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Cosmic reionization corresponds to the epoch in which the first stars and black holes reionize the neutral intergalactic medium (IGM) that pervades the Universe following recombination, within a few hundred million years of the Big Bang. The epoch of reionization, and the preceding ‘dark ages’ prior to the formation of the first stars, represent the last unexplored phases of cosmic evolution to be tested and explored. Scientists at the Cavendish Astrophysics Group are involved in a major international program to explore this ‘last frontier’ in observational cosmology, using the unique properties of the 21cm emission line from neutral Hydrogen. Direct observation of the large scale structure of the primordial IGM, and its evolution with time, via the HI 21cm line will have a profound impact on our understanding of the birth of the first galaxies and black holes, their influence on the surrounding gas, and cosmology.

Fig. 1: The PAPER configuration now – the EoR array.
Power Spectrum
Fig.2: Power spectra at z = 7.7 derived from a 55-day PAPER observation. In both panels, solid cyan depicts upper limits derived from PAPER observations without the removal of off-diagonal covariance terms, and black indicates the final measured power spectrum with confidence intervals.


The Precision Array to Probe the Epoch of Reionization is an array of 128 dipole elements operating between 120MHz and 180MHz, corresponding to HI redshifts between 11 and 7 (universal age between 0.4 Myr and 0.8Myr).  The array is situated in the very low terrestrial interference environment of the SKA site in the Karoo region of South Africa. The array grid-design (Fig.1) exploits our new understanding of the chromatic response of an interferometer to perform an optimal statistical (power spectrum) search for the HI 21cm signal, while avoiding the strong radio continuum foreground emission that would otherwise overwhelm the cosmic HI signal. PAPER has currently set the best limits to the HI signal from reionization (Fig.2). PAPER has also produced unprecedented wide field images of the radio sky at 120MHz to 150MHz (Fig.3). In the next two years, we will obtain deep cosmological integrations with PAPER with the potential to make the first detection of the HI 21cm signal from cosmic reionization.

Fig.3: Image of the Centaurus A (Cen A) field obtained from PAPER data. The Cen A radio galaxy dominates the centre, while the Galactic plane crosses the lower half of the image.
Fig.4: Members of the Cambridge and SA teams reconfiguring the array.
PAPER is a collaboration between the Cavendish, Berkeley, NRAO, Pennsylvania, the SKA-South Africa, and the University of KwaZulu-Natal, SA (Fig.4).


The Hydrogen Epoch of Reionization Array represents the next phase of our reionization experiments.  Building on lessons learned with PAPER and the Murchison Widefield Array, we have established a broader collaboration to construct a new array in South Africa with a sensitivity close to two orders of magnitude greater than PAPER, operating between 50MHz and 225MHz (Fig.5). HERA will move beyond simple statistical detection of the HI signal, to a full characterization of the HI 21cm power spectrum through reionization. HERA will also push power spectral studies back to the end of the ‘Dark Ages’, when the first sources of new light in the Universe warm the neutral IGM. Ultimately, HERA-331 will have the sensitivity to perform direct imaging of the largest scale structures in the Cosmos during reionization (Fig.6). We expect first science observations with HERA in late 2017. HERA is a collaboration between the PAPER institutions above, with the U. of Washington, Arizona State Univ, and MIT.

HERA array
Fig.5: The configuration of HERA’s 500 antennas.


Fig.6: Simulation by Dr. Daniel Jacobs of the imaging capabilities of HERA.

Cavendish Reionization Team

Prof. Chris Carilli: project lead at Cavendish

Irina Stefan: Imaging and calibration analysis

Dr. Nima Razavi-Ghods: Field engineering, deployment, testing

Dr. Jack Hickish: Digital design

Prof. Paul Alexander: oversight and science

Dr. Eloy de Lera Acedo: Array RF modeling

Lindley Lentati: Bayesian power spectral analysis

Peter Sims: 21cm and continuum sky simulations

Selected publications