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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-

The Spitzer Space Telescope has revealed a significant population of high-redshift (z $ 2) dust-obscured galaxies with large mid-infrared to ultraviolet luminosity ratios. Due to their optical faintness, these galaxies have been previously missed in traditional optical studies of the distant universe. We present a simple method for selecting this high-redshift population based solely on the ratio of the observed mid-infrared 24 m to optical R-band flux density. We apply this method to observations of the %8.6 deg 2 NOAO Deep Wide-Field Survey Boötes field, and uncover %2600 dust-obscured galaxy candidates [i.e., 0.089 arcmin À2 ) with 24 m flux densities F 24 m ! 0:3 mJy and (R À ½24) ! 14 (i.e., F (24 m)/F (R) k1000]. These galaxies have no counterparts in the local universe. They represent 7% AE 0:6% of the 24 m source population at F 24 m ! 1 mJy but increase to %13% AE 1% of the population at %0.3 mJy. These galaxies exhibit evidence of both star formation and AGN activity, with the brighter 24 m sources being more AGN-dominated. We have measured spectroscopic redshifts for 86 of these galaxies, and find a broad redshift distribution centered atz % 1:99 AE 0:05. The space density of this population is AE DOG (F 24m ! 0:3 mJy) ¼ (2:82 AE 0:05) ; 10 À5 h 3 70 Mpc À3 , similar to that of bright submillimeter-selected galaxies at comparable redshifts. These redshifts imply large luminosities, with median L (8 m) % 4 ; 10 11 L . The infrared luminosity density contributed by this relatively rare dust-obscured galaxy population is log (IRLD) % 8:23 þ0:18 À0:30 . This is %60 þ40 À15 % of that contributed by z $ 2 ultraluminous infrared galaxies (ULIRGs, with L IR > 10 12 L ); our simple selection thus identifies a significant fraction of z $ 2 ULIRGs. This IRLD is %26% AE 14% of the total contributed by all z $ 2 galaxies. We suggest that these dust-obscured galaxies are the progenitors of luminous ($4L Ã ) present-day galaxies, seen undergoing an extremely luminous, short-lived phase of both bulge and black hole growth. They may represent a brief evolutionary phase between submillimeter-selected galaxies and less obscured quasars or galaxies.

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.

Full low resolution (65