonlinear way; see [38,41] for details.…”

Redshifted 21-cm emission signal from the halos in Dark Ages

“…The main parameters of LOFAR observations, which are important in our analysis, are as follows: field of view is 12 deg 2 (beam diameter 3.8 deg), the frequency range is 54 ÷ 68 MHz, the redshift range is z = 19.8 ÷ 25.2, the integration time is 14 hours and the frequency resolution is 3 kHz [10].…”

“…Since the differential spectral flux from halos is proportional to the square beam size of the antenna, we present the estimations for the antenna with the radius of beam θ beam = 1.9 o , 10 arcmin and 2.1 arcmin. The first is the radius of LOFAR antenna beam [10], the last is MWA beam radius [9]. One can see that average differential flux per unit frequency from all halos observed within a given antenna beam fast decreases with increasing of z.…”

“…The upper limits on the brightness temperature power spectrum of the redshifted 21-cm signal provided by several instruments. In[10] the LOFAR LBA results are presented for two celestial fields: 3C220.3 galaxy vicinity and North Celestial Pole (in before last and last rows accordingly).…”