Greg Engargiola

Giant Molecular Clouds in M33. II. High-Resolution Observations

We present 12CO (J = 1 → 0) observations of 45 giant molecular clouds (GMCs) in M33 made with the BIMA array. The observations have a linear resolution of 20 pc, sufficient to measure the sizes of most GMCs in the sample. We place upper limits on the specific angular momentum of the GMCs and find the observed values to be nearly an order of magnitude below the values predicted from simple formation mechanisms. The velocity gradients across neighboring, high-mass GMCs appear preferentially aligned on scales less than 500 pc. If the clouds are rotating, 40% are counterrotating with respect to the galaxy. GMCs require a braking mechanism if they form from the large-scale radial accumulation of gas. These observations suggest that molecular clouds form locally out of atomic gas, with significant braking by magnetic fields to dissipate the angular momentum imparted by galactic shear. The observed GMCs share basic properties with those found in the Galaxy, such as similar masses, sizes, and line widths, as well as a constant surface density of 120 M☉ pc-2. The size-line width relationship follows ΔV ∝ r0.45±0.02, consistent with that found in the Galaxy. The cloud virial masses imply that the CO-to-H2 conversion factor has a value of 2 × 1020 H2 cm-2 (K km s-1)-1 and does not change significantly over the disk of M33, despite a change of 0.8 dex in the metallicity.

The Allen Telescope Array: The First Widefield, Panchromatic, Snapshot Radio Camera for Radio Astronomy and SETI

The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in northern California. Scientists and engineers are actively exploiting all of the flexibility designed into this innovative instrument for simultaneously conducting surveys of the astrophysical sky and conducting searches for distant technological civilizations. This paper summarizes the design elements of the ATA, the cost savings made possible by the use of commercial off-the-shelf components, and the cost/performance tradeoffs that eventually enabled this first snapshot radio camera. The fundamental scientific program of this new telescope is varied and exciting; some of the first astronomical results will be discussed.

An antenna-coupled bolometer with an integrated microstrip bandpass filter

We describe the fabrication and testing of antenna-coupled superconducting transition-edge bolometers for use at millimeter wavelengths. The design uses a double-slot dipole antenna connected to superconductingniobium microstrip. Band defining filters are implemented in the microstrip, which is then terminated with a load resistor. The power dissipated in the load resistor is measured by a superconducting transition-edge sensor (TES). The load resistor and TES are thermally well connected and are supported by a silicon nitride substrate. The substrate is suspended by four narrow silicon nitride legs for thermal isolation. The bolometers have been optically characterized and the spectral response is presented. This detector is a prototype element for use in an array designed for studies of the cosmic microwave backgroundpolarization.

A dual-polarized broadband planar antenna and channelizing filter bank for millimeter wavelengths

We describe the design, fabrication, and testing of a broadband log-periodic antenna coupled to multiple cryogenic bolometers. This detector architecture, optimized here for astrophysical observations, simultaneously receives two linear polarizations with two octaves of bandwidth at millimeter wavelengths. The broad bandwidth signal received by the antenna is divided into sub-bands with integrated in-line frequency-selective filters. We demonstrate two such filter banks: a diplexer with two sub-bands and a log-periodic channelizer with seven contiguous sub-bands. These detectors have receiver efficiencies of 20-40% and percent level polarization isolation. Superconducting transition-edge sensor bolometers detect the power in each sub-band and polarization. We demonstrate circularly symmetric beam patterns, high polarization isolation, accurately positioned bands, and high optical efficiency. The pixel design is applicable to astronomical observations of intensity and polarization at millimeter through sub-millimeter wavelengths. As compared with an imaging array of pixels measuring only one band, simultaneous measurements of multiple bands in each pixel has the potential to result in a higher signal-to-noise measurement while also providing spectral information. This development facilitates compact systems with high mapping speeds for observations that require information in multiple frequency bands.

Primary Beam and Dish Surface Characterization at the Allen Telescope Array by Radio Holography

The Allen Telescope Array (ATA) is a cm-wave interferometer in California, comprising 42 antenna elements with 6-m diameter dishes. We characterize the antenna optical accuracy using two-antenna interferometry and radio holography. The distortion of each telescope relative to the average is small, with RMS differences of 1% of beam peak value. Holography provides images of dish illumination, characterizing as-built mirror surfaces. Maximal distortions across ~ 2 meter lengths appear to result from mounting stresses or solar radiation. Experimental RMS errors are 0.7 mm at night and 3 mm under worst-case solar illumination. For frequencies 4, 10, and 15 GHz, the nighttime values indicate sensitivity losses of 1, 10 and 20%, respectively. ATAs wide-bandwidth receiver permits observations over a continuous range 0.5-11.2 GHz. We probe the antenna optical gain and beam pattern stability as a function of focus position and observation frequency, concluding that ATA can produce high fidelity images over a decade of simultaneous observation frequencies. We quantify solar heating effects on antenna sensitivity and pointing accuracy. We find that during the day, observations >;5 GHz will suffer some sensitivity loss and it may be necessary to make antenna pointing corrections on a 1-2 hourly basis.

A Log-Periodic Channelizer for Multichroic Antenna-Coupled TES-Bolometers

We have fabricated a dual-polarized multichroic antenna-coupled bolometer prototype pixel for use in focal planes of Cosmic Microwave Background (CMB) polarimetry experiments and for submillimeter observations of high redshift galaxies. We couple millimeter and submillimeter power onto the detector with a dual polarized sinuous antenna that has a continuous bandwidth of 60-250 GHz and we use a contacting extended hemispherical lens to boost the gain high enough to match the optics of a typical telescope. We partition each polarization channel into seen contiguous channels with a channelizer circuit that mimics the physiology of the human ear and then terminate each channel on a separate TES bolometer. Fourier Transform Spectroscopy demonstrates that the bands are near the designed locations with total receiver throughputs of 20-30%.

A dual-polarized multichroic antenna-coupled TES bolometer for terrestrial CMB Polarimetry

We are developing dual-polarized multi-channel antenna-coupled Transition Edge Sensor (TES) Bolometers for Cosmic Microwave Background (CMB) Polarimetry in terrestrial experiments. Each pixel of the array couples incident power into the lithographed microstrip circuits with a dual-polarized broadband planar sinuous antenna whos gain is increased with a contacting extended hemispherical lens. Microstrip filter manifolds partition the two-octave bandwidth into narrow channels before terminating at separate TES bolometers. We describe the design methodology and fabrication methods used, and also the results of optical tests that show high optical throughput in properly located bands, as well as high cross-polarization rejection. We have explored two antenna feeding schemes that result in different quality beams and we comment on the relative merits of each. Finally, we quantify the increases in mapping speed that an array of our multichroic pixels might realize over traditional monochromatic pixels.

Sinuous‐Antenna coupled TES bolometers for Cosmic Microwave Background Polarimetry

We are developing antenna‐coupled TES bolometers for CMB polarimetry that receive both linear polarizations over nearly two octaves of bandwidth. This ultra‐wide bandwidth is achieved with a novel adaptation of the sinuous antenna that integrates microstrip feed‐lines onto the arms of the antenna and uses a contacting extended hemispherical lens to focus the beam. It is challenging to achieve desirable antenna performance over such a wide band and our version of the sinuous antenna offers a unique solution. We have integrated this antenna with TES‐bolometers and report on a series of optical tests that demonstrate the antenna beams’s high symmetry, cross‐polarization rejection, gain, and optical efficiency over the operating band.

Sinuous antennas for cosmic microwave background polarimetry

We are developing antenna-coupled Transition Edge Sensor (TES) bolometers to be used in the focal planes of telescopes mapping Cosmic Microwave Background (CMB) polarization anisotropies. These detectors will be both dual-polarized and ultra-wide band, each containing several frequency channels. Arrays of such detectors could realize mapping speeds nearly an order of magnitude higher than previously deployed technology while naturally facilitating foreground removal. For such detectors to be useful, the antennas must have a high gain and a low cross-polarization. We have designed a novel modification of DuHamels Sinuous antenna that couples to a contacting lens and is driven by integrated microstrip feed-lines. The integrated feed lines allow the antenna to interface with microstrip circuits and bolometers in a way that is planar and scalable to kilo-pixel arrays. We have demonstrated the polarization and beam properties with scale model antennas that operate at 1-12 GHz.

The Allen Telescope Array

The Allen Telescope Array, a joint project between the SETI Institute and the Radio Astronomy Laboratory at the University of California Berkeley, is currently under development and construction at the Hat Creek Radio Observatory in northern California. It will consist of 350 6.1-m offset Gregorian antennas in a fairly densely packed configuration, with minimum baselines of less than 10 m and a maximum baseline of about 900 m. The dual-polarization frequency range spans from about 500 MHz to 11 GHz, both polarizations of which are transported back from each antenna. The first generation processor will provide 32 synthesized beams of 104 MHz bandwidth, eight at each of four tunings, as well as outputs for a full-polarization correlator at two of the tunings at the same bandwidth. This paper provides a general description of the Allen Telescope Array.