Experimental radio cosmology

Abstract:

We present an analysis of the diffuse emission at 5 GHz in the first quadrant of the Galactic plane using two months of preliminary intensity data taken with the C-Band All-Sky Survey (C-BASS) northern instrument at the Owens Valley Radio Observatory, California. Combining C-BASS maps with ancillary data tomake temperature-temperature plots, we find synchrotron spectral indices of β = -2.65 ± 0.05 between 0.408 and 5 GHz and β = -2.72 ± 0.09 between 1.420 and 5 GHz for -10° > |b| > -4°, 20° > l > 40°. Through the subtraction of a radio recombination line free-free template, we determine the synchrotron spectral index in the Galactic plane (|b|>4°) to be β =-2.56±0.07 between 0.408 and 5 GHz, with a contribution of 53±8 per cent from free-free emission at 5 GHz. These results are consistent with previous low-frequency measurements in the Galactic plane. By including C-BASS data in spectral fits, we demonstrate the presence of anomalous microwave emission (AME) associated with the HII complexes W43, W44 and W47 near 30 GHz, at 4.4Σ, 3.1Σ and 2.5Σ, respectively. The CORNISH (Co-Ordinated Radio 'N' Infrared Survey for High mass star formation) VLA 5-GHz source catalogue rules out the possibility that the excess emission detected around 30 GHz may be due to ultracompact HII regions. Diffuse AME was also identified at a 4Σ level within 30° > l > 40°, -2° > b > 2° between 5 and 22.8 GHz.

Abstract:

Efficiently mapping large areas of the sky with high spectral resolution at mm and sub-mm wavelengths will require a new generation of heterodyne focal-plane array receivers. The number of pixels in such arrays has not increased particularly rapidly in the last two decades, with maximum achieved pixel numbers between 16 and 64 (e.g. [1] and [2]). Thus new approaches are needed to address such problems as local oscillator (LO) injection, feed horn fabrication and SIS mixer design, fabrication and repeatability.

Here we describe a prototype focal-plane array unit based on unilateral finline SIS mixers, fed with smooth-walled feed horns. LO injection power diplexing is achieved by a combination of directly machined waveguide Y-power splitters and bow-tie cross waveguide couplers. The 1×4 prototype array, currently under construction, will demonstrate several technologies relevant to the construction of large format arrays.

Abstract:

The presence of spurious frequency tones emanating from a harmonically generated local oscillator (LO) signal can significantly degrade the performance of a SuperconductorInsulator-Superconductor (SIS) tunnel junction mixer [1]. The existence of harmonics is often revealed by distorted I-V curves obtained when pumping the SIS device with the LO. We have investigated this effect by using a commercially available source and correlating the photon step induced I-V curve structure with a direct frequency measurement made via a Fourier Transform Spectrometer (FTS) and a spectrum analyser. Our results suggest that intermixing frequency products are generated within the first, low frequency, stage of the LO chain, and these unwanted signals are converted to high frequency tones through subsequent stages of LO amplification and frequency up-conversion. The experimental work has been performed by using measurement facilities available at the University of Oxford and the Rutherford Appleton Laboratory (RAL), UK.