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Scientific paper
Oct 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt........13d&link_type=abstract
PhD dissertation, University of Manchester, 2002.
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1
Scientific paper
In this thesis I describe work related to the investigation of CMB foregrounds, particularly at frequencies of ≈ 30 GHz where the Very Small Array (VSA) operates. During the commissioning period at Tenerife, the VSA was shown to approach the theoretical sensitivity. The first season (1 year) of CMB observations measures the ℓ-range 150-900. The first peak in the CMB power spectrum is clearly detected at ℓ ≈ 220 with Δ T ≈ 80 μK and there is evidence for a second peak at ℓ ≈ 550. At higher ℓ, the fluctuations drop in amplitude to ≈ 30 μK at ℓ ≈ 800, in agreement with current inflationary cosmologies. The VSA results agree exceptionally well with other recent CMB experiments (BOOMERanG, MAXIMA, DASI, CBI). With priors from type Ia supernovae data, we find Ω_{b}h^{2}=0.029_{-0.009}^{+0.009}, n_{s}=1.02_{-0.06}^{+0.12}, Ω_{tot}=1.02_{-0.06}^{+0.08}, Ω_{cdm}h^{2}=0.09_{-0.04}^{+0.05}, Ω_{m}=0.32_{-0.06}^{+0.09} and Ω_{Λ}=0.71_{-0.07}^{+0.07}.
Foreground contamination from residual discrete radio sources is calculated to be less than 2.5% tep{Scott02}. This is achieved by measuring at the same frequency and time, the brightest sources in each VSA region using a dedicated source-subtraction baseline. We have opted to observe at a single frequency therefore it is critical to know the amplitude of Galactic emission. The contamination from diffuse Galactic radiation is calculated to be ≤ 5 μK at angular scales of 1° using r.m.s. predictions from foreground maps. Further to this, I have developed a cross-correlation method to calculate the correlation of the interferometer data with foreground maps. I find no correlation with the 100 μm map tep{Schlegel98}, with an upper limit of 0.84 μK (2σ) in the VSA data.
A full-sky Hα map at 1deg; resolution is produced by combining data from the WHAM and SHASSA surveys; the Hα line is well-known to be a good tracer of free-free emission. A quantitative method of calculating the absorption of Hα by dust is given, based on the 100 μm maps of tet{Schlegel98}. The greatest uncertainty is the fraction of dust, f_{d}, along the line of sight actually absorbing. The conversion from Hα to radio free-free continuum is re-evaluated and equations in the literature are corrected. The conversion can be applied to produce free-free template maps for frequencies 1-1000 GHz with a precision of ≈ 10%.
The free-free template is tested for several regions of the sky using low frequency data. The free-free estimates appear to be too large by up to ˜ 30%. This can be largely explained by variations in the electron temperature. At high Galactic latitudes (|b|>20°), the 408 MHz synchrotron map has ˜ 6% in the form of free-free emission; the correction is therefore important. Finally, the power spectrum of Hα/free-free seems to be much flatter (power-law index of ≤ 2) than previously thought, but is well below the CMB anisotropy for ℓ=10-70.
I also describe work associated with the Manchester Wide-Field Camera (MWFC). The MWFC has a 32° field of view with 7 arcmin resolution which allows large-scale (≥ 1°) nebulosities to be imaged. Hα observations of the Eridanus region are presented and the correlations with the far infrared and X-ray emission are described. There is evidence for a superposition of Hα shells along the line of sight. The camera has been modified to allow stand-alone operation using its own mount and CCD.
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