H i intensity mapping: a single dish approach

We discuss the detection of large scale HI intensity fluctuations using a single dish approach with the ultimate objective of measuring the Baryonic Acoustic Oscillations (BAO) and constraining the properties of dark energy. To characterise the signal we present 3D power spectra, 2D angular power spectra for individual redshift slices, and also individual line-of-sight spectra, computed using the S3 simulated HI catalogue which is based on the Millennium Simulation. We consider optimal instrument design and survey strategies for a single dish observation at low and high redshift for a fixed sensitivity. For a survey corresponding to an instrument with Tsys = 50 K, 50 feed horns and 1 year of observations, we find that at low redshift (z ≈ 0.3), a resolution of ∼ 40 arcmin and a survey of ∼ 5000 deg2 is close to optimal, whereas at higher redshift (z ≈ 0.9) a resolution of ∼ 10 arcmin and ∼ 500 deg2 would be necessary-something which would be difficult to achieve cheaply using a single dish. Continuum foreground emission from the Galaxy and extragalactic radio sources are potentially a problem. In particular, we suggest that it could be that the dominant extragalactic