IRAS flux densities, redshifts, and infrared luminosities are reported for all sources identified in the IRAS Revised Bright Galaxy Sample (RBGS), a complete flux-limited survey of all extragalactic objects with total 60 lm flux density greater than 5.24 Jy, covering the entire sky surveyed by IRAS at Galactic latitudes |b| > 5 . The RBGS includes 629 objects, with median and mean sample redshifts of 0.0082 and 0.0126, respectively, and a maximum redshift of 0.0876. The RBGS supersedes the previous two-part IRAS Bright Galaxy Samples (BGS 1 +BGS 2 ), which were compiled before the final (Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGS also makes use of more accurate and consistent automated methods to measure the flux of objects with extended emission. The RBGS contains 39 objects that were not present in the BGS 1 +BGS 2 , and 28 objects from the BGS 1 +BGS 2 have been dropped from RBGS because their revised 60 lm flux densities are not greater than 5.24 Jy. Comparison of revised flux measurements for sources in both surveys shows that most flux differences are in the range $5%-25%, although some faint sources at 12 and 25 lm differ by as much as a factor of 2. Basic properties of the RBGS sources are summarized, including estimated total infrared luminosities, as well as updates to cross identifications with sources from optical galaxy catalogs established using the NASA/IPAC Extragalactic Database. In addition, an atlas of images from the Digitized Sky Survey with overlays of the IRAS position uncertainty ellipse and annotated scale bars is provided for ease in visualizing the optical morphology in context with the angular and metric size of each object. The revised bolometric infrared luminosity function, (L ir ), for infrared-bright galaxies in the local universe remains best fit by a double power law, (L) / L , with = À0.6(AE0.1) and = À2.2(AE0.1) below and above the '' characteristic '' infrared luminosity L Ã ir $ 10 10:5 L , respectively. A companion paper provides IRAS High Resolution (HIRES) processing of over 100 RBGS sources where improved spatial resolution often provides better IRAS source positions or allows for deconvolution of close galaxy pairs.

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-

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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

We have obtained the first detection of spectral absorption lines in one of the high-velocity stars in the vicinity of the Galaxy's central supermassive black hole. Both Brg (2.1661 mm) and He i (2.1126 mm) are seen in absorption in S0-2 with equivalent widths ( and Å ) and an inferred stellar rotational velocity 2.8 ‫ע‬ 0.3 1.7 ‫ע‬ 0.4 ( k ms Ϫ1 ) that are consistent with that of an O8-B0 dwarf, which suggests that it is a massive 220 ‫ע‬ 40 (∼15 M , ) young (less than 10 Myr) main-sequence star. This presents a major challenge to star formation theories, given the strong tidal forces that prevail over all distances reached by S0-2 in its current orbit (130-1900 AU) and the difficulty in migrating this star inward during its lifetime from farther out where tidal forces should no longer preclude star formation. The radial velocity measurements ( km s Ϫ1 ) and our reported Av S p Ϫ510 ‫ע‬ 40 z proper motions for S0-2 strongly constrain its orbit, providing a direct measure of the black hole mass of . The Keplerian orbit parameters have uncertainties that are reduced by a factorof 2-3 compared to previously reported values and include, for the first time, an independent solution for the dynamical center; this location, while consistent with the nominal infrared position of Sgr A*, is localized to a factor of 5 more precisely ‫2ע(‬ mas). Furthermore, the ambiguity in the inclination of the orbit is resolved with the addition of the radial velocity measurement, indicating that the star is behind the black hole at the time of closest approach and counterrevolving against the Galaxy. With further radial velocity measurements in the next few years, the orbit of S0-2 will provide the most robust estimate of the distance to the Galactic center.

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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.