Title: Probing the cosmological 21 cm global signal from the Antarctic ice sheet
First author: Shijie Sun
First author institution: National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China
Status: Available on ArXiv, published in Astronomical Techniques and Instruments
Cosmologists often use galaxies, which can be viewed through a telescope, to gain a picture of how matter is distributed throughout the Universe. However, one can also trace radiation emitted by hydrogen gas to gain a picture of the matter distribution in the Universe. The electron in a hydrogen atom can spontaneously emit 21 cm radiation (radio wavelength) when it moves from a higher energy state to a lower energy state by flipping its intrinsic spin angular momentum. As such, measurements of 21 cm radiation across the sky allows us to map out structures in the Universe like galaxies, filaments, clusters, and voids through the detection of hydrogen gas using radio telescopes.
One of the goals of radio astronomy is to study the redshifted 21 cm radiation emitted from hydrogen in the distant Universe to probe Reionization and Cosmic Dawn (see a past bite on this topic). Cosmic Dawn refers to the period in which the first stars and galaxies began to form, while Reionization refers to the event in which the strong ultraviolet (short-wavelength) emission from the first stars during Cosmic Dawn caused much of the gas in the Universe to become strongly ionized. The signatures of Cosmic Dawn and Reionization are expected to be very weak compared to the radio emission from our own galaxy, and this poses a challenge. It requires us to work with extremely sensitive radio instruments and have a thorough understanding of systematic uncertainties and noise that can affect the measurements.
Today’s authors point out a variety of problems that can impact radio astronomy measurements. However some of these problems become much less significant when the instrument is placed in the Antarctic environment.
The benefits of placing your radio telescope in Antarctica
Radio frequency interference (RFI) refers to unwanted electromagnetic interference from some external source, and this can impact our ability to detect the cosmological 21 cm radiation we are interested in. This can be a problem for low-frequency radio experiments, due to human activities that generate RFI in the range of frequencies that might affect their experiment (such as due to phones, radios, other devices). In Antarctica however, it is much less likely for human generated RFI to be a problem for an experiment. Additionally, the air in central Antarctica is dry and stable due to the desert-like conditions which is ideal for astronomical measurements.
Another problem that can cause unwanted issues in radio astronomy measurements is reflection of radio waves off the ground, which can interfere with the radio signal from the sky and distort the data. However, the thick ice in Antarctica is less reflective of radio waves and can reduce this impact. Aside from reflection of the waves, there can also be antenna-ground couplings that reduce the performance of the radio antenna. In soil, the conductivity (the ability of a material to conduct electricity) is dependent on humidity and temperature., As such, at different depths that the radio wave propagates into the ground there can be reflection of the waves; this can create standing waves between the ground and the antenna, creating systematic errors in the measured signal. The temperature of the ground can also affect the signal by its emission of thermal radiation. The conditions of the ground in Antarctica help to reduce these impacts.
The other benefit is related to the placement of a telescope in Antarctica in relation to the rotation of the Earth. Telescopes can suffer from chromatic errors, for a radio telescope, it refers to the fact that the sensitivity of the antenna changes with the frequency of the radiation, and this can impact the ability of the telescope to measure the faint signal of interest. In this experiment, the authors want to measure the sky-averaged 21 cm signal from the distant Universe, but the radio radiation from our galaxy can induce chromatic errors in the measurements that are difficult to correct for due to the anisotropy of the frequency of this radiation. By placing the telescope in Antarctica, there is less impact due to the fact that the visible part of the sky changes very little. This is another way in which by placing the telescope in Antarctica, one can avoid sources of systematic uncertainty.
Designing the telescope and an Antarctic expedition
Once a year the Chinese National Antarctic Expedition Program goes to inland Antarctica, and installs instruments for various scientific endeavours. The authors took advantage of this opportunity to install their radio telescope, the Antarctic global spectrum measurement experiment. They choose a site that is flat, open, and not too close to sources of RFI, such as from other scientific instruments. This instrument is able to operate automatically, is designed for the Antarctic weather, and only requires maintenance once a year. The physical size of the instrument is designed to detect wavelengths that correspond to the redshifted 21 cm radiation from the Cosmic Dawn era, at a frequency of about 50-100 MHz.
Due to the cold and harsh weather, the instrument has been designed to have a strong frame to tolerate strong winds, while still being lightweight and easily transportable to the remote installation site. The instrument is solar powered; it can lose power during long nights when there is little sunlight available but is able to regain power at a later time. They tested that the specially reinforced components of the instrument are able to tolerate temperatures down to -70 degrees Celsius while off and -50 degrees Celsius while operating.
The expedition team also conducted a survey to measure the ice layers around the telescope site and measured the RFI that might occur between the two Antarctic stations. They find the RFI is sufficiently low between 30-400 MHz, which is suitable for their experiment.
The expedition team also used ground-penetrating radar (GPR) to survey ice layers. They moved the GPR along the ice and beamed radar downwards. The results found that there were randomly reflected waves but there was no significant structure in the reflection spectrum for the inland ice that could be confused with the Cosmic Dawn signal.
Outlook for the future of the Global Spectrum Experiment
Overall, the authors find that the Antarctic environment offers a suitable location to conduct low frequency radio astronomy measurements, despite the harsh temperatures and isolation. The telescope has been successfully installed in Antarctica. We can hope in the future to learn more about conducting radio astronomy in this environment and gain scientific data informing us on the faint cosmological signatures of Cosmic Dawn and the epoch of Reionization.
Edited by: Lindsey Gordon
Featured image: Figure 3 from the paper, the Antarctic global spectrum measurement experiment.
