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Mapping the universe in radio, by observing the 21 cm hyperfine transition of atomic hydrogen (HI), is a complementary approach to optical surveys for the study of large scale structures (LSS), in particular baryonic acoustic oscillations (BAO). The challenge is to extract the cosmological signal, with an amplitude of less than a mK, in the presence of foregrounds with a brightness several thousand times greater and instrumental noise, characterized by the system temperature (Tsys), of a few tens of Kelvin. Instrumental noise can be reduced thanks to long integration times, a few hours for each sky direction. Component separation or foreground subtraction is based on the absence of structuring along the frequency axis on the scale of a few MHz for the foregrounds. Packed interferometric arrays, observing in transit mode, are well adapted for intensity mapping surveys. Analyses are generally carried out in the spatial Fourier domain, where (k⊥ , k∥) correspond to the wave numbers or spatial frequencies in the transverse plane and along the radial direction, which corresponds to the redshift.

The proposed studies will mainly be carried out in the framework of the international Tianlai collaboration, led by the NAOC (National Astronomical Observatories – Chinese Academy of Sciences).

Two interferometric instruments were built for Tianlai on a dedicated site at Hongliuxia in Xinjiang province (China): an array of 3 cylinders (15m x 40m each), and a dense array of 16 reflectors, each 6m in diameter. Observations have been carried out with these two instruments for several years. The 16-antenna array offers the advantage of being able to observe at different declinations, and towards the North Celestial Pole in particular. It is therefore possible to carry out a deep survey over a small area of the sky in order to achieve the sensitivities required for the detection of the extragalactic 21 cm signal. Both instruments are currently equipped with receivers sensitive in the 700-800 MHz frequency band, suitable for a cosmological signal around the redshift z ~ 1, as well as a system for detecting transient radio signals (FRB – Fast Radio Burst). It is planned to modify the electronic filters on the dish array receivers over the next few months, in order to observe in the 1300-1400 MHz band, around z=0 – 0.1 . The instrument’s sensitivity would then be sufficient to observe the 21cm signal from the cosmic structures.

FAST (Five-hundred-meter Aperture Spherical Radio Telescope) is the world’s largest radio telescope, with a single 500-meter diameter reflector, built in a natural depression in Guizhou province, China. One of the observation programs is run by our Tianlai collaborators and is dedicated to intensity mapping. It uses the L-band receiver, equipped with 19 horns, covering the 1050-1450 MHz frequency band, divided into three sub-bands.

In the longer term, it will be possible to carry out 21cm intensity mapping programs using the new-generation SKA instruments. Two very large radio telescopes are currently under construction by the SKAO international observatory (SKA: Square Kilometre Array) and should be operational in 2028. The low-frequency instrument (SKA-LOW), which covers the 50-350 MHz frequency band, is located in Australia and is targeted towards the search for signals from the Epoch of Reionization (EoR). The second instrument, SKA-MID, located in South Africa, will be composed of around 200 radio dishes, each around 15 metres in diameter, covering the 350 MHz-15GHz frequency band.