Frank J. Low

The Spitzer Space Telescope Mission

The Spitzer Space Telescope, NASAs Great Observatory for infrared astronomy, was launched 2003 August 25 and is returning excellent scientific data from its Earth-trailing solar orbit. Spitzer combines the intrinsic sensitivity achievable with a cryogenic telescope in space with the great imaging and spectroscopic power of modern detector arrays to provide the user community with huge gains in capability for exploration of the cosmos in the infrared. The observatory systems are largely performing as expected, and the projected cryogenic lifetime is in excess of 5 years. This paper summarizes the on-orbit scientific, technical, and operational performance of Spitzer. Subsequent papers in this special issue describe the Spitzer instruments in detail and highlight many of the exciting scientific results obtained during the first 6 months of the Spitzer mission.

THE MULTIBAND IMAGING PHOTOMETER FOR SPITZER (MIPS)

The Multiband Imaging Photometer for Spitzer (MIPS) provides long-wavelength capability for the mission in imaging bands at 24, 70, and 160 ?m and measurements of spectral energy distributions between 52 and 100 ?m at a spectral resolution of about 7%. By using true detector arrays in each band, it provides both critical sampling of the Spitzer point-spread function and relatively large imaging fields of view, allowing for substantial advances in sensitivity, angular resolution, and efficiency of areal coverage compared with previous space far-infrared capabilities. The 24 ?m array has excellent photometric properties, and measurements with rms relative errors of about 1% can be obtained. The two longer-wavelength arrays use detectors with poor photometric stability, but a system of onboard stimulators used for relative calibration, combined with a unique data pipeline, produce good photometry with rms relative errors of less than 10%.

NICMOS Imaging of Infrared-Luminous Galaxies

We present near-infrared images obtained with the Hubble Space Telescope NICMOS camera for a sample of nine luminous [LIGs: LIR(8?1000 ?m) ? 1011 L?] and 15 ultraluminous (ULIGS: LIR ? 1012 L?) infrared galaxies. The sample includes representative systems classified as warm (f25 ?m/f60 ?m > 0.2) and cold (f25 ?m/f60 ?m ? 0.2) based on the mid-infrared colors and systems with nuclear emission lines classified as H II (i.e., starburst), QSO, Seyfert, and LINER. The morphologies of the sample galaxies are diverse and provide further support for the idea that they are created by the collision or interactions of spiral galaxies. Although no new nuclei are seen in the NICMOS images, the NICMOS images do reveal new spiral structures, bridges, and circumnuclear star clusters. The colors and the luminosities of the observed clusters are consistent with them being young (107?108 yr), formed as a result of galactic interactions, and having masses much greater than those of Galactic globular clusters. In NGC 6090 and VV 114, they are preferentially situated along the area of overlap of the two galactic disks. With the exception of IR 17208-0018, all of the ULIGs have at least one compact (2.2??m FWHM ? 200 pc) nucleus. Analysis of the near-infrared colors (i.e., m1.1?1.6 vs. m1.6?2.2) derived from 11 diameter apertures suggests that the warm galaxies have near-infrared colors consistent with QSO+hot dust emission and the cold galaxies, as a group, have near-infrared colors consistent with reddened starlight. In addition, the cold ULIG UGC 5101 (and possibly three others) have near-infrared colors suggesting additional active galactic nucleus?like near-infrared components in their nuclei. In a 2 kpc diameter aperture measurement, the global colors of all of the cold galaxies except UGC 5101 are consistent with starlight with a few magnitudes of visual extinction. The general dichotomy of the near-infrared properties of the warm and the cold galaxies are further supported by the light distributions: seven of the eight warm galaxies have unresolved nuclear emission that contributes significantly (i.e., ?30%?40%) to the total near-infrared luminosity. The smooth, more extended light observed in all of the galaxies is most likely composed of giant and supergiant stars, but evidence at longer wavelengths suggests that these stars contribute little to the high 8?1000 ?m luminosity of these galaxies. Finally, light profiles of nine of the 24 systems were fitted well by an r1/4 law (and not so well by an exponential disk profile). Whether these star systems eventually become massive central bulges or giant elliptical galaxies will depend on how efficiently the present ISM is converted into stars.

Low-Temperature Germanium Bolometer

A bolometer, using gallium-doped single crystal germanium as the temperature-sensitive resistive element, has been constructed and operated at 2°K with a noise equivalent power of 5×10−13 w and a time constant of 400 μsec. Sensitivities approaching the limits set by thermodynamics have been achieved, and it is shown that the background radiation limited or BLIP condition can be satisfied at 4.2°K. An approximate theory is developed which describes the performance of the device and aids in the design of bolometers with specific properties. The calculated noise equivalent power at 0.5°K, for a time constant of 10−3 sec, is 10−15 w. The detector is suitable for use in both infrared and microwave applications.

9.7 μm Silicate Features in Active Galactic Nuclei: New Insights into Unification Models

We describe observations of 9.7 μm silicate features in 97 AGNs, exhibiting a wide range of AGN types and of X-ray extinction toward the central nuclei. We find that the strength of the silicate feature correlates with the H I column density estimated from fitting the X-ray data, such that low H I columns correspond to silicate emission, while high columns correspond to silicate absorption. The behavior is generally consistent with unification models in which the large diversity in AGN properties is caused by viewing-angle-dependent obscuration of the nucleus. Radio-loud AGNs and radio-quiet quasars follow roughly the correlation between H I columns and the strength of the silicate feature defined by Seyfert galaxies. The agreement among AGN types suggests a high-level unification with similar characteristics for the structure of the obscuring material. We demonstrate the implications for unification models qualitatively with a conceptual disk model. The model includes an inner accretion disk (<0.1 pc in radius), a middle disk (0.1-10 pc in radius) with a dense diffuse component and with embedded denser clouds, and an outer clumpy disk (10-300 pc in radius).

The discovery of dust trails in the orbits of periodic comets

Analysis of data from the Infrared Astronomical Satellite has yielded evidence for narrow trails of dust coincident with the orbits of periodic comets Tempel 2, Encke, and Gunn. Dust was found both ahead of and behind the orbital positions of these comets. This dust was produced by the low-velocity ejection of large particles during perihelion passage. More than 100 additional dust trails are suggested by the data, almost all near the detection limits of the satellite. Many of these dust trails may be derived from previously unobserved comets.

EXPLORING TERRESTRIAL PLANET FORMATION IN THE TW HYDRAE ASSOCIATION

Spitzer Space Telescope infrared measurements are presented for 24 members of the TW Hya association (TWA). High signal-to-noise ratio 24 μm photometry is presented for all these stars, including 20 stars that were not detected by IRAS. Among these 20 stars, only a single object, TWA 7, shows excess emission at 24 μm at the level of only 40% above the stars photosphere. TWA 7 also exhibits a strong 70 μm excess that is a factor of 40 brighter than the stellar photosphere at this wavelength. At 70 μm, an excess of similar magnitude is detected for TWA 13, although no 24 μm excess was detected for this binary. For the 18 stars that failed to show measurable IR excesses, the sensitivity of the current 70 μm observations does not rule out substantial cool excesses at levels 10-40 times above their stellar continua. Measurements of two T Tauri stars, TW Hya and Hen 6-300, confirm that their spectacular IR spectral energy distributions (SEDs) do not turn over even by 160 μm, consistent with the expectation for their active accretion disks. In contrast, the Spitzer data for the luminous planetary debris systems in the TWA, HD 98800B and HR 4796A, are consistent with single-temperature blackbody SEDs and agree with previous IR, submillimeter, and millimeter measurements. The major new result of this study is the dramatic bimodal distribution found for the association in the form of excess emission at a wavelength of 24 μm, indicating negligible amounts of warm (100 K) dust and debris around 20 of 24 stars in this group of very young stars. This bimodal distribution is especially striking given that the four stars in the association with strong IR excesses are 100 times brighter at 24 μm than their photospheres. Clearly, two terrestrial planetary systems, HD 98800B and HR 4796A, exist in some form. In addition, there are at least two active accreting objects, TW Hya and Hen 6-300, that may still be forming planetesimals. The remaining stars may possess significant amounts of cold dust, as in TWA 7 and 13, that have yet to be found.

Aromatic Features in AGNs: Star-forming Infrared Luminosity Function of AGN Host Galaxies

We describe observations of aromatic features at 7.7 and 11.3 μm in AGNs of three types, including PG, 2MASS, and 3CR objects. The feature has been demonstrated to originate predominantly from star formation. Based on the aromatic-derived star-forming luminosity, we find that the far-IR emission of AGNs can be dominated by either star formation or nuclear emission; the average contribution from star formation is around 25% at 70 and 160 μm. The star-forming infrared luminosity functions of the three types of AGNs are flatter than those of field galaxies, implying that nuclear activity and star formation tend to be enhanced together. The star-forming luminosity function is also a function of the strength of nuclear activity from normal galaxies to the bright quasars, with luminosity functions becoming flatter for more intense nuclear activity. Different types of AGNs show different distributions in the level of star formation activity, with 2MASS > PG > 3CR star formation rates.

The internal powers and effective temperatures of Jupiter and Saturn.

Internal powers and effective temperatures of Jupiter and Saturn measured by aircraft mounted telescope, noting restraints on planetary structure and atmosphere models

The 2.8-14-micron spectrum of Jupiter.

Jupiter spectrum observations in 2.8-14 micron range, describing absorption strength and brightness temperature, basing analysis on ammonia, methane and hydrogen absorption