Radio emission from early‐type galaxies and cosmic microwave background experiments

We investigate the possible contribution from the emission of accretion flows around supermassive black holes in early‐type galaxies to current measurements of the cosmic microwave background (CMB) at radio frequencies. We consider a range of luminosities suggested by targeted radio observations and accretion models, and compute the residual contribution of these sources to the spectrum and bispectrum of the observed CMB maps. As for high‐resolution CMB experiments, we find that the unresolved component of these sources could make up to ∼40–50 per cent of the observed Cosmic Background Imager (CBI) and Berkeley Illinois Maryland Association (BIMA) array power spectrum at l > 2000 . As a consequence, the inferred σ SZ 8 value could be biased high by up to 6–7 per cent. As for all‐sky experiments, we find that the contribution of accretion‐flow sources to the Wilkinson Microwave Anisotropy Probe ( WMAP ) bispectrum is at the 2–3 per cent level at most. At the flux limit that Planck will achieve, however, these sources may contribute up to 15 per cent of the bispectrum in the 60–100 GHz frequency range. Moreover, Planck should detect hundreds of these sources in the 30–300 GHz frequency window. These detections, possibly coupled with galaxy type confirmation from optical surveys, will allow number counts to put tighter constraints on the radio luminosity and accretion‐flow properties of early‐type galaxies. These sources may also contribute up to the 30 per cent level to the residual radio sources power spectrum in future high‐resolution Sunyaev–Zeldovich (SZ) surveys (like the Atacama Cosmology Telescope or the Atacama Pathfinder Experiment) reaching mJy flux limits.