Experimental radio cosmology

Abstract:

Here we describe our work designing multiple flare-angle horns, optimised using a genetic algorithm. Several horns designs will be described and experimentally measured beam patterns for horns at 230 GHz will be presented. In addition, we will present new, wide bandwidth horns offering good performance over a ∼30% bandwidth. © 2013 IEEE.

Abstract:

Here we present the design and performance of a novel unilateral finline Nb-AlOx-Nb SIS (Superconductor-Insulator-Superconductor) mixer, operating around 230 GHz, with a target IF bandwidth of 2-13 GHz. The mixer is intended to be used in GUBBINS [1], a prototype high brightness sensitivity, low spatial resolution heterodyne interferometer. A key feature of the mixer design is the use of a unilateral finline taper to provide a smooth transition from high-impedance waveguide to low-impedance microstrip suitable for feeding a 1μm2 SIS junction. The use of a finline transmission line allows other complicated planar circuits to be compactly integrated on the substrate and allows the use of an easy-to-fabricate mixer block. Also the employment of the silicon substrate allows trenches to be fabricated around individual SIS mixer devices on the wafer, allowing the devices to be separated easily without dicing. To realise the wide IF bandwidth, a separate IF matching board, consisting of a few sections of microstrip, was designed to match the dynamic output impedance of SIS mixer to the LNA. In this paper, the SIS mixer design will be described in detail, including the electromagnetic simulations of the passive circuit with HFSS. We have fabricated and tested several of these SIS mixers over RF bandwidth 190-260 GHz. We have obtained a best noise temperature of 75 K at 208.8 GHz over IF bandwidth 4-6 GHz. However, the noise temperature measured across IF bandwidth 2-18 GHz shows that an IF resonance exists around 8 GHz, caused by an excess capacitance due to the large surface area of the microstrip transition and RF matching circuitry. An improved design is described, suitably modified to shift the IF resonance out of the target IF band.

Abstract:

The James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey (NGLS) comprises an Hi-selected sample of 155 galaxies spanning all morphological types with distances less than 25Mpc. We describe the scientific goals of the survey, the sample selection and the observing strategy. We also present an atlas and analysis of the CO J=3 - 2 maps for the 47 galaxies in the NGLS which are also part of the Spitzer Infrared Nearby Galaxies Survey. We find a wide range of molecular gas mass fractions in the galaxies in this sample and explore the correlation of the far-infrared luminosity, which traces star formation, with the CO luminosity, which traces the molecular gas mass. By comparing the NGLS data with merging galaxies at low and high redshift, which have also been observed in the CO J=3 - 2 line, we show that the correlation of far-infrared and CO luminosity shows a significant trend with luminosity. This trend is consistent with a molecular gas depletion time which is more than an order of magnitude faster in the merger galaxies than in nearby normal galaxies. We also find a strong correlation of the LFIR/LCO(3-2) ratio with the atomic-to-molecular gas mass ratio. This correlation suggests that some of the far-infrared emission originates from dust associated with atomic gas and that its contribution is particularly important in galaxies where most of the gas is in the atomic phase. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.