Epoch of Reionization

The last few weeks have definitely been an exciting time for the 21-cm community: The Experiment to Detect the Global EoR Step (EDGES) has reported a detection of a deep 21-cm absorption trough at 78 MHz (z ~ 17, Figure 1).

Figure 1: The 21-cm absorption trough recently reported by the EDGES team (Bowman et al. 2018).

EDGES is an experiment built to observe the sky-average 21-cm signal in the 6 < z < 26 redshift range using two different dipoles. Observations using the high band (i.e., 6 < z < 12) dipole recently placed upper limits on the signal from the epoch where cosmic reionization is expected to happen. Observations using the low band dipole revealed a most surprising signal. The 21-cm line is always observed as a contrast against the cosmic microwave background (CMB) radiation: if the intergalactic gas is colder (warmer) than the CMB the signal will appear in absorption (emission). As long as no luminous sources appear in our Universe, the gas keeps cooling down as the Universe expands, therefore we would expect to see the 21-cm signal in absorption… although not as bright as reported by Bowman et al. (2018). The big surprise that came with their findings is indeed that the magnitude of the absorption trough is a factor of about two larger than the most optimistic model in the literature, implying that the current theoretical scenario has been missing some physical ingredients.

These results have sparked a renewed theoretical effort, particularly in the view of a possible role of dark matter in collisionally cooling the gas. Given the theoretical challenges that such measurement poses, the community is awaiting confirmation from experiments similar to EDGES in the near future. If confirmed, pre-reionization 21-cm fluctuations will be a few times brighter than previously anticipated and therefore, easier to be detected by future SKA observations.

Meanwhile, the interferometers MWA, LOFAR and HERA are working towards improving the constraints on the 21-cm signal power spectrum for lower redshifts (z < 10). The MWA project has been focusing efforts on improving the calibration of the data, and applying strong quality control metrics to choose the best data for power spectrum processing (ionospheric activity, calibration quality, RFI metrics, flagged tiles). The collaboration is aiming to publish upper limits across a range of redshifts in 2018.

The LOFAR EoR Key Science Project is in the process of improving its published one night upper limit result (Patil et al. 2017) by calibrating additional nights. During the past year changes in the calibration routines have resulted in a substantial reduction of the excess noise reported in Patil et al. (2017) and the derived upper limits display the expected behaviour with integration time, at least up to the number of nights which have been processed which is currently ten. The aim is to publish another paper with upper limits for the power spectrum based on these ten nights of data later this year.

HERA construction is well underway in South Africa where more than 60 dishes have been deployed and early science observations have started. New feeds, extending the sensitivity down to 70 MHz (z ~ 19), are currently being developed.

Report provided by the EoR SWG