The Hydrogen Epoch of Reionization Array (HERA) is a
radio telescope
A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency ...
dedicated to observing large scale structure during and prior to the epoch of
reionization
In the fields of Big Bang theory and cosmology, reionization is the process that caused matter in the universe to reionize after the lapse of the " dark ages".
Reionization is the second of two major phase transitions of gas in the universe (t ...
. HERA is a
Square Kilometre Array
The Square Kilometre Array (SKA) is an intergovernmental international radio telescope project being built in Australia (low-frequency) and South Africa (mid-frequency). The combining infrastructure, the Square Kilometre Array Observatory (SKA ...
(SKA) precursor instrument, intended to observe the early universe and to assist in the design of the full SKA. Along with
MeerKAT
MeerKAT, originally the Karoo Array Telescope, is a radio telescope consisting of 64 antennas in the Meerkat National Park, in the Northern Cape of South Africa. In 2003, South Africa submitted an expression of interest to host the Square Kilom ...
, also in South Africa, and two
radio telescope
A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency ...
s in
Western Australia
Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to th ...
, the
Australian SKA Pathfinder
The Australian Square Kilometre Array Pathfinder (ASKAP) is a radio telescope array located at Murchison Radio-astronomy Observatory (MRO) in the Mid West region of Western Australia.
The facility began as a technology demonstrator for the int ...
(ASKAP) and the
Murchison Widefield Array
The Murchison Widefield Array (MWA) is a joint project between an international consortium of organisations to construct and operate a low-frequency radio array. 'Widefield' refers to its very large field of view (on the order of 30 degrees ac ...
(MWA), the HERA is one of four precursors to the final SKA. It is located in the
Meerkat National Park
Meerkat National Park is a large National Park in the Northern Cape, South Africa, that encompasses the Square Kilometre Array, MeerKAT and HERA telescopes. To ensure long term viability of the Karoo site for the MeerKAT and the SKA, and ...
.
Motivation
There is a large time interval between the universe's
recombination epoch (
z=1100) and the time (z = 7±1) after which intergalactic gas is largely reionized. Studies of the
cosmic microwave background
In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...
(CMB) have shown the structure of the universe at the start of this interval, and deep optical surveys such as the
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project began in 2000 a ...
show the structure after this time interval, but there is little data available from the time when the
first generation of stars and the earliest black holes were appearing. Since hydrogen was by far the most common element in the early universe, a natural way to examine the epoch when stars appeared is to look at the fraction of hydrogen that is ionized. Astronomers observe cold atomic hydrogen with the
21 cm (1420 MHz) spectral line. Assuming that reionization occurs at z = 6 to 8, one would expect to see this spectral line redshifted into a frequency range around 150 to 200 MHz. A series of instruments such as
PAPER
Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre evenly distributed ...
,
LOFAR and
MWA MWA may refer to:
* Machas with Attitude, Indian hip hop group
* Mandatory Work Activity, UK
* Married Women's Association, UK
* May Week#May Week Alternative, May Week Alternative, a Cambridge University student charity
* Metropolitan Waterfront ...
have been looking for this radiation. HERA is a project to look for this radiation with improved sensitivity.
The CMB provides a background screen upon which the hydrogen reionization signal will be seen. Very cold hydrogen gas will absorb CMB photons, and produce a dip in the CMB signal. Warm hydrogen will emit 21 cm (rest frame) photons, increasing the sky brightness above what the CMB provides. After the hydrogen is fully reionized, the CMB will no longer be affected by the 21 cm line transitions of neutral hydrogen. Observing how absorption and emission vary as a function of z, and as a function of sky position, will provide tight constraints on models of the formation of stars, galaxies and supermassive black holes.
Technical challenges
HERA will observe in the frequency band from 50 to 250 MHz.
This frequency range includes broadcast signals for
FM radio
FM broadcasting is a method of radio broadcasting using frequency modulation (FM). Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to provide high fidelity sound over broadcast radio. FM broadcasting is cap ...
,
television
Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertisin ...
, and many other terrestrial sources. These signals will be many orders of magnitude stronger than the signal HERA is searching for, so to minimize this interference HERA is being constructed in the sparsely populated
Karoo
The Karoo ( ; from the Afrikaans borrowing of the South Khoekhoe !Orakobab or Khoemana word ''ǃ’Aukarob'' "Hardveld") is a semi-desert natural region of South Africa. No exact definition of what constitutes the Karoo is available, so its ext ...
desert of South Africa, near
Carnarvon.
Interference from the Sun also disrupts observations, which restricts observations to night time hours. The galactic plane of the Milky Way also must be avoided. These two constraints limit HERA's science observations to an annual observing window approximately 4 months long.
Another major technical challenge is emission from foreground sources such as quasars and ionized gas within the Milky Way. This emission is expected to be four to five orders of magnitude stronger than the recombination epoch signals HERA will try to detect. However this foreground is composed of
synchrotron
A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed closed-loop path. The magnetic field which bends the particle beam into its closed p ...
and
free-free emission, which has no narrow spectral features. In contrast HERA's goal is to detect the formation of warm neutral and ionized regions within the intergalactic gas, and those regions should produce narrow spectral features. The HERA array must have a smooth spectral bandpass so that when the foreground emission is subtracted from the detected signal, the remaining narrow features come from the astronomical objects rather than the instrument's spectral response.
Telescope design
HERA is a radio interferometer, cross-correlating the signals from pairs of individual antennas. Radio interferometers intended for imaging are usually designed to minimize the number of identical baselines, in order to measure emission at the largest possible number of spatial frequencies for image reconstruction. In contrast, the antennas in the HERA array are deployed in a
hexagonal tiling
In geometry, the hexagonal tiling or hexagonal tessellation is a regular tiling of the Euclidean plane, in which exactly three hexagons meet at each vertex. It has Schläfli symbol of or (as a truncated triangular tiling).
English mathemat ...
pattern, which provides a large number of identical baselines. While this geometry reduces the quality of images that HERA can produce, it allows signals from identical baselines to be summed in order to improve the signal to noise ratio.
The individual antennas have a cross-dipole feed suspended above a 14 meter wire mesh
parabolic dish
A parabolic (or paraboloid or paraboloidal) reflector (or dish or mirror) is a reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid, that is, the surface generated ...
. The antennas are not steerable, they point towards the zenith. The size of the antenna was selected to be large enough that any standing waves within the antenna structure would be at frequencies below 50 MHz, outside of the frequency band of interest. The antennas are constructed from low cost materials such as wood and
PVC pipes.
When completed, the array will consist of 350 antennas (318 in a densely-packed hexaconal core 300 meters across, and 32 in more distant outriggers). The total collecting area will be 54,000 m², similar to that of
Arecibo Observatory
The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...
.
Results
Because of the large
fractional bandwidth
Bandwidth is the difference between the upper and lower frequencies in a continuous band of frequencies. It is typically measured in hertz, and depending on context, may specifically refer to ''passband bandwidth'' or ''baseband bandwidth''. ...
of the array (which complicates foreground signal subtraction) and the weakness of the cosmological signal (roughly 10 millikelvin), it is unlikely that HERA will produce high quality images of individual structures within the reionizing gas. Instead, the array will measure the power spectrum of fluctuations in the gas, much as early CMB instruments did.
References
External links
*
*{{YouTube, HvCvqcSzIHE, The Hydrogen Epoch of Reionization Array (2016 CfA colloquium about HERA) 1:04:22
Radio telescopes