We present 5 − 38µm mid-infrared spectra at a spectral resolution of R ≈ 65 − 130 of a large sample of 22 starburst nuclei taken with the Infrared Spectrograph IRS on board the Spitzer Space Telescope. The spectra show a vast range in starburst SEDs. The silicate absorption ranges from essentially no absorption to heavily obscured systems with an optical depth of τ 9.8µm ∼ 5. The spectral slopes can be used to discriminate between starburst and AGN powered sources. The monochromatic continuum fluxes at 15µm and 30µm enable a remarkably accurate estimate of the total infrared luminosity of the starburst. We find that the PAH equivalent width is independent of the total starburst luminosity L IR as both continuum and PAH feature scale proportionally. However, the luminosity of the 6.2µm feature scales with L IR and can be used to approximate the total infrared luminosity of the starburst. Although our starburst sample covers about a factor of ten difference in the [Ne III] / [Ne II] ratio, we found no systematic correlation between the radiation field hardness and the PAH equivalent width or the 7.7µm / 11.3µm PAH ratio. These results are based on spatially integrated diagnostics over an entire starburst region, and local variations may be "averaged out". It is presumably due to this effect that unresolved starburst nuclei with significantly different global properties appear spectrally as rather similar members of one class of objects.are expected to depend on numerous parameters such as the initial stellar mass function (IMF), the duration and epoch of the individual starburst(s), the metallicity of the ISM, the size and distribution of the dust grains, the strength of the magnetic fields, gas pressure and temperature of the ISM, galactic shear, total luminosity, and total mass. Furthermore, nearby starbursts, for which high resolution imaging is possible, have revealed complex substructures -in both stellar distributions and ISM -ranging from ultra-compact H ii regions (UCHIIR) to large complexes of super star clusters (SSC), suggesting small-scale variations of the observables across a starburst region.We use the low resolution mode of the Infrared Spectrograph 3 (IRS) ) on board the Spitzer Space Telescope (Werner et al. 2004) to observe the central regions of 22 starburst galaxies. Our objects represent a sample of "classical" starbursts for which a wealth of literature exists. The sample includes both purely starburst and starbursts with weak AGN activity (as determined from X-ray, optical, or radio observations). The summary in Table 1 lists the observed targets, their general properties, the classifications we adopt, and the references from which they are derived. The continuous 5 − 38µm IRS spectra include the silicate bands around 10µm and 18µm, a large number of PAH emission features, and information on the slope of the spectral con-

We present spectra taken with the Infrared Spectrograph 8 on Spitzer covering the 5 − 38µm region of the ten Ultraluminous Infrared Galaxies (ULIRGs) found in the IRAS Bright Galaxy Sample. Among the BGS ULIRGs, we find a factor of 50 spread in the rest-frame 5.5 − 60µm spectral slope. The 9.7µm silicate optical depths range from at least τ 9.7 ≤ 0.4 to τ 9.7 ≥ 4.2, implying line of sight extinctions of A V ∼ 8 mag to nearly A V ≥ 78 mag. There is evidence for water ice and hydrocarbon absorption and C 2 H 2 and HCN absorption features in four and possibly six of the 10 BGS ULIRGs, indicating shielded molecular clouds and a warm, dense ISM. We have detected [NeV] emission in three of the ten BGS ULIRGs, at flux levels of 5 − 18 × 10 −14 erg cm −2 sec −1 and [NeV] 14.3/[NeII] 12.8 line flux ratios of 0.12 − 0.85. The remaining BGS ULIRGs have limits on their [NeV]/[NeII] line flux ratios which range from ≤ 0.15 to ≤ 0.01. Among the BGS ULIRGs, the AGN fractions implied by either the [NeV]/[NeII] or [OIV]/[NeII] line flux ratios (or their upper limits) are significantly lower than implied by

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.

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We present a Spitzer-based mid-infrared study of a large sample of Blue Compact Dwarf galaxies (BCD) using the Infrared Spectrograph (IRS), including the first mid-IR spectrum of IZw18, the archetype for the BCD class and among the most metal poor galaxies known. We show the spectra of Polycyclic Aromatic Hydrocarbon (PAH) emission in low-metallicity environment. We find that the equivalent widths (EW) of PAHs at 6.2, 7.7, 8.6 and 11.2 µm are generally weaker in BCDs than in typical starburst galaxies and that the fine structure line ratio, [NeIII]/[NeII], has a weak anti-correlation with the PAH EW. A much stronger anti-correlation is shown between the PAH EW and the product of the [NeIII]/[NeII] ratio and the UV luminosity density divided by the metallicity. We conclude that PAH EW in metal-poor high-excitation environments is determined by a combination of PAH formation and destruction effects.

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 have surveyed a field covering 9.0 deg 2 within the NOAO Deep Wide-Field Survey region in Bootes with the Multiband Imaging Photometer on the Spitzer Space Telescope (SST) to a limiting 24 mm flux density of 0.3 mJy. Thirty-one sources from this survey with mJy that are optically very faint ( mag)have been observed with the low-resolution modules of the Infrared Spectrograph on SST (IRS). Redshifts derived primarily from strong silicate absorption features are reported here for 17 of these sources; 10 of these are optically invisible ( mag), with no counterpart in , R, or I. The observed redshifts for 16 sources are R տ 26 B 1.7 ! W . These represent a newly discovered population of highly obscured sources at high redshift with extreme z ! 2.8 infrared-to-optical ratios. Using IRS spectra of local galaxies as templates, we find that a majority of the sources have mid-infrared spectral shapes most similar to ultraluminous infrared galaxies powered primarily by active galactic nuclei. Assuming that the same templates also apply at longer wavelengths, bolometric luminosities exceed 10 13 L , .

We have used the Infrared Spectrograph (IRS) on the Spitzer Space Telescope to obtain spectra of HD 100764, an apparently single carbon star with a circumstellar disk. The spectrum shows emission features from polycyclic aromatic hydrocarbons (PAHs) that are shifted to longer wavelengths than normally seen, as characteristic of "class C" systems in the classification scheme of Peeters et al. All seven of the known class C PAH sources are illuminated by radiation fields that are cooler than those which typically excite PAH emission features. The observed wavelength shifts are consistent with hydrocarbon mixtures containing both aromatic and aliphatic bonds. We propose that the class C PAH spectra are distinctive because the carbonaceous material has not been subjected to a strong ultraviolet radiation field, allowing relatively fragile aliphatic materials to survive.