B. Rownd

The global rate and efficiency of star formation in spiral galaxies as a function of morphology and environment

CCD images of Ha: and R-band emission in 120 spiral galaxies were obtained using the now-retired No. 1-0.9 m telescope of Kitt Peak National Observatory. These images were used to derive the distribution and total flux of continuum-subtracted Ha: line emission, and therefore the Ha: surface brightnesses and high mass star formation rates in these galaxies. We find a small but significant variation in the mean Ha surface brightness for spiral galaxies along the Hubble sequence; the Sd-Ir galaxies exhibit a mean Ha surface brightness 1.4 times higher than the Sbc-Scd galaxies, and 2-3 times higher than the Sa-Sb galaxies. Estimates for the total formation rate for high mass stars have been compared with global molecular gas masses to determine the global efficiency of high mass star formation as a function of morphological type and environment. We find that the mean efficiency of high mass star formation in this sample of spiral galaxies shows little dependence on morphological type for galaxies of type Sa through Scd, although there is a wide range in star formation efficiencies within each type. Galaxies in disturbed environments (i.e., strongly interacting systems) are found to have a mean star formation efficiency ~4 times higher than in isolated spiral galaxies, uncorrected for extinction. This confirms previous findings (Young et al 1986a,b; Sanders et al 1986; Solomon & Sage 1988; Tinney et al 1990), based on the far-inffared luminosity rather than the Ha luminosity to trace the rate of high mass star formation, that the mean star formation efficiency among isolated galaxies is significantly lower than that among interacting systems. This result provides further confirmation that the rate of high mass star formation is reasonably well traced by both the Ha and the IR luminosity in spiral galaxies.

A three-stage helium sorption refrigerator for cooling of infrared detectors to 280 mK

Abstract We review some of the applications of submillimetre instruments for astronomy and Earth atmospheric sciences where helium sorption refrigerators have been used. We describe a three-stage helium refrigerator which uses a 4He–3He double-stage refrigerator to precool and intercept the parasitic heatload on a final 3He single-stage refrigerator. This multi-stage configuration permits operation of the refrigerator system without externally pumping on a 4He bath. This results in simpler integration with the instrument and allows scientific observations to be more easily made. We have achieved a base temperature of 234 mK for a hold time of 20.4 h. The heat lift at 280 mK is 15 μW.

A Determination of the Hubble Constant Using Measurements of X-Ray Emission and the Sunyaev-Zeldovich Effect at Millimeter Wavelengths in the Cluster Abell 1835

We present a determination of the Hubble constant and central electron density in the cluster Abell 1835 (z = 0.2523) from measurements of X-ray emission and millimeter-wave observations of the Sunyaev-Zeldovich (S-Z) effect with the Sunyaev-Zeldovich Infrared Experiment (SuZIE) multifrequency array receiver. Abell 1835 is a well studied cluster in the X-ray with a large central cooling flow. Using a combination of data from ROSAT PSPC and HRI images and millimeter wave measurements we fit a King model to the emission from the ionized gas around Abell 1835 with θ0 = 0farcm22 ± 0farcm02 and β = 0.58 ± 0.02. Assuming the cluster gas to be isothermal with a temperature of 9.8img1.gif keV, we find a y-parameter of 4.9 ± 0.6 × 10-4 and a peculiar velocity of 500 ± 1000 km s-1 from measurements at three frequencies, 145, 221, and 279 GHz. Combining the S-Z measurements with X-ray data, we determine a value for the Hubble constant of H0 = 59img2.gif km s-1 Mpc-1 and a central electron density for Abell 1835 of ne0 = 5.64img3.gif × 10-2 cm-3 assuming a standard cosmology with Ωm = 1 and ΩΛ = 0. The error in the determination of the Hubble constant is dominated by the uncertainty in the temperature of the X-ray emitting cluster gas.

Design and performance of feedhorn-coupled bolometer arrays for SPIRE

This paper reviews the design, modeling, and testing of feedhorn arrays coupled to bolometric detector arrays being developed for the ESA Herschel Space Observatorys SPIRE instrument. SPIRE will incorporate five arrays of silicon nitride micromesh bolometers, in three broadband photometers and two Fourier-Transform spectrometers covering 200-700 μm, with a total of 326 feedhorn-coupled bolometers. The precision feedhorn arrays are formed by close-packing individually fabricated conical feedhorns, which terminate in waveguides and integrating cavities. The detector array is efficiently packaged by mounting it between a metallized silicon backshort array and the feedhorn array, which encloses the bolometers in precisely tuned integrating cavities. The absorption efficiency, bandwidth, and cross talk were first investigated with numerical simulations of the electromagnetic fields, and then measured for prototype arrays in a test facility. This discussion describes the design goals, simulations, fabrication, and measurements of optical efficiencies, spectral properties, beam shapes, and cross talk between bolometers.