The Infrared Spectrograph (IRS) is one of three science instruments on the Spitzer Space Telescope. The IRS comprises four separate spectrograph modules covering the wavelength range from 5.3 to 38 m with spectral resolutions, R ¼ k=Ák % 90 and 600, and it was optimized to take full advantage of the very low background in the space environment. The IRS is performing at or better than the prelaunch predictions. An autonomous target acquisition capability enables the IRS to locate the mid-infrared centroid of a source, providing the information so that the spacecraft can accurately offset that centroid to a selected slit. This feature is particularly useful when taking spectra of sources with poorly known coordinates. An automated data-reduction pipeline has been developed at the Spitzer Science Center.

SMART is a software package written in IDL to reduce and analyze Spitzer data from all four modules of the Infrared Spectrograph, including the peak-up arrays. The software is designed to make full use of the ancillary files generated in the Spitzer Science Center pipeline so that it can either remove or flag artifacts and corrupted data and maximize the signal-to-noise in the extraction routines. It may be run in both interactive and batch mode. The software and Users Guide will be available for public release in December 2004. We briefly describe some of the main features of SMART including: visualization tools for assessing the data quality, basic arithmetic operations for either 2-d images or 1-d spectra, extraction of both point and extended sources and a suite of spectral analysis tools.

We also mis-reported the temperature of the silicate and carbon grains in our fit to the HR 7012 IRS spectrum; the grains have a temperature 550 K, not 520 K as reported previously. Lastly, the composition of the enstatite used to fit the HR 7012 spectrum is Mg 0.7 Fe 0.3 SiO 3 , not Mg 0.7 Fe 0.3 SiO 4 . In addition, we noticed an error in the minimum blow-out size for silicate, carbon, and silica grains around HD 113766 and HR 7012; the blow-out sizes are smaller than previously estimated. For HD 113766, we originally estimated minimum silicate and carbon sizes of 1.4 and 1.9 m, respectively; we now estimate 0.35 and 0.46 m, respectively. With the exception of forsterite, all of the grains used to model the HD 113766 spectrum are larger than the minimum grain sizes. The forsterite grains (submicron) possess radii that are similar to the minimum silicate grain size. For HR 7012, we originally estimated minimum silicate, carbon, and silica sizes of 1.1, 1.4, and 1.6 m, respectively; we now estimate 0.9, 1.2, and 1.3 m, respectively. The enstatite and cristobalite grains used to model the infrared HR 7012 spectrum are still smaller than the minimum grain size. We had previously concluded that the minimum grain sizes (>1 m) were inconsistent with presence of submicron-sized grains inferred from the structure of the silicate emission features, suggesting that a recent massive collision must have occurred around HD 113766 and HR 7012. Our new minimum grain size estimates are more consistent with our models for the infrared spectra and do not require a recent massive collision around HD 113766. However, our models do indicate the presence of submicron-sized particles significantly smaller than the blow-out size around HR 7012, suggesting that a recent massive collision may have occurred in this system.

We present the spectral atlas of sources observed in low resolution with the Infrared Spectrograph on board the Spitzer Space Telescope. More than 11,000 distinct sources were extracted using a dedicated algorithm based on the SMART software with an optimal extraction (AdOpt package). These correspond to all 13,000 low-resolution observations of fixed objects (both single source and cluster observations). The pipeline includes image cleaning, individual exposure combination, and background subtraction. Particular attention is given to bad pixel and outlier rejection at the image and spectra levels. Most sources are spatially unresolved so that optimal extraction reaches the highest possible signal-to-noise ratio. For all sources, an alternative extraction is also provided that accounts for all of the source flux within the aperture. CASSIS provides publishable quality spectra through an online database together with several important diagnostics, such as the source spatial extent and a quantitative measure of detection level. Ancillary data such as available spectroscopic redshifts are also provided. The database interface will eventually provide various ways to interact with the spectra, such as on-the-fly measurements of spectral features or comparisons among spectra.

We report the spectroscopic detection of silicate emission at 10 and 18 mm in five PG quasars, the first detection of these two features in galaxies outside the Local Group. This finding is consistent with the unification model for active galactic nuclei (AGNs), which predicts that an AGN torus seen pole-on should show a silicate emission feature in the mid-infrared. The strengths of the detected silicate emission features range from 0.12 to 1.25 times the continuum at 10 mm and from 0.20 to 0.79 times the continuum at 18 mm. The silicate grain temperatures inferred from the ratio of 18 mm to 10 mm silicate features under the assumption of optically thin emission range from 140 to 220 K.