Jamie Leech

Abstract:

Modern ground-based radio-telescopes have stringent requirements on the receiver's feed-horn performance. These include a bandwidth covering a full atmospheric window from good sites (∼ 30%), low cross polarization and high beam circularity. In addition the unprecedented sensitivity required by recent proposed experiments requires large format focal plane arrays consisting of a large number of high performance feeds. While these feeds are straightforward to fabricate at microwave frequencies, the tight tolerances required in the THz region makes the realization of a large format array both expensive and time consuming. In this paper we describe feed designs that can be fabricated cheaply and rapidly without compromising the science requirements within the operating bandwidth. We present simulated and measured far-field beam patterns showing low sidelobe levels, good beam circularity and low cross-polarization levels over a fractional bandwidth of 20%. Having demonstrated the efficacy of our horn designs and fabricating technique experimentally, we have designed, fabricated and tested a prototype focal plane array of 37 hexagonally close packed horns milled in a single block of aluminum. Experimental testing of the radiation patterns of the array feeds demonstrated that they were essentially identical to the patterns of the feeds fabricated individually and that cross coupling between adjacent feeds was negligible. © 2013 IEEE.

Abstract:

We present, for the first time, a statistical study of [N II] 205 μm line emission for a large sample of local luminous infrared galaxies using Herschel Spectral and Photometric Imaging Receiver Fourier Transform Spectrometer (SPIRE FTS) data. For our sample of galaxies, we investigate the correlation between the [N II] luminosity (L [N II]) and the total infrared luminosity (L IR), as well as the dependence of L [N II]/L IR ratio on L IR, far-infrared colors (IRAS f 60/f100), and the [O III] 88 μm to [N II] luminosity ratio. We find that L [N II] correlates almost linearly with L IR for non-active galactic nucleus galaxies (all having L IR < 1012 L ⊙) in our sample, which implies that L [N II] can serve as a star formation rate tracer which is particularly useful for high-redshift galaxies that will be observed with forthcoming submillimeter spectroscopic facilities such as the Atacama Large Millimeter/submillimeter Array. Our analysis shows that the deviation from the mean L [N II]-L IR relation correlates with tracers of the ionization parameter, which suggests that the scatter in this relation is mainly due to the variations in the hardness, and/or ionization parameter, of the ambient galactic UV field among the sources in our sample. © 2013. The American Astronomical Society. All rights reserved..

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.