The understanding and removal of CMB foregrounds has become an important topic in CMB data analysis. Foregrounds are ANY emissions that confuse the primordial CMB signal after the time of last scattering. The study of CMB foregrounds is critical in obtaining accurate CMB power spectrum measurements. They include extragalactic discrete point sources, synchrotron emission, free-free emission, dust (vibrational) emission, spinning dust emission and atmospheric emission. The level at which foreground contributes to CMB observations depends on the observing frequency and angular scale. The study of CMB foregrounds is critical in obtaining accurate CMB power spectrum measurements. Furthermore, the study of each foreground reveals interesting information about our Galaxy and other Galaxies. At Jodrell Bank, we are working on many aspects of foregrounds, including:
Foreground estimates for CMB observations
At Jodrell Bank, we have made much progress in understanding the foreground emissions. The Tenerife experiments at frequencies of 10, 15 and 33 GHz (Gutierrez et al., 2000) have been crucial not only in providing CMB power spectra measurements, but also in measuring the foregrounds at these relatively unexplored frequency bands. With the COBE-DMR data (Smoot et al. 1992), the Tenerife data has been critical in understanding the anomalous emission (de Oliveira-Costa et al, 2000), attributed to spinning dust grains (Drainge & Lazarian, 1998). The Jodrell 5 GHz broad-band interferometer has also helped in determining the spectral index of Galactic emission.
The results of the first season of VSA observations show that the effect of Galactic foregrounds at 34 GHz, for the fields observed with the VSA over angular scales of 2 degrees - 0.4 arcmin, are approx. 1 micro Kelvin, compared to a CMB signal of 20-80 micro Kelvins. This results in us only needing to observe at a single frequency.
At Jodrell Bank, we are particularly interested in producing the "best" foreground templates for each of the Galactic foregrounds. Fig. 1 shows an improved version of the widely-used Haslam et al. 408 MHz map of the sky. Davies et al. (1996) removed the hundreds of radio sources on the map and reduced the effect of striations, an artifact of scanning the sky with a radio telescope. More recently, we have produced for the first time, a free-free template for the whole-sky as shown in Fig. 2. The map is a composite map made from two Halpha surveys; the Wisconsin Halpha Mapper (WHAM) and the Southern H-Alpha Sky Survey Atlas (SHASSA). The optical Halpha line is well-known to be a tracer of free-free radio continuum emission. We have our own instrument for measuring the large-scale distribution of Halpha; the Manchester Wide-Field Camera (MWFC). With a 32 degree field of view and 7 arcmin resolution it is ideal for covering large-areas with adequate resolution for upcoming CMB experiments. However, dust absorption can reduce the Halpha intensity, particularly near or on the Galactic plane. We have produced a first-order absorption template based on the 100 micron dust template (Schlegel, Finkbeiner & Davis, 1998). For more details, see Dickinson, Davies & Davis (2002). This template has many uses including testing the Halpha/free-free relations, correcting low-frequency synchrotron templates, calculating the power spectrum of free-free emission.
In collaboration with Anthony Banday and Krystof Gorski, we have used this free-free template to investigate the contribution of free-free emission on the COBE-DMR data. In particularly, we studied the spectral behaviour of the anomalous emission detected with COBE-DMR (Kogut et al. 1996. This work continued with new data from NASA's WMAP satellite. An analysis of different high-latitude emission features lead to interesting results both for the anomalous dust-correlated foreground as well as for the free-free emission.
Very Small Array
We are using the Very Small Array to help us understand Galactic emission at frequencies of about 30 GHz. As well as the foreground estimates from the deep CMB observations, we have made separate pointed observations of particular regions of the sky. These include regions of higher dust emission, to investigate the recently proposed spinning dust grains to account for the excess emission.
We have also observed a portion of the Galactic plane with the VSA, requiring many overlapping pointings. The data is currently being analyzed. Combining the data with other multi-wavelength data, including Radio Recombination Line data from the HIPASS survey, will give us detailed information about the structure of our Galaxy.