The dust in the Small Magellanic Cloud (SMC), an ideal analog of primordial galaxies at high redshifts, differs markedly from that in the Milky Way by exhibiting a steeply rising far-ultraviolet extinction curve, an absence of the 2175 Å extinction feature, and a local minimum at ~12 μm in its infrared emission spectrum, suggesting the lack of ultrasmall carbonaceous grains (i.e., polycyclic aromatic hydrocarbon molecules), which are ubiquitously seen in the Milky Way. While current models for the SMC dust all rely heavily on silicates, recent observations of the SMC line of sight toward Sk 155 have indicated that Si and Mg are essentially undepleted and that the depletions of Fe range from mild to severe, suggesting that metallic grains or iron oxides, instead of silicates, may dominate the SMC dust. However, here we apply the Kramers-Kronig relation to demonstrate that neither metallic grains nor iron oxides are capable of accounting for the observed extinction; silicates remain as an important contributor to the extinction, consistent with current models for the SMC dust.

We analyze a sample of ~2600 Spitzer MIPS 24 μm sources brighter than ~80 μJy and located in the Chandra Deep Field-South to characterize the evolution of the comoving infrared (IR) energy density of the universe up to z ~ 1. Using published ancillary optical data, we first obtain a nearly complete redshift determination for the 24 μm objects associated with R ≲ 24 mag counterparts at z ≲ 1. These sources represent ~55%-60% of the total MIPS 24 μm population with f24 μm ≳ 80 μJy, the rest of the sample likely lying at higher redshifts. We then determine an estimate of their total IR luminosities using various libraries of IR spectral energy distributions. We find that the 24 μm population at 0.5 ≲ z ≲ 1 is dominated by "luminous infrared galaxies" (i.e., 1011 L☉ ≤ LIR ≤ 1012 L☉), the counterparts of which appear to be also luminous at optical wavelengths and tend to be more massive than the majority of optically selected galaxies. A significant number of fainter sources (5 × 1010 L☉ ≲ LIR ≤ 1011 L☉) are also detected at similar distances. We finally derive 15 μm and total IR luminosity functions (LFs) up to z ~ 1. In agreement with the previous results from the Infrared Space Observatory (ISO) and SCUBA and as expected from the MIPS source number counts, we find very strong evolution of the contribution of the IR-selected population with look-back time. Pure evolution in density is firmly excluded by the data, but we find considerable degeneracy between strict evolution in luminosity and a combination of increases in both density and luminosity [L ∝ (1 + z), ϕ ∝ (1 + z)]. A significant steepening of the faint-end slope of the IR luminosity function is also unlikely, as it would overproduce the faint 24 μm source number counts. Our results imply that the comoving IR energy density of the universe evolves as (1 + z)3.9±0.4 up to z ~ 1 and that galaxies luminous in the infrared (i.e., LIR ≥ 1011 L☉) are responsible for 70% ± 15% of this energy density at z ~ 1. Taking into account the contribution of the UV luminosity evolving as (1 + z)~2.5, we infer that these IR-luminous sources dominate the star-forming activity beyond z ~ 0.7. The uncertainties affecting these conclusions are largely dominated by the errors in the k-corrections used to convert 24 μm fluxes into luminosities.

We measured the spectrum of polarization for three proto - planetary nebulae (PPNs), IRAS 17411 - 2411, IRAS 08005 - 2356, and IRAS 04296+ 3429, and made model calculations with the dust-scattering Monte Carlo code DIRTY and the dust emission code 2Dust. We show that high levels of polarization in these PPNs correlate with extreme asymptotic giant branch (AGB) superwind mass-loss rates in excess of 10(-4) M-. yr(-1). All three objects show evidence for evacuated lobes cleared by collimated fast winds, and two indicate a significant equatorial mass enhancement. Our best-fit models require sharply peaked grain size distributions, suggesting that most of the light is being scattered by grains of a characteristic size in IRAS 17441 - 2411 and IRAS 08005 - 2356. IRAS 17441 - 2411 and IRAS 08005 - 2356 have lobes with wide opening angles, perhaps produced by deflection of a polar jet from an accreting companion by the AGB superwind. Modeling the spectropolarimetry of IRAS 04296+ 3429 indicates a point-symmetric, multipolar morphology in the PPN phase. The modeling of spectropolarimetry and other observations of PPNs provides a powerful way to constrain circumstellar morphology and dust parameters.

We describe the data reduction algorithms for the Multiband Imaging Photometer for Spitzer (MIPS). These algorithms were based on extensive preflight testing and modeling of the Si:As (24 μm) and Ge:Ga (70 and 160 μm) arrays in MIPS and have been refined based on initial flight data. The behaviors we describe are typical of state‐of‐the‐art infrared focal planes operated in the low backgrounds of space. The Ge arrays are bulk photoconductors and therefore show a variety of artifacts that must be removed to calibrate the data. The Si array, while better behaved than the Ge arrays, does show a handful of artifacts that must also be removed to calibrate the data. The data reduction to remove these effects is divided into three parts. The first part converts the nondestructively read data ramps into slopes while removing artifacts with time constants of the order of the exposure time. The second part calibrates the slope measurements while removing artifacts with time constants longer than the exposure time. The third part uses the redundancy inherent in the MIPS observing modes to improve the artifact removal iteratively. For each of these steps, we illustrate the relevant laboratory experiments or theoretical arguments, along with the mathematical approaches taken to calibrate the data. Finally, we describe how these preflight algorithms have performed on actual flight data.

We present high spatial resolution mid- and far-infrared images of the Vega debris disk obtained with the Multiband Imaging Photometer for Spitzer (MIPS). The disk is well resolved, and its angular size is much larger than found previously. The radius of the disk is at least 43'' (330 AU), 70'' (543 AU), and 105'' (815 AU) in extent at 24, 70, and 160 μm, respectively. The disk images are circular, smooth, and without clumpiness at all three wavelengths. The radial surface brightness profiles follow radial power laws of r-3 or r-4 and imply an inner boundary at a radius of 11′′ ± 2′′ (86 AU). Assuming an amalgam of amorphous silicate and carbonaceous grains, the disk can be modeled as an axially symmetric and geometrically thin disk, viewed face-on, with the surface particle number density following an inverse radial power law. The disk radiometric properties are consistent with a range of models using grains of sizes ~1 to ~50 μm. The exact minimum and maximum grain size limits depend on the adopted grain composition. However, all of these models require an r-1 surface number density profile and a total mass of × 10-3 M⊕ in grains. We find that a ring, containing grains larger than 180 μm and at radii of 86-200 AU from the star, can reproduce the observed 850 μm flux, while its emission does not violate the observed MIPS profiles. This ring could be associated with a population of larger asteroidal bodies analogous to our own Kuiper Belt. Cascades of collisions starting with encounters among these large bodies in the ring produce the small debris that is blown outward by radiation pressure to much larger distances, where we detect its thermal emission. The relatively short lifetime (<1000 yr) of these small grains and the observed total mass, ~3 × 10-3 M⊕, set a lower limit on the dust production rate, ~1015 g s-1. This rate would require a very massive asteroidal reservoir for the dust to be produced in a steady state throughout Vega's life. Instead, we suggest that the disk we imaged is ephemeral and that we are witnessing the aftermath of a large and relatively recent collisional event, and a subsequent collisional cascade.

Spitzer provides substantial advances for studies of external galaxies: 1.) it has the very high sensitivity toward extended, low-surface-brightness sources made possible by a cryogenic telescope in space; 2.) the use of large format arrays and projected pixel scales that sample the telescope image well provide substantial improvements in angular resolution compared with previous space infrared telescopes; and 3.) its spectrograph has wavelength resolution well-suited to extragalactic studies, as well as high performance arrays that provide a major advance in sensitivity. We show how these capabilities are already leading to new possibilities in studying the structure of nearby galaxies. Combined with recent breakthroughs in high quality imaging at other wavelengths, we can anticipate a substantial improvement in our understanding of bow different components of a galaxy relate to each other and bow their interactions influence galaxy evolution. In addition, Spitzer is advancing our understanding of the behavior of the polyaromatic hydrocarbon (PAH) emission features, as a function of environment and metallicity. New features are also being discovered due to the high-sensitivity mapping capabilities of the spectrograph. An additional direction for research with Spitzer arises from its deep survey capabilities and the possibility of testing how the spectral energy distributions of galaxies evolve at large look-back times.

We present an extinction analysis of nine reddened/comparison star pairs in the Large and Small Magellanic Clouds (LMC and SMC) based on Far-Ultraviolet Spectroscopic Explorer (FUSE) FUV observations. To date, just two LMC sight lines have probed dust grain composition and size distributions in the Magellanic Clouds using spectral data for wavelengths as short as 950 Å. We supplement these two with data from four regions distinguished by their IR through UV extinction curves and grouped as LMCAvg, LMC2, SMC bar, and SMC wing. Despite the distinct characters of extinction in the Clouds and Milky Way, our results are generally analogous to those found for Galactic curves—namely, that the FUSE portions of each extinction curve are described reasonably well by Fitzpatrick & Massa curves fitted only to longer wavelength data and lack any dramatic new extinction features, and any deviations from the Cardelli, Clayton, & Mathis (CCM) formalism continue into FUV wavelengths. A maximum entropy method analysis of all of these curves suggests that LMCAvg and SMC wing sight lines, whose extinction parameters more closely resemble those for Galactic paths, require more silicon and/or carbon in dust than current abundance measurements would indicate are available. The requirements for LMC2 and SMC bar sight lines do not fully tax the available reservoirs, in part because large grains contribute less to the extinction in these directions. An intermediate product of this extinction analysis is the measurement of new H2 abundances in the Magellanic Clouds. Collectively considering Cloud sight lines that possess significant H2 column densities, E(B - V)/N(H ) ratios are reduced by significant factors relative to the Galactic mean, whereas the corresponding E(B - V)/N(H2) values more closely resemble their Galactic counterpart. These trends reflect the fact that among these sight lines f(H2) values are lower than those common in the Milky Way for paths with similar degrees of reddening.

We investigate extremely red objects (EROs) using near- and mid-infrared observations in five passbands (3.6 to 24 μm) obtained from the Spitzer Space Telescope, and deep ground-based R and K imaging. The great sensitivity of the Infrared Array Camera (IRAC) camera allows us to detect 64 EROs (a surface density of 2.90 ± 0.36 arcmin-2; [3.6]AB < 23.7) in only 12 minutes of IRAC exposure time, by means of an R - [3.6] color cut (analogous to the traditional red R - K cut). A pure infrared K - [3.6] red cut detects a somewhat different population and may be more effective at selecting z > 1.3 EROs. We find ∼17% of all galaxies detected by IRAC at 3.6 or 4.5 μm to be EROs. These percentages rise to about 40% at 5.8 μm, and about 60% at 8.0 μm. We utilize the spectral bump at 1.6 μm to divide the EROs into broad redshift slices using only near-infrared colors (2.2/3.6/4.5 μm). We conclude that two-thirds of all EROs lie at redshift z > 1.3. Detections at 24 μm imply that at least 11% of 0.6 < z < 1.3 EROs and at least 22% of z > 1.3 EROs are dusty star-forming galaxies.

Galaxy source counts in the infrared provide strong constraints on the evolution of the bolometric energy output from distant galaxy populations. We present the results from deep 24 mum imaging from Spitzer surveys, which include approximate to5 x 10(4) sources to an 80% completeness of similar or equal to 60 muJy. The 24 mum counts rapidly rise at near-Euclidean rates down to 5 mJy, increase with a super-Euclidean rate between 0.4 and 4 mJy, and converge below similar to 0.3 mJy. The 24 mum counts exceed expectations from nonevolving models by a factor of greater than or similar to10 at S-nu similar to 0.1 mJy. The peak in the differential number counts corresponds to a population of faint sources that is not expected from predictions based on 15 mum counts from the Infrared Space Observatory. We argue that this implies the existence of a previously undetected population of infrared-luminous galaxies at z similar to 1-3. Integrating the counts to 60 muJy, we derive a lower limit on the 24 mum background intensity of 1.9 +/- 0.6 nW m(-2) sr(-1) of which the majority (similar to60%) stems from sources fainter than 0.4 mJy. Extrapolating to fainter flux densities, sources below 60 muJy contribute 0.8(-0.4)(+0.9) nW m(-2) sr(-1) to the background, which provides an estimate of the total 24 mum background of 2.7(-0.7)(+1.1) nW m(-2) sr(-1).

Well-resolved infrared observations of nearby galaxies are of fundamental importance to the study of the processes that affect galactic evolution. In this paper we report on the first imaging results from the Spitzer Infrared Nearby Galaxies Survey ( SINGS) using observations of the Sb galaxy NGC 7331. We present images of NGC 7331 over a large range of wavelengths that allow us to compare the distributions of gas, stars, and dust in unprecedented detail. As an example of the types of information that the full SINGS will provide, we use three methods to determine that the interstellar medium mass in the ring of NGC 7331 is similar to5 x 10(9) M-circle dot. We also present the first images showing emission from small hot (similar to1000 K) dust grains, but we show that these dust grains contribute only a small fraction of the integrated 4.5 mum emission from NGC 7331.

The Multiband Imaging Photometer for Spitzer (MIPS) provides long wavelength capability for the mission, in imaging bands at 24, 70, and 160 microns and measurements of spectral energy distributions between 52 and 100 microns 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 Si:As BIB 24 micron array has excellent photometric properties, and measurements with rms relative errors of 1% or better can be obtained. The two longer wavelength arrays use Ge:Ga detectors with poor photometric stability. However, the use of 1.) a scan mirror to modulate the signals rapidly on these arrays, 2.) a system of on-board stimulators used for a relative calibration approximately every two minutes, and 3.) specialized reduction software result in good photometry with these arrays also, with rms relative errors of less than 10%.

We present observations of the star formation region NGC 7129 taken with the Multiband Imaging Photometer for Spitzer (MIPS). A significant population of sources, likely pre-main-sequence members of the young stellar cluster, are revealed outside the central photoionization region. Combining Infrared Array Camera (IRAC) and ground-based near-infrared images, we have obtained colors and spectral energy distributions for some 60 objects. The [3.6]-[4.5] versus [8]-[24] color-color plane shows sources clustered at several different loci, which roughly correspond to the archetypal evolutionary sequence Class 0, I, II, and III. We obtain preliminary classifications for 36 objects and find significant numbers of both Class I and II objects. Most of the pre-main-sequence candidates are associated with the densest part of the molecular cloud surrounding the photoionization region, indicating active star formation over a broad area outside the central cluster. We discuss three Class II candidates that exhibit evidence of inner disk clearing, which would be some of the youngest known examples of a transition from accretion to optically thin quiescent disks.

We present preliminary results on 24 mum detections of luminous infrared galaxies at z greater than or similar to 1 with the Multiband Imaging Photometer for Spitzer (MIPS). Observations were performed in the Lockman Hole and the Extended Groth Strip (EGS) and were supplemented by data obtained with the Infrared Array Camera (IRAC) between 3 and 9 mum. The positional accuracy of less than or similar to2" for most MIPS/IRAC detections provides unambiguous identifications of their optical counterparts. Using spectroscopic redshifts from the Deep Extragalactic Evolutionary Probe survey, we identify 24 mum sources at z greater than or similar to 1 in the EGS, while the combination of the MIPS/IRAC observations with BVRIJHK ancillary data in the Lockman Hole also shows very clear cases of galaxies with photometric redshifts at 1 less than or similar to z less than or similar to 2.5. The observed 24 mum fluxes indicate infrared luminosities greater than 10(11) L-circle dot, while the data at shorter wavelengths reveal rather red and probably massive (M greater than or similar to M*) galaxy counterparts. It is the first time that this population of luminous objects is detected up to z similar to 2.5 in the infrared. Our work demonstrates the ability of the MIPS instrument to probe the dusty universe at very high redshift and illustrates how the forthcoming Spitzer deep surveys will offer a unique opportunity to illuminate a dark side of cosmic history not explored by previous infrared experiments.

We present 3.6-24 μm Spitzer observations of an unbiased sample of nine luminous, dusty galaxies selected at 1200 μm by MAMBO on the IRAM 30 m telescope, a population akin to the well-known submillimeter or SCUBA galaxies (hereafter SMGs). Owing to the coarse resolution of submillimeter/millimeter cameras, SMGs have traditionally been difficult to identify at other wavelengths. We compare our multiwavelength catalogs to show that the overlap between 24 and 1200 μm must be close to complete at these flux levels. We find that all (4/4) of the most secure ≥4 σ SMGs have ≥4 σ counterparts at 1.4 GHz, while the fraction drops to 7/9 using all ≥3 σ SMGs. We show that combining mid-infrared (MIR) and marginal (≥3 σ) radio detections provides plausible identifications in the remaining cases, enabling us to identify the complete sample. Accretion onto an obscured central engine is betrayed by the shape of the MIR continuum emission for several sources, confirming Spitzer's potential to weed out active galaxies. We demonstrate the power of an S24 μm/S8 μm versus S8 μm/S4.5 μm color-color plot as a diagnostic for this purpose. However, we conclude that the majority (∼75%) of SMGs have rest-frame mid/far-IR spectral energy distributions commensurate with obscured starbursts. Sensitive 24 μm observations are clearly a useful route to identify and characterize reliable counterparts to high-redshift far-IR-bright galaxies, complementing what is possible via deep radio imaging.

We present submillimeter statistical detections of galaxies discovered in the 5′ × 5′ Spitzer Early Release Observations (to ∼4-15 μJy 5 σ at 3.6-8 μm, 170 μJy at 24 μm) through a stacking analysis of our reanalyzed SCUBA 8 mJy survey maps and a Spitzer identification of a new submillimeter point source in the 8 mJy survey region. For sources detected at 5.8 or 8 μm (154 and 111 sources, respectively), we detect positive skews in the submillimeter flux distributions at 99.2%-99.8% confidence using Kolmogorov-Smirnov tests, at both 850 and 450 μm. We also marginally detect the Spitzer 24 μm galaxies at 850 μm at 97% confidence and place limits on the mean submillimeter fluxes of the 3.6 and 4.5 μm sources. Integrating the submillimeter fluxes of the Spitzer populations, we find the 5.8 μm galaxies contribute 0.12 ± 0.05 nW m-2 sr-1 to the 850 μm background and 2.4 ± 0.7 nW m-2 sr-1 to the 450 μm background; similar contributions are made by the 8 μm-selected sample. We infer that the populations dominating the 5.8 and 8 μm extragalactic background light also contribute around a quarter of the 850 μm background and the majority of the 450 μm background.

We present Multiband Imaging Photometer for Spitzer (MIPS) observations of M81 at 24, 70, and 160 μm. The grand design nature of M81 is clearly seen, showing two well-resolved spiral arms containing numerous bright star-forming regions. The MIPS images reveal a significant amount of cold dust associated with the spiral arms. We investigate the variation of the ultraviolet (UV), Hα, and infrared (IR) luminosities and star formation rate (SFR) indicators across the face of M81 using the MIPS images and archival UV and Hα images. For regions in M81, we find that UV and Hα SFRs (uncorrected for dust attenuation) are always lower than the IR SFR. The cause of this behavior is dust attenuation and/or using SFR calibrations appropriate for entire galaxies, not regions in galaxies. The characteristics of the dust attenuation for the regions indicate the dust grains and/or geometry are different from those in starburst galaxies. The behavior of the infrared-radio correlation in M81 is seen to vary from the global average, with variations correlated with the morphology of M81.

We present Spitzer Space Telescope imaging observations at 3.6, 4.5, 5.8, 8.0, and 24 mum of the SCUBA submillimeter sources and muJy VLA radio sources in a 5' x 5' area in the Lockman Hole East region. Out of the similar to 40 SCUBA/VLA sources in the field, Spitzer counterparts were detected for nearly all except for the few low-weight SCUBA detections. We show that the majority (80% - 90%) of the detected sources are cold (i.e., starburst-like) infrared-luminous galaxies (L-IR > 10(11) L-.) at redshift 0.5 < z < 3.5, whose star-formation rate density (SFRD) is comparable to that of the optically selected star-forming galaxies.

The Multiband Imaging Photometer for Spitzer (MIPS) provides long-wavelength capability for the mission in imaging bands at 24, 70, and 160 μm and measurements of spectral energy distributions between 52 and 100 μm 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 μm 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%.

We use the source counts measured with the Multiband Imaging Photometer for Spitzer at 24, 70, and 160 μm to determine the 5 σ confusion limits due to extragalactic sources: 56 μJy, 3.2 mJy, and 40 mJy at 24, 70, and 160 μm, respectively. We also make predictions for confusion limits for a number of proposed far-infrared missions of larger aperture (3.5-10 m diameter).

We derive galaxy source counts at 70 and 160 μm using the Multiband Imaging Photometer for Spitzer (MIPS) to map the Chandra Deep Field-South (CDF-S) and other fields. At 70 μm, our observations extend upward about 2 orders of magnitude in flux density from a threshold of 15 mJy, and at 160 μm they extend about an order of magnitude upward from 50 mJy. The counts are consistent with previous observations on the bright end. Significant evolution is detected at the faint end of the counts in both bands, by factors of 2-3 over no-evolution models. This evolution agrees well with models that indicate that most of the faint galaxies lie at redshifts between 0.7 and 0.9. The new Spitzer data already resolve about 23% of the cosmic far-infrared background at 70 μm and about 7% at 160 μm.