Mark Dragovan

Interferometric Observation of Cosmic Microwave Background Anisotropies

We present a formalism for analyzing interferometric observations of the cosmic microwave background anisotropy and polarization. The formalism is based on the l-space expansion of the angular power spectrum favored in recent years. Explicit discussions of maximum likelihood analysis, power spectrum reconstruction, parameter estimation, imaging, and polarization are given. As an example, several calculations for the Degree Angular Scale Interferometer and Cosmic Background Interferometer experiments are presented.

Measurement of polarization with the Degree Angular Scale Interferometer

Measurements of the cosmic microwave background (CMB) radiation can reveal with remarkable precision the conditions of the Universe when it was ∼400,000 years old. The three most fundamental properties of the CMB are its frequency spectrum (which determines the temperature), and the fluctuations in both the temperature and polarization across a range of angular scales. The frequency spectrum has been well determined, and considerable progress has been made in measuring the power spectrum of the temperature fluctuations. But despite many efforts to measure the polarization, detection of this property of the CMB has hitherto been beyond the reach of even the most sensitive observations. Here we describe the Degree Angular Scale Interferometer (DASI), an array of radio telescopes, which for the past two years has conducted polarization-sensitive observations of the CMB from the Amundsen–Scott South Pole research station.

Detection of submillimeter polarization in the Orion nebula

Linear polarization has been observed in the submillimeter radiation (270 microns) from two regions of Orion: one centered in the Kleinmann-Low nebula and one centered 1.5 arcmin south of the nebula. The observations were performed in September of 1983 and January of 1984 with the NASA Kuiper Airborne Observatory (KAO). The polarizations measured for the two regions were both 1.7 percent, plus or minus 0.4 and 0.5 percent, respectively. The angle of the outflow from both sources was 27 degrees, plus or minus seven degrees. An upper limit for polarization in the submillimeter radiation from the nebula W3(OH) was established at 1.6 percent. The observational data are compared with results from several other recent polarimetric observations of Orion, and some of their implications are discussed.

Z-Spec: a broadband millimeter-wave grating spectrometer: design, construction, and first cryogenic measurements

We present the design, integration, and first ryogenic testing of our new broad-band millimeter-wave spectrometer, Z-Spec. Z-Spec uses a novel architecture called WaFIRS (Waveguide Far-IR Spectrometer), which employs a curved diffraction grating in a parallel-plate waveguide propagation medium. The instrument will provide a resolving power betwee 200 and 350 across an instantaneous bandwidth of 190-310 GHz, all packaged within a cryostat that is of order 1 meter in size. For background-limited astronomical observations in the 1mm terrestrial window, Z-Spec uses 160 silicon nitride micro-mesh bolometers and the detectors and waveguide grating are cooled to ~0.1 K. Our first cryogenic measurements at 225 GHz show resolving power greater than 200, and the end-to-end throughput is estimated to be greater than 30%, possibly as high as 40%. Z-Spec represents the first systematic approach to cosmological redshift measurement that is not based on optical or near-IR identifications. With its good sensitivity and large bandwidth, Z-Spec provides a new capability for millimeter-wave astrophysics. The instrument will be capable of measureing rotational carbon monoxide line emission from bright dusty galaxies at redshifts of up to 4, and the broad bandwidth insures that at least two lines will be simultaneously detected, providing an unambiguous redshift determination. In addition to Z-Specs observations over the next 1-3 years, the WaFIRS spectrometer architecture makes an excellent candidate for mid-IR to millimeter-wave spectrometers on future space-borned and suborbital platforms such as SPICA and SAFIR. The concept is dramatically more compact and lightweight than conventional free-space grating spectrometers, and no mirrors or lenses are used in the instrument. After the progress report on Z-Spec we highlight this capability.

Anisotropy in the cosmic microwave background at degree angular scales: Python V results

Observations of the microwave sky using the Python telescope in its fifth season of operation at the Amundsen-Scott South Pole Station in Antarctica are presented. The system consists of a 0.75 m off-axis telescope instrumented with a HEMT amplifier-based radiometer having continuum sensitivity from 37 to 45 GHz in two frequency bands. With a 091 × 102 beam, the instrument fully sampled 598 deg2 of sky, including fields measured during the previous four seasons of Python observations. Interpreting the observed fluctuations as anisotropy in the cosmic microwave background, we place constraints on the angular power spectrum of fluctuations in eight multipole bands up to l ~ 260. The observed spectrum is consistent with both the COBE experiment and previous Python results. There is no significant contamination from known foregrounds. The results show a discernible rise in the angular power spectrum from large (l ~ 40) to small (l ~ 200) angular scales. The shape of the observed power spectrum is not a simple linear rise, but has a sharply increasing slope starting at l ~ 150.

High-Energy Gamma-Ray Observations of the Crab Nebula and Pulsar with the Solar Tower Atmospheric Cerenkov Effect Experiment

The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is a new ground-based atmospheric Cherenkov telescope for gamma-ray astronomy. STACEE uses the large mirror area of a solar heliostat facility to achieve a low energy threshold. A prototype experiment which uses 32 heliostat mirrors with a total mirror area of ~ 1200\unit{m^2} has been constructed. This prototype, called STACEE-32, was used to search for high energy gamma-ray emission from the Crab Nebula and Pulsar. Observations taken between November 1998 and February 1999 yield a strong statistical excess of gamma-like events from the Crab, with a significance of

WaFIRS: a waveguide far-IR spectrometer enabling spectroscopy of high-z galaxies in the far-IR and submillimeter

+6.75\sigma

Z-Spec: a broadband, direct-detection, millimeter-wave spectrometer

in 43 hours of on-source observing time. No evidence for pulsed emission from the Crab Pulsar was found, and the upper limit on the pulsed fraction of the observed excess was E_{th}) = (2.2 \pm 0.6 \pm 0.2) \times 10^{-10}\unit{photons cm^{-2} s^{-1}}. The observed flux is in agreement with a continuation to lower energies of the power law spectrum seen at TeV energies.

ANISOTROPY IN THE MICROWAVE SKY AT INTERMEDIATE ANGULAR SCALES

The discovery of galaxies beyond z~1 which emit the bulk of their luminosity at long wavelengths has demonstrated the need for high-sensitivity, broad-band spectroscopy in the far-IR/submm/mm bands. Because many of these sources are not detectable in the optical, long-wavelength spectroscopy is key to measuring their redshifts and ISM conditions. The continuum source list will increase in the coming decade with new ground-based instruments (SCUBA2, Bolocam, MAMBO), and the surveys of HSO and SIRTF. Yet the planned spectroscopic capabilities lag behind, in part due to the difficulty in scaling existing IR spectrograph designs to longer wavelengths. To overcome these limitations, we are developing WaFIRS, a novel concept for long-wavelength spectroscopy which utilizes a parallel-plate waveguide and a curved diffraction grating. WaFIRS provides the large (~60%) instantaneous bandwidth and high throughput of a conventional grating system, but offers a dramatic reduction in volume and mass. WaFIRS requires no space overheads for extra optical elements beyond the diffraction grating itself, and is two-dimensional because the propagation is confined between two parallel plates. Thus several modules could be stacked to multiplex either spatially or in different frequency bands. The size and mass savings provide opportunities for spectroscopy from space-borne observatories which would be impractical with traditional spectrographs. With background-limited detectors and a cooled 3.5 m telescope, the line sensitivity would be comparable to that of ALMA, with instantaneous broad-band coverage. We present the spectrometer concept, performance verification with a mm-wave prototype, and our progress toward a cryogenic astronomical instrument

The science case and mission concept for the Single Aperture Far-Infrared (SAFIR) Observatory

Z-Spec is a broadband (195 - 310 GHz), direct-detection, millimeter-wave spectrometer with moderate resolution (R ~ 350) that we are building to observe CO rotational lines and atomic fine-structure lines in the recently discovered population of submillimeter galaxies. A large fraction of these sources cannot be identified optically and thus redshift determination is extremely difficult. The large instantaneous bandwidth of Z-Spec will allow measurement of redshifts up to z~4 via detection of two or more CO lines in a single spectrum. The spectrometer is based on a parallel-plate waveguide grating architecture that is substantially more compact than a conventional free-space grating system. The spectrometer and an array of 160 silicon nitride micromesh bolometers will be cooled to 100 mK to provide background-limited sensitivity. In addition to measuring the redshifts of sources discovered in submillimeter continuum surveys, Z-Spec will demonstrate a novel spectrometer concept well-suited for future far-infrared space missions.