Extracting H I cosmological signal with generalized needlet internal linear combination

H I intensity mapping is a new observational technique to map fluctuations in the large-scale structure of matter using the 21 cm emission line of atomic hydrogen (H I). Sensitive HI intensity mapping experiments have the potential to detect Baryon Acoustic Oscillations at low redshifts (z less than or similar to 1) in order to constrain the properties of dark energy. Observations of the HI signal will be contaminated by instrumental noise and, more significantly, by astrophysical foregrounds, such as Galactic synchrotron emission, which is at least four orders of magnitude brighter than the HI signal. Foreground cleaning is recognized as one of the key challenges for future radio astronomy surveys. We study the ability of the Generalized Needlet Internal Linear Combination (GNILC) method to subtract radio foregrounds and to recover the cosmological HI signal for a general HI intensity mapping experiment. The GNILC method is a new technique that uses both frequency and spatial information to separate the components of the observed data. Our results show that the method is robust to the complexity of the foregrounds. For simulated radio observations including HI emission, Galactic synchrotron, Galactic free-free, radio sources, and 0.05 mK thermal noise, we find that the GNILC method can reconstruct the HI power spectrum for multipoles 30 < l < 150 with 6 per cent accuracy on 50 per cent of the sky for a redshift z similar to 0.25.