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 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.

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 present R $ 600, 10Y37 m spectra of 53 ultraluminous infrared galaxies (ULIRGs), taken using the Infrared Spectrograph on board Spitzer. The spectra show fine-structure emission lines of neon, oxygen, sulfur, silicon, argon, chlorine, iron, and phosphorous; molecular hydrogen lines, and C 2 H 2 , HCN, and OH À absorption features. We employ diagnostics based on the fine-structure lines, the polycyclic aromatic hydrocarbon (PAH) features and the 9.7 m silicate absorption feature, to show that the infrared emission from most ULIRGs is powered mostly by star formation, with only $20% of ULIRGs hosting an AGN with a greater IR luminosity than the starburst. The detection of [Ne v] k14.32 in just under half the sample, however, implies that an AGN contributes significantly to the mid-IR flux in $42% of ULIRGs. The starbursts and AGNs in ULIRGs appear more extincted, and for the starbursts more compact than those in lower luminosity systems. The excitations and electron densities in the narrow-line regions of ULIRGs appear comparable to those of starbursts with LP10 11.5 L , although the NLR gas in ULIRGs may be more dense. We show that the [Ne ii] k12.81 + [Ne iii] k15.56 luminosity correlates with both infrared luminosity and the luminosity of the 6.2 and 11.2 m PAH features, and derive a calibration between PAH luminosity and star formation rate. Finally, we show that ULIRGs with silicate absorption strengths S sil of 0:8 P S sil P 2:4 are likely to be powered mainly by star formation, but that ULIRGs with S sil P 0:8, and possibly those with S sil k 2:4, contain an IR-luminous AGN.

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 , .

Full low resolution (65