Towards a free-free template for CMB foregrounds

A full-sky template map of the Galactic free-free foreground emission component is increasingly important for high-sensitivity cosmic microwave background (CMB) experiments. We use the recently published H data of both the northern and southern skies as the basis for such a template. The first step is to correct the Halpha maps for dust absorption using the 100-mum dust maps of Schlegel, Finkbeiner & Davis. We show that for a range of longitudes, the Galactic latitude distribution of absorption suggests that it is 33 per cent of the full extragalactic absorption. A reliable absorption-corrected Halpha map can be produced for similar to95 per cent of the sky; the area for which a template cannot be recovered is the Galactic plane area | b | < 5 degrees, l = 260 degrees-0 degrees-160 degrees and some isolated dense dust clouds at intermediate latitudes. The second step is to convert the dust-corrected H alpha data into a predicted radio surface brightness. The free-free emission formula is revised to give an accurate expression (1 per cent) for the radio emission covering the frequency range 100 MHz-100 GHz and the electron temperature range 3000-20 000 K. The main uncertainty when applying this expression is the variation of electron temperature across the sky. The emission formula is verified in several extended H II regions using data in the range 408-2326 MHz. A full-sky free-free template map is presented at 30 GHz; the scaling to other frequencies is given. The Haslam et al. all-sky 408-MHz map of the sky can be corrected for this free-free component, which amounts to a 6 per cent correction at intermediate and high latitudes, to provide a pure synchrotron all-sky template. The implications for CMB experiments are discussed.