Hydrogen Epoch of Reionisation Array radio telescope
The HERA (Hydrogen Epoch of Reionisation Array) radio telescope will be instrumental in detecting the distinctive signature that would allow astronomers to understand the formation and evolution of the very first luminous sources: the first stars and galaxies in the Universe.
To solve the mystery of how we came to be where we are, astronomers must look back in time to a transitional period of cosmic history that has been dubbed the “epoch of reionisation”. Nobody has yet detected a signal from this period, but the Precision Array for Probing the Epoch of Reionisation (PAPER) pathfinder instrument has come the closest so far. HERA is expected to measure such signal throughout the epoch of reionization.
What is HERA
HERA is an American, South African and British collaboration to build a telescope capable of making a solid detection of the Epoch Of Reionisation (EOR) red-shifted hydrogen power spectrum signature, as well as conducting initial EOR science and launching this new scientific field of the observational cosmic dawn. The discovery and the early advancement of this science is a likely candidate for a Nobel Prize.
How HERA works
Unlike KAT-7, MeerKAT and eventually the SKA, which are general-purpose instruments intended to perform many different scientific observations, HERA has just one goal: to characterise the epoch of reionisation. It will give us a 3D map of the universe during this era. HERA is designed to detect radio waves in the low-frequency range of 100–200 MHz, which allows it to detect fluctuations in the emissions from neutral hydrogen gas that was found throughout the universe before stars, galaxies and black holes formed. Being a low-frequency instrument, the field-of-view for an antenna of a particular size is much larger than it would be if high frequencies were being detected. In other words, one can see more of the sky in one go at low frequencies.
This, along with the fact that the sought signal is everywhere in the sky (much like the Cosmic Microwave Background signal), means that individual antennas do not have to move around or be pointed at specific locations to scan for signals. No expensive moving parts are thus required. Whereas MeerKAT and SKA dishes rotate both vertically and horizontally to survey the sky, HERA antennas are immobile, pointing straight up at all times.
“HERA is expected to be the most sensitive SKA pathfinder to study the EOR, the period when the first galaxies were formed and started to shine – a cutting edge research field in modern cosmology and one of the main science cases for the SKA. We expect that HERA will take the field one leap forward and will be able to inform the SKA in terms of searching for the HI signal at very high redshift,” explains Dr Gianni Bernardi, SKA SA Senior Astronomer.
PAPER and HERA
HERA is located in the South African Karoo Astronomy Reserve, with a nominal array centre of 30°43’17″S and 21°25’42″E – the location of the existing Precision Array for Probing the Epoch of Reionisation (PAPER).
PAPER listens in on both the Northern and Southern hemisphere skies. The primary 128-antenna instrument is at the SKA site in the Karoo, while its smaller 32-antenna cousin is in West Virginia in the United States.
While PAPER is a pathfinder experiment (also known as HERA phase I) consisting of small antennas only a few metres across, each of HERA’s 331 antennas will measure 14 metres in diameter and boast a collecting area roughly 30 times larger. Bigger antennas mean that HERA will have a smaller field-of-view than PAPER, but it will be much more sensitive to the faint signals from the epoch of reionisation. Each antenna measures dual polarisation and they are referred to as dipole antennas.
HERA works on the same interferometry principles as KAT-7, MeerKAT and the SKA: essentially, data from all its antennas are combined. The instrument also shares much of the supporting infrastructure being constructed for MeerKAT and the SKA, including the underlying technology of the digital back-end, which processes and packages the incoming analogue radio signals so that astronomers can glean information and create visual images using digital data.
The current PAPER container will be moved to a location just west of HERA and the correlator will be moved to the Karoo Array Processor Building (KAPB). PAPER will be decommissioned over the HERA construction period.
HERA collaboration under way
The US National Science Foundation is the primary funder of the current HERA prototype phase, with significant funds from the University of Cambridge Cavendish Laboratory, the University of California at Berkeley and the National Radio Astronomy Observatory, with site and logistical support from SKA South Africa.
Construction will be sourced and constructed from within South Africa – predominantly from the Carnarvon area. South African scientists from SKA SA and numerous South African universities (the University of the Witwatersrand, University of KwaZulu-Natal, University of Cape Town and the University of the Western Cape) are significantly involved with the science, and researchers and engineers from SKA SA, the Stellenbosch University and the Durban University of Technology are expected to be heavily involved in the design and production of the array. Rhodes University, through the Research Associate Dr Gianni Bernardi, is also involved in research using PAPER and HERA.
PAPER (HERA phase I)
- PAPER has collected data for five years, with the aim of establishing how strongly the signal from the Epoch Of Reionisation (EOR) can be detected using the pioneering approach planned for HERA.
- PAPER has produced the best upper limits on the EOR signal to date.
- Also thanks to PAPER, scientists now have a roadmap for the next phases of HERA, directing them along the best routes for configuring the array, processing and storing the data, and preventing interference from other radio wave sources that would distort the signal.