B. T. Soifer

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.

Thermal infrared and nonthermal radio - Remarkable correlation in disks of galaxies

A tight, linear correlation is established between the far-infrared flux measured by IRAS and the nonthermal radio flux density (at 1.4 GHz) from disks of spiral galaxies. This correlation defines a ratio of infrared to radio fluxes that is characteristic of star formation activity. Galaxies with nuclear starbursts seem to follow the correlation. If the far-infrared is reradiated luminosity from young massive stars, then the supernova remnants alone account for less than 10 percent of the radio emission. These results indicate a close coupling between dust heating and cosmic-ray generation and confinement in a wide range of conditions. 32 references.

THE INFRARED ASTRONOMICAL SATELLITE (IRAS) MISSION

The Infrared Astronomical Satellite (IRAS) consists of a spacecraft and a liquid helium cryostat that contains a cooled IR telescope. The telescopes focal plane assembly is cooled to less than 3 K, and contains 62 IR detectors in the survey array which are arranged so that every source crossing the field of view can be seen by at least two detectors in each of four wavelength bands. The satellite was launched into a 900 km-altitude near-polar orbit, and its cryogenic helium supply was exhausted on November 22, 1983. By missions end, 72 percent of the sky had been observed with three or more hours-confirming scans, and 95 percent with two or more hours-confirming scans. About 2000 stars detected at 12 and 25 microns early in the mission, and identified in the SAO (1966) catalog, have a positional uncertainty ellipse whose axes are 45 x 9 arcsec for an hours-confirmed source.

Optical Spectroscopy of Luminous Infrared Galaxies. II. Analysis of the Nuclear and Long-Slit Data

A spectroscopic survey of a sample of 201 luminous IRAS galaxies was caried out using the Palomar 5-meter amd University of Hawaii 2.2-m telescopes.

Obscured and unobscured active galactic nuclei in the Spitzer Space Telescope First Look Survey

Selection of active galactic nuclei (AGNs) in the infrared facilitates the discovery of AGNs whose optical emission is extinguished by dust. In this paper, we use the Spitzer Space Telescope First Look Survey (FLS) to assess the fraction of AGNs with mid-infrared (MIR) luminosities that are comparable to quasars and that are missed in optical quasar surveys because of dust obscuration. We begin by using the Sloan Digital Sky Survey (SDSS) database to identify 54 quasars within the 4 deg^2 extragalactic FLS. These quasars occupy a distinct region in MIR color space by virtue of their strong, red continua. This has allowed us to define an MIR color criterion for selecting AGN candidates. About 2000 FLS objects have colors that are consistent with them being AGNs, but most are much fainter in the MIR than the SDSS quasars, which typically have 8 μm flux densities S_(8.0) ~ 1 mJy. We have investigated the properties of 43 objects with S_(8.0) ≥ 1 mJy that satisfy our AGN color selection. This sample should contain both unobscured quasars as well as AGNs that are absent from the SDSS survey because of extinction in the optical. After removing 16 known quasars, three probable normal quasars, and eight spurious or confused objects from the initial sample of 43, we are left with 16 objects that are likely to be obscured quasars or luminous Seyfert 2 galaxies. This suggests that the numbers of obscured and unobscured AGNs are similar in samples selected in the MIR at S_(8.0) ~ 1 mJy.

Observations of Ultraluminous Infrared Galaxies with the Infrared Spectrograph on the Spitzer Space Telescope. II. The IRAS Bright Galaxy Sample

We present spectra taken with the Infrared Spectrograph on Spitzer covering the 5-38 μm region of the 10 ultraluminous infrared galaxies (ULIRGs) found in the IRAS Bright Galaxy Sample (BGS). There is a factor of 50 spread in the rest-frame 5.5-60 μm spectral slopes, and the 9.7 μm silicate optical depths range from at least τ_(9.7) ≤ 0.4 (A_V ~ 8) to τ_(9.7) ≥ 4.2 (A_V ≥ 78). There is evidence for water ice and hydrocarbon absorption and C_2H_2 and HCN absorption features in 4 and possibly 6 of the 10 BGS ULIRGs, indicating shielded molecular clouds and a warm, dense ISM. We have detected [Ne V] emission in 3 of the 10 BGS ULIRGs, at flux levels of 5-18 × 10^(-14) ergs cm^(-2) s^(-1) and [Ne V] 14.3/[Ne II] 12.8 line flux ratios of 0.12-0.85. The remaining BGS ULIRGs have limits on their [Ne V]/[Ne II]line flux ratios, which range from ≤0.15 to ≤0.01. Among the BGS ULIRGs, the AGN fractions implied by either the [Ne V]/[Ne II] or [O IV]/[Ne II] line flux ratios (or their upper limits) are significantly lower than implied by the MIR slope or strength of the 6.2 μm PAH EQW feature. There is evidence for hot (T > 300 K) dust in five of the BGS ULIRGs, with the fraction of hot dust to total dust luminosity ranging from ~1% to 23%, before correcting for extinction. When integrated over the IRAC-8, IRS blue peak-up, and MIPS-24 filter bandpasses, the IRS spectra imply very blue colors for some ULIRGs at z ~ 1.3. The large range in diagnostic parameters among the nearest ULIRGs suggests that matching survey results to a small number of templates may lead to biased results about the fraction of luminous dusty starbursts and AGNs at high z.

Molecular gas in luminous infrared galaxies

Radio observations of 60 bright IRAS galaxies with redshifts of 1500-25,000 km/sec are reported. Data obtained in the 1-0 line of CO using the 12-m NRAO radio telescope during 1985-1988 are presented in extensive tables, graphs, and line profiles and analyzed along with similar data on 29 less distant IRAS bright galaxies (Tinney et al., 1990). The galaxies are found to have H2 masses of (1-60) x 10 to the 9th solar mass and a mean ratio of H2 to warm dust of 540 + or - 290, corresponding to a total gas/dust ratio of 900-1100. The discrepancy between this value and that for the Galaxy (about 150) is tentatively attributed to the presence of undetected cold dust or errors in interpreting the IR data. The mechanisms which might be responsible for the high ratios of IR luminosity to H2 mass (2-220 solar luminosity per solar mass) are discussed.

The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy's Central Black Hole: A Paradox of Youth

We have obtained the first detection of spectral absorption lines in one of the high-velocity stars in the vicinity of the Galaxys central supermassive black hole. Both Brγ (2.1661 μm) and He I (2.1126 μm) are seen in absorption in S0-2 with equivalent widths (2.8 ± 0.3 and 1.7 ± 0.4 A) and an inferred stellar rotational velocity (220 ± 40 km s^(-1)) that are consistent with that of an O8-B0 dwarf, which suggests that it is a massive (~15 M_☉) young (less than 10 Myr) main-sequence star. This presents a major challenge to star formation theories, given the strong tidal forces that prevail over all distances reached by S0-2 in its current orbit (130-1900 AU) and the difficulty in migrating this star inward during its lifetime from farther out where tidal forces should no longer preclude star formation. The radial velocity measurements (v_z) = -510 ± 40 km s^(-1)) and our reported proper motions for S0-2 strongly constrain its orbit, providing a direct measure of the black hole mass of 4.1(±0.6) × 10^6 (Ro/8kpc)^3 M_☉. The Keplerian orbit parameters have uncertainties that are reduced by a factor of 2-3 compared to previously reported values and include, for the first time, an independent solution for the dynamical center; this location, while consistent with the nominal infrared position of Sgr A^*, is localized to a factor of 5 more precisely (±2 mas). Furthermore, the ambiguity in the inclination of the orbit is resolved with the addition of the radial velocity measurement, indicating that the star is behind the black hole at the time of closest approach and counterrevolving against the Galaxy. With further radial velocity measurements in the next few years, the orbit of S0-2 will provide the most robust estimate of the distance to the Galactic center.

Infrared cirrus - New components of the extended infrared emission

Extended sources of far-infrared emission superposed on the zodiacal and galactic backgrounds are found at high galactic latitudes and near the ecliptic plane. Clouds of interstellar dust at color temperatures as high as 35 K account for much of this complex structure, but the relationship to H I column density is not simple. Other features of the extended emission show the existence of warm structures within the solar system. Three bands of dust clouds at temperatures of 150-200 K appear within 10 deg on both sides of the ecliptic plane. Their ecliptic latitudes and derived distances suggest that they are associated with the main asteroid belt. A third component of the 100-micron cirrus, poorly correlated with H I, may represent cold material in the outer solar system or a new component of the interstellar medium.

The IRAS Bright Galaxy Sample. IV. Complete IRAS observations

Total flux densities, peak flux densities, and spatial extents at 12, 25, 60, and 100 microns are presented for the 330 sources in the IRAS Bright Galaxy Sample. The flux density ratios S{sub nu} (60 microns)/S{sub nu} (100 microns) and S{sub nu} (12 microns)/S{sub n} (25 microns) are found to correlate with both the infrared luminosity and the ratio of IR to visible flux. The relation between these two flux density ratios is shown to follow that found previously, with different slopes appearing for the warmer and colder galaxies in the sample. The results suggest that single photon heating of small grains (often the dominant source of 12 and 25 micron radiation from galaxies) significantly affects the emission of some galaxies at 60 microns, and that optical depth effects may alter the emergent radiation at 12 and 25 microns. 31 refs.