Towards a LOFAR detection of the 21cm line from the Epoch of Reionization

July 01, 2020

Recently, in correspondence with the 10th birthday of LOFAR, a core group of researchers including MPA scientists published the most stringent upper limits on the reionization signal from the early Universe. These observations are able to exclude some reionization models and constrain the thermal and ionization state of the intergalactic medium when the Universe was still in its infancy.

Aerial view of the MPA LOFAR station.

In the current cosmological framework, the diffuse gas (IGM) in the Universe, was initially very hot and in a highly ionized state. About 450 thousand years after the Big Bang it was cooling down, started to recombine and form neutral atoms. It then remained neutral until the first sources of ionizing radiation, such as stars or black holes formed and started to ionize it once again, about 500 millions years later. This marks a major phase transition in our Universe, which is known as reionization process.

A plethora of observations of distant galaxies, quasars and gamma ray bursts provide estimates of the amount of neutral hydrogen towards the end of reionization, while experiments on the cosmic microwave background radiation give a measure of the abundance of electrons produced by the process. However, observations that map the temporal and spatial evolution of reionization are not yet available.

It has long been known that neutral hydrogen in the IGM may be directly detectable at frequencies that fall in the radio band (in the range ~50-200 MHz) and measurements at different frequencies should allow us to accurately probe the structure and the evolution of the reionizing gas. This experiment is particularly attractive as a new generations of radio telescopes are operational (e.g. LOFAR, MWA, HERA) or under construction (e.g. SKA). The LOFAR core is located in the Netherlands, but several European countries are hosting remote antenna fields. Germany, represented by the GLOW Consortium, has six LOFAR stations. One of these (see Fig. 1) has been built and is operated by MPA.  

In addition to the LOFAR station, MPA is strongly involved in the scientific exploitation of LOFAR data as MPA scientists are core members of the LOFAR Epoch of Reionization (EoR) Key Science Project. Recently, this team has reported an improved upper limit on the 21cm signal from the primordial universe – the characteristic radio signal from neutral hydrogen - as well as its theoretical interpretation.

Left panel: map of 21cm signal at z=9.1 for one of the models studied in Ghara et al. (2020).
Right panel: the curves show the power spectra of the 21cm signal at different scales for the 1495 models studied in Ghara et al. (2020). The red points with error-bars show the upper limits from the LOFAR observations by Mertens et al. (2020), while the blue dashed curve represents the model corresponding to the map shown in the left panel. All the models above the observational points are excluded by LOFAR observations.

The analysis is based on 141 h of data collected with LOFAR in the frequency range 134-146 MHz and gives the best upper limits on the 21cm signal power spectrum of neutral hydrogen at z=9.1, or when the Universe was about 500 million years old. The larger number of hours processed compared to the previously reported upper limit, together with the updated calibration pipeline, show a reduction of a factor of about 8 in the value of the upper limit. This will further improve with the next milestone, which includes future refinements to the signal processing chain and about a 1000 h of processed data.

Using a combination of state-of-the-art N-body simulations, 1D radiative transfer calculations and a Bayesian inference framework to constrain the parameters, which describe the physical state of the intergalactic medium, the LOFAR EoR team found that the new upper limits are already able to exclude some reionization models (see Fig. 2). This exciting result shows that in the near future, once more data will be processed (also at different redshifts), observations with LOFAR of the 21cm line from neutral hydrogen we will be able to constrain the physical properties of the IGM at high redshift and the history of reionization.

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