J. Leech

Experimental Investigation of a Low-Cost, High Performance Focal-Plane Horn Array

In previous work, we have described novel smooth-walled multiple flare-angle horns designed using a genetic algorithm. A key feature of these horns is that they can be manufactured very rapidly and cheaply in large numbers, by repeated direct drilling into a single plate of aluminum using a shaped machine tool. The rapid manufacturing technique will enable the construction of very low cost focal-plane arrays, offering an alternative to conventional electroformed corrugated horn arrays. In order to experimentally demonstrate the new technology, we constructed a 230 GHz focal-plane array comprising 37 smooth- walled horns fabricated by direct drilling. We present the measured beam patterns for a large sample of these horns across the array, demonstrating the suitability of our manufacturing techniques for large format arrays. We have measured the cross coupling between adjacent feeds and have shown that it is negligible. We also present high quality beam patterns measured for a much smaller 700 GHz horn, showing the promise of the extending this technology to THz frequencies.

The James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey. I. Star-Forming Molecular Gas in Virgo Cluster Spiral Galaxies

We present large-area maps of the CO J = 3-2 emission obtained at the James Clerk Maxwell Telescope for four spiral galaxies in the Virgo Cluster. We combine these data with published CO J = 1-0, 24 μm, and Hα images to measure the CO line ratios, molecular gas masses, and instantaneous gas depletion times. For three galaxies in our sample (NGC 4254, NGC 4321, and NGC 4569), we obtain molecular gas masses of 7 × 108 – 3 × 109 M ☉ and disk-averaged instantaneous gas depletion times of 1.1-1.7 Gyr. We argue that the CO J = 3-2 line is a better tracer of the dense star-forming molecular gas than the CO J = 1-0 line, as it shows a better correlation with the star formation rate surface density both within and between galaxies. NGC 4254 appears to have a larger star formation efficiency (smaller gas depletion time), perhaps because it is on its first passage through the Virgo Cluster. NGC 4569 shows a large-scale gradient in the gas properties traced by the CO J = 3-2/J = 1-0 line ratio, which suggests that its interaction with the intracluster medium is affecting the dense star-forming portion of the interstellar medium directly. The fourth galaxy in our sample, NGC 4579, has weak CO J = 3-2 emission despite having bright 24 μm emission; however, much of the central luminosity in this galaxy may be due to the presence of a central active galactic nucleus.

The JCMT Nearby Galaxies Legacy Survey – III. Comparisons of cold dust, polycyclic aromatic hydrocarbons, molecular gas and atomic gas in NGC 2403

We used 3.6, 8.0, 70, 160 µm Spitzer Space Telescope data, James Clerk Maxwell Telescope HARP-B COJ =(3-2) data, National Radio Astronomy Observatory 12 meter telescope CO J =(1-0) data, and Very Large Array HI data to investigate the relations among PAHs, cold (� 20 K) dust, molecular gas, and atomic gas within NGC 2403, an SABcd galaxy at a distance of 3.13 Mpc. The dust surface density is mainly a function of the total (atomic and molecular) gas surface density and galactocentric radius. The gas-to-dust ratio monotonically increases with radius, varying from � 100 in the nucleus to � 400 at 5.5 kpc. The slope of the gas-to-dust ratio is close to that of the oxygen abunda nce, suggesting that metallicity strongly affects the gas-to-dust ratio within this galaxy. The exponential scale length of the radial profile for the CO J =(3-2) emission is statistically identical to the scale len gth for the stellar continuum-subtracted 8 µm (PAH 8 µm) emission. However, CO J =(3-2) and PAH 8 µm surface brightnesses appear uncorrelated when examining sub-kpc sized regions.

The James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey. II. Warm molecular gas and star formation in three field spiral galaxies

Original article can be found at: http://iopscience.iop.org/0004-637X/ Copyright American Astronomical Society. [Full text of this article is not available in the UHRA]

The JCMT Nearby Galaxies Legacy Survey – IV. Velocity dispersions in the molecular interstellar medium in spiral galaxies

The definitive version can be found at: http://onlinelibrary.wiley.com/ Copyright Wiley-Blackwell and Royal Astronomical Society

A CO(3–2) survey of a merging sequence of luminous infrared galaxies

Luminous infrared galaxies (L(IR) > 10(11) L(circle dot)) are often associated with interacting galactic systems and are thought to be powered by merger-induced starbursts and/or dust-enshrouded active galactic nucleus. in such systems, the evolution of the dense, star-forming molecular gas as a function of merger separation is of particular interest. Here, we present observations of the CO(3-2) emission from a sample of luminous infrared galaxy mergers that span a range of galaxy-galaxy separations. The excitation of the molecular gas is studied by examining the CO(3-2)/CO( 1 -0) line ratio, r(31), as a function of merger extent. We find these line ratios, r(31), to be consistent with kinetic temperatures of T(k) = (30-50) K and gas densities of n(H2) = 10(3) cm(-3). We also find weak correlations between r(31) and both merger progression and star formation efficiency [L(FIR)/L(CO(1-0))]. These correlations show a tendency for gas excitation to increase as the merger progresses and the star formation efficiency rises. To conclude, we calculate the contributions of the CO(3-2) line to the 850-mu m fluxes measured with SCUBA (Submillimetre Common-User Bolometer Array), which are seen to be significant (similar to 22 per cent).

A Herschel Survey of the [N II] 205 μm Line in Local Luminous Infrared Galaxies: The [N II] 205 μm Emission as a Star Formation Rate Indicator

We present, for the first time, a statistical study of [N II] 205 mu m 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)/f(100)), and the [O III] 88 mu m 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 < 10(12) L-circle dot) 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.

Warm Molecular Gas in Luminous Infrared Galaxies

We present our initial results on the CO rotational spectral line energy distribution (SLED) of the J to J-1 transitions from J = 4 up to 13 from Herschel SPIRE spectroscopic observations of 65 luminous infrared galaxies (LIRGs) in the Great Observatories All-Sky LIRG Survey. The observed SLEDs change on average from one peaking at J <= 4 to a broad distribution peaking around J similar to 6 to 7 as the IRAS 60-to-100 mu m color, C(60/100), increases. However, the ratios of a CO line luminosity to the total infrared luminosity, L-IR, show the smallest variation for J around 6 or 7. This suggests that, for most LIRGs, ongoing star formation (SF) is also responsible for a warm gas component that emits CO lines primarily in the mid-J regime (5 less than or similar to J less than or similar to 10). As a result, the logarithmic ratios of the CO line luminosity summed over CO (5-4), (6-5), (7-6), (8-7) and (10-9) transitions to L-IR, log R-midCO, remain largely independent of C(60/100), and show a mean value of -4.13 ( log R-midCO(SF)) and a sample standard deviation of only 0.10 for the SF-dominated galaxies. Including additional galaxies from the literature, we show, albeit with a small number of cases, the possibility that galaxies, which bear powerful interstellar shocks unrelated to the current SF, and galaxies, in which an energetic active galactic nucleus contributes significantly to the bolometric luminosity, have their R-midCO higher and lower than R-midCO(SF), respectively.

The JCMT Nearby Galaxies Legacy Survey – VIII. CO data and the LCO(3-2)–LFIR correlation in the SINGS sample

The James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey (NGLS) comprises an H i-selected sample of 155 galaxies spanning all morphological types with distances less than 25 Mpc. 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.

Multiple flare-angle horn feeds for sub-mm astronomy and cosmic microwave background experiments

Context. The use of large-format focal plane imaging arrays employing multiple feed horns is becoming increasingly important for the next generation of single dish sub-mm telescopes and cosmology experiments. Such receivers are being commissioned on both general purpose, common user telescopes and telescopes specifically designed for mapping intensity and polarisation anisotropies in the cosmic microwave background (CMB). Telescopes are currently being constructed to map the CMB polarisation that employ hundreds of feeds and the cost of manufacturing these feeds has become a significant fraction of the total cost of the telescope. Aims. We have developed and manufactured low-cost easy-to-machine smooth-walled horns that have a performance comparable to the more traditional corrugated feed horns that are often used in focal plane arrays. Our horns are much easier to fabricate than corrugated horns enabling the rapid construction of arrays with a large number of horns at a very low cost. Methods. Our smooth walled horns use multiple changes in flare angle to excite higher order waveguide modes. They are designed using a genetic algorithm to optimise the positions and magnitudes of these flare angle discontinuities. We have developed a fully parallelised software suite for the optimisation of these horns. We have manufactured prototype horns by traditional electroforming and also by a new direct drilling technique and we have measured their beam patterns using a far-field antenna test range at 230 GHz. Results. We present simulated and measured far-field beam patterns for one of our horn designs. They exhibit low sidelobe levels, good beam circularity and low cross-polarisation levels over a fractional bandwidth of 20%. These results offer experimental confirmation of our design technique, allowing us to proceed confidently in the optimisation of horns with a wider operational bandwidth. The results also show that the new manufacturing technique using drilling is successful, enabling the fabrication of large format arrays by repeatedly drilling into a single aluminium plate. This will enable the construction of focal plane arrays at a very low cost per horn. Conclusions. We have developed a new type of high performance feed horn that is fast and easy to fabricate. Having demonstrated the efficacy of our horn designs experimentally, we are building and testing a prototype focal plane array of 37 hexagonally close packed horns. This prototype array will be an important step towards building a complete CMB mapping receiver using these feed horns.