21 cm observation of large-scale structures at

Context. Large-scale structures (LSS) in the universe can be traced using the neutral atomic hydrogen HI through its 21 cm emission. Such a 3D matter distribution map can be used to test the cosmological model and to constrain the dark energy properties or its equation of state. A novel approach, called intensity mapping, can be used to map the HI distribution, using radio interferometers with a large instantaneous field of view and waveband. Aims. We study the sensitivity of different radio interferometer configurations, or multi-beam instruments for observing LSS and baryon acoustic oscillations (BAO) in 21 cm, and we discuss the problem of foreground removal. Methods. For each configuration, we determined instrument response by computing the (u, v) or Fourier angular frequency plane coverage using visibilities. The (u, v) plane response determines the noise power spectrum, hence the instrument sensitivity for LSS P(k) measurement. We also describe a simple foreground subtraction method of separating LSS 21 cm signal from the foreground due to the galactic synchrotron and radio source emission. Results. We have computed the noise power spectrum for different instrument configurations, as well as the extracted LSS power spectrum, after separating the 21 cm-LSS signal from the foregrounds. We have also obtained the uncertainties on the dark energy parameters for an optimized 21 cm BAO survey. Conclusions. We show that a radio instrument with few hundred simultaneous beams and a collecting area of ~10 000 m2 will be able to detect BAO signal at redshift z ~ 1 and will be competitive with optical surveys