William T. Reach

The Infrared Array Camera (IRAC) for the Spitzer Space Telescope

The Infrared Array Camera (IRAC) is one of three focal plane instruments on the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broadband images at 3.6, 4.5, 5.8, and 8.0 � m. Two nearly adjacent 5A2 ; 5A2 fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 � m; 4.5 and 8 � m). All four detector arrays in the camera are 256 ; 256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper summarizes the in-flight scientific, technical, and operational performance of IRAC.

The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background. I. Limits and Detections

The Diffuse Infrared Background Experiment (DIRBE) on the Cosmic Background Explorer (COBE) spacecraft was designed primarily to conduct a systematic search for an isotropic cosmic infrared background (CIB) in 10 photometric bands from 1.25 to 240 μm. The results of that search are presented here. Conservative limits on the CIB are obtained from the minimum observed brightness in all-sky maps at each wavelength, with the faintest limits in the DIRBE spectral range being at 3.5 μm (νIν < 64 nW m-2 sr-1, 95% confidence level) and at 240 μm (νIν < 28 nW m-2 sr-1, 95% confidence level). The bright foregrounds from interplanetary dust scattering and emission, stars, and interstellar dust emission are the principal impediments to the DIRBE measurements of the CIB. These foregrounds have been modeled and removed from the sky maps. Assessment of the random and systematic uncertainties in the residuals and tests for isotropy show that only the 140 and 240 μm data provide candidate detections of the CIB. The residuals and their uncertainties provide CIB upper limits more restrictive than the dark sky limits at wavelengths from 1.25 to 100 μm. No plausible solar system or Galactic source of the observed 140 and 240 μm residuals can be identified, leading to the conclusion that the CIB has been detected at levels of νIν = 25 ± 7 and 14 ± 3 nW m-2 sr-1 at 140 and 240 μm, respectively. The integrated energy from 140 to 240 μm, 10.3 nW m-2 sr-1, is about twice the integrated optical light from the galaxies in the Hubble Deep Field, suggesting that star formation might have been heavily enshrouded by dust at high redshift. The detections and upper limits reported here provide new constraints on models of the history of energy-releasing processes and dust production since the decoupling of the cosmic microwave background from matter.

Exploring the Optical Transient Sky with the Palomar Transient Factory

The Palomar Transient Factory (PTF) is a wide-field experiment designed to investigate the optical transient and variable sky on time scales from minutes to years. PTF uses the CFH12k mosaic camera, with a field of view of 7.9 deg^2 and a plate scale of 1″ pixel^(-1), mounted on the Palomar Observatory 48 inch Samuel Oschin Telescope. The PTF operation strategy is devised to probe the existing gaps in the transient phase space and to search for theoretically predicted, but not yet detected, phenomena, such as fallback supernovae, macronovae, .Ia supernovae, and the orphan afterglows of gamma-ray bursts. PTF will also discover many new members of known source classes, from cataclysmic variables in their various avatars to supernovae and active galactic nuclei, and will provide important insights into understanding galactic dynamics (through RR Lyrae stars) and the solar system (asteroids and near-Earth objects). The lessons that can be learned from PTF will be essential for the preparation of future large synoptic sky surveys like the Large Synoptic Survey Telescope. In this article we present the scientific motivation for PTF and describe in detail the goals and expectations for this experiment.

Absolute Calibration of the Infrared Array Camera on the Spitzer Space Telescope

ABSTRACT The Infrared Array Camera (IRAC) on the Spitzer Space Telescope is absolutely calibrated by comparing photometry of a set of A stars near the north ecliptic pole to predictions based on ground‐based observations and a stellar atmosphere model. The brightness of point sources is calibrated to an accuracy of 3%, relative to models for A‐star stellar atmospheres, for observations performed and analyzed in the same manner as for the calibration stars. This includes corrections for the location of the star in the array and the location of the centroid within the peak pixel. Long‐term stability of the IRAC photometry was measured by monitoring the brightness of A dwarfs and K giants (near the north ecliptic pole) observed several times per month; the photometry is stable to 1.5% (rms) over a year. Intermediate‐timescale stability of the IRAC photometry was measured by monitoring at least one secondary calibrator (near the ecliptic plane) every 12 hr while IRAC was in nominal operations; the intermediat...

The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background. II. Model of the Interplanetary Dust Cloud

The COBE Diffuse Infrared Background Experiment (DIRBE) was designed to search for the cosmic infrared background (CIB) radiation. For an observer confined to the inner solar system, scattered light and thermal emission from the interplanetary dust (IPD) are major contributors to the diffuse sky brightness at most infrared wavelengths. Accurate removal of this zodiacal light foreground is a necessary step toward a direct measurement of the CIB. The zodiacal light foreground contribution in each of the 10 DIRBE wavelength bands ranging from 1.25 to 240 μm is distinguished by its apparent seasonal variation over the whole sky. This contribution has been extracted by fitting the brightness calculated from a parameterized physical model to the time variation of the all-sky DIRBE measurements over 10 months of liquid He cooled observations. The model brightness is evaluated as the integral along the line of sight of the product of a source function and a three-dimensional dust density distribution function. The dust density distribution is composed of multiple components: a smooth cloud, three asteroidal dust bands, and a circumsolar ring near 1 AU. By using a directly measurable quantity that relates only to the IPD cloud, we exclude other contributors to the sky brightness from the IPD model. High-quality maps of the infrared sky with the zodiacal foreground removed have been generated using the IPD model described here. Imperfections in the model reveal themselves as low-level systematic artifacts in the residual maps that correlate with components of the IPD. The most evident of these artifacts are located near the ecliptic plane in the mid-IR and are less than 2% of the zodiacal foreground brightness. Uncertainties associated with the model are discussed, including implications for the CIB search.

Spitzer survey of the large magellanic cloud: Surveying the agents of a Galaxy's evolution (SAGE). I. Overview and initial results

We are performing a uniform and unbiased imaging survey of the Large Magellanic Cloud (LMC; ~7° × 7°) using the IRAC (3.6, 4.5, 5.8, and 8 μm) and MIPS (24, 70, and 160 μm) instruments on board the Spitzer Space Telescope in the Surveying the Agents of a Galaxys Evolution (SAGE) survey, these agents being the interstellar medium (ISM) and stars in the LMC. This paper provides an overview of the SAGE Legacy project, including observing strategy, data processing, and initial results. Three key science goals determined the coverage and depth of the survey. The detection of diffuse ISM with column densities >1.2 × 10^(21) H cm^(-2) permits detailed studies of dust processes in the ISM. SAGEs point-source sensitivity enables a complete census of newly formed stars with masses >3 M_☉ that will determine the current star formation rate in the LMC. SAGEs detection of evolved stars with mass-loss rates >1 × 10^(-8) M_☉ yr^(-1) will quantify the rate at which evolved stars inject mass into the ISM of the LMC. The observing strategy includes two epochs in 2005, separated by 3 months, that both mitigate instrumental artifacts and constrain source variability. The SAGE data are nonproprietary. The data processing includes IRAC and MIPS pipelines and a database for mining the point-source catalogs, which will be released to the community in support of Spitzer proposal cycles 4 and 5. We present initial results on the epoch 1 data for a region near N79 and N83. The MIPS 70 and 160 μm images of the diffuse dust emission of the N79/N83 region reveal a similar distribution to the gas emissions, especially the H I 21 cm emission. The measured point-source sensitivity for the epoch 1 data is consistent with expectations for the survey. The point-source counts are highest for the IRAC 3.6 μm band and decrease dramatically toward longer wavelengths, consistent with the fact that stars dominate the point-source catalogs and the dusty objects detected at the longer wavelengths are rare in comparison. The SAGE epoch 1 point-source catalog has ~4 × 10^6 sources, and more are anticipated when the epoch 1 and 2 data are combined. Using Milky Way (MW) templates as a guide, we adopt a simplified point-source classification to identify three candidate groups—stars without dust, dusty evolved stars, and young stellar objects—that offer a starting point for this work. We outline a strategy for identifying foreground MW stars, which may comprise as much as 18% of the source list, and background galaxies, which may comprise ~12% of the source list.

Far-infrared spectral observations of the galaxy by COBE

We derive Galactic continuum spectra from 5-96 cm(-1) fromCOBE/FIRAS observations. The spectra are dominated by warm dust emission,which may be fitted with a single temperature in the range 16-21 K (fornu(2) emissivity) along each line of sight. Dust heated by the attenuatedradiation field in molecular clouds gives rise tointermediate-temperature (10-14 K) emission in the inner Galaxy only. Awidespread, very cold component (4-7 K) with optical depth that isspatially correlated with the warm component is also detected. The coldcomponent is unlikely to be due to very cold dust shielded from starlightbecause it is present at high latitude. We consider hypotheses that thecold component is due to enhanced submillimeter emissivity of the dustthat gives rise to the warm component, or that it may be due to verysmall, large, or fractal particles. Lack of substantial power above theemission from warm dust places strong constraints on the amount of coldgas in the Galaxy. The microwave sky brightness due to interstellar dustis dominated by the cold component, and its angular variation could limitour ability to discern primordial fluctuations in the cosmic microwavebackground radiation.

Detection and Characterization of Cold Interstellar Dust and Polycyclic Aromatic Hydrocarbon Emission, from COBE Observations

Using data obtained by the DIRBE instrument on the COBE spacecraft, we present the mean 3.5-240 μm spectrum of high-latitude dust. Combined with a spectrum obtained by the FIRAS instrument, these data represent the most comprehensive wavelength coverage of dust in the diffuse interstellar medium, spanning the 3.5-1000 μm wavelength regime. At wavelengths shorter than ~60 μm the spectrum shows an excess of emission over that expected from dust heated by the local interstellar radiation field and radiating at an equilibrium temperature. The DIRBE data thus extend the observations of this excess, first detected by the IRAS satellite at 25 and 12 μm, to shorter wavelengths. The excess emission arises from very small dust particles undergoing temperature fluctuations. However, the 3.5-4.9 μm intensity ratio cannot be reproduced by very small silicate or graphite grains. The DIRBE data strongly suggest that the 3.5-12 μm emission is produced by carriers of the ubiquitous 3.3, 6.2, 7.7, 8.6, and 11.3 μm solid state emission features that have been detected in a wide variety of astrophysical objects. The carriers of these features have been widely identified with polycyclic aromatic hydrocarbons (PAHs). Our dust model consists of a mixture of PAH molecules and bare astronomical silicate and graphite grains with optical properties given by Draine & Lee. We obtain a very good fit to the DIRBE spectrum, deriving the size distribution, abundances relative to the total hydrogen column density, and relative contribution of each dust component to the observed IR emission. At wavelengths above 140 μm the model is dominated by emission from T ≈ 17-20 K graphite and 15-18 K silicate grains. The model provides a good fit to the FIRAS spectrum in the 140-500 μm wavelength regime but leaves an excess Galactic emission component at 500-1000 μm. The nature of this component is still unresolved. We find that (C/H) is equal to (7.3 ± 2.2) × 10-5 for PAHs and equal to (2.5 ± 0.8) × 10-4 for graphite grains, requiring about 20% of the cosmic abundance of carbon to be locked up in PAHs, and about 70% in graphite grains [we adopt (C/H)☉ = 3.6 × 10-4]. The model also requires all of the available magnesium, silicon, and iron to be locked up in silicates. The power emitted by PAHs is 1.6 × 10-31 W per H atom, by graphite grains 3.0 × 10-31 W per H atom, and by silicates 1.4 × 10-31 W per H atom, adding up to a total infrared intensity of 6.0 × 10-31 W per H atom, or ~2 L☉ M. The [C II] 158 μm line emission detected by the FIRAS provides important information on the gas phase abundance of carbon in the diffuse ISM. The 158 μm line arises predominantly from the cold neutral medium (CNM) and shows that for typical CNM densities and temperatures C+/H = (0.5-1.0) × 10-4, which is ~14%-28% of the cosmic carbon abundance. The remaining carbon abundance in the CNM, which must be locked up in dust, is about equal to that required to provide the observed IR emission, consistent with notion that most (75%) of this emission arises from the neutral component of the diffuse ISM. The model provides a good fit to the general interstellar extinction curve. However, at UV wavelengths it predicts a larger extinction. The excess extinction may be the result of the UV properties adopted for the PAHs. If real, the excess UV extinction may be accounted for by changes in the relative abundances of PAHs and carriers of the 2200 A extinction bump.

Dust in the Local Interstellar Wind

The gas-to-dust mass ratios found for interstellar dust within the solar system, versus values determined astronomically for the cloud around the solar system, suggest that large and small interstellar grains have separate histories and that large interstellar grains preferentially detected by spacecraft are not formed exclusively by mass exchange with nearby interstellar gas. Observations by the Ulysses and Galileo satellites of the mass spectrum and flux rate of interstellar dust within the heliosphere are combined with information about the density, composition, and relative flow speed and direction of interstellar gas in the cloud surrounding the solar system to derive an in situ value for the gas-to-dust mass ratio, Rg/d = 94. This ratio is dominated by the larger near-micron-sized grains. Including an estimate for the mass of smaller grains, which do not penetrate the heliosphere owing to charged grain interactions with heliosheath and solar wind plasmas, and including estimates for the mass of the larger population of interstellar micrometeorites, the total gas-to-dust mass ratio in the cloud surrounding the solar system is half this value. Based on in situ data, interstellar dust grains in the 10-14 to 10-13 g mass range are underabundant in the solar system, compared to a Mathis, Rumple, & Nordsiek mass distribution scaled to the local interstellar gas density, because such small grains do not penetrate the heliosphere. The gas-to-dust mass ratios are also derived by combining spectroscopic observations of the gas-phase abundances in the nearest interstellar clouds. Measurements of interstellar absorption lines formed in the cloud around the solar system, as seen in the direction of CMa, give Rg/d = 427 for assumed solar reference abundances and Rg/d = 551 for assumed B star reference abundances. These values exceed the in situ value suggesting either that grain mixing or grain histories are not correctly understood or that sweptup stardust is present. Such high values for diffuse interstellar clouds are strongly supported by diffuse cloud data seen toward λ Sco and 23 Ori, provided B star reference abundances apply. If solar reference abundances prevail, however, the surrounding cloud is seen to have greater than normal dust destruction compared to higher column density diffuse clouds. The cloud surrounding the solar system exhibits enhanced gas-phase abundances of refractory elements such as Fe+ and Mg+, indicating the destruction of dust grains by shock fronts. The good correlation locally between Fe+ and Mg+ indicates that the gas-phase abundances of these elements are dominated by grain destruction, while the poor correlation between Fe+ and H0 indicates either variable gas ionization or the decoupling of neutral gas and dust over parsec scale lengths. These abundances, combined with grain destruction models, indicate that the nearest interstellar material has been shocked with shocks of velocity ~150 km s-1. If solar reference abundances are correct, the low Rg/d value toward λ Sco may indicate that at least one cloud component in this direction contains dust grains that have retained their silicate mantles and are responsible for the polarization of the light from nearby stars seen in this general region. Weak frictional coupling between gas and dust in nearby low density gas permit inhomogeneities to be present.

A Spitzer Space Telescope Infrared Survey of Supernova Remnants in the Inner Galaxy

Using Infrared Array Camera (IRAC) images at 3.6, 4.5, 5.8, and 8 ?m from the GLIMPSE Legacy science program on the Spitzer Space Telescope, we searched for infrared counterparts to the 95 known supernova remnants that are located within Galactic longitudes 65? > |l| > 10? and latitudes |b| < 1?. Eighteen infrared counterparts were detected. Many other supernova remnants could have significant infrared emission but are in portions of the Milky Way too confused to allow the separation of bright H II regions and pervasive mid-infrared emission from atomic and molecular clouds along the line of sight. Infrared emission from supernova remnants originates from synchrotron emission, shock-heated dust, atomic fine-structure lines, and molecular lines. The detected remnants are G11.2-0.3, Kes?69, G22.7-0.2, 3C?391, W44, 3C?396, 3C?397, W49B, G54.4-0.3, Kes?17, Kes?20A, RCW 103, G344.7-0.1, G346.6-0.2, CTB?37A, G348.5-0.0, and G349.7+0.2. The infrared colors suggest emission from molecular lines (nine remnants), fine-structure lines (three remnants), polycyclic aromatic hydrocarbons (four remnants), or a combination; some remnants feature multiple colors in different regions. None of the remnants are dominated by synchrotron radiation at mid-infrared wavelengths. The IRAC-detected sample emphasizes remnants interacting with relatively dense gas, for which most of the shock cooling occurs through molecular or ionic lines in the mid-infrared.