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 Hα data of both the northern and southern skies as the basis for such a template.

The first step is to correct the Hα maps for dust absorption using the 100-μm 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 Hα map can be produced for ~95 per cent of the sky; the area for which a template cannot be recovered is the Galactic plane area |b| < 5°, l= 260°-0°-160° and some isolated dense dust clouds at intermediate latitudes.

The second step is to convert the dust-corrected Hα 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 3000-20000 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 HII 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.