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

We present a new diagnostic diagram for mid-infrared spectra of infrared galaxies based on the equivalent width of the 6.2 µm PAH emission feature and the strength of the 9.7 µm silicate feature. Based on the position in this diagram we classify galaxies into 9 classes ranging from continuum-dominated AGN hot dust spectra and PAH-dominated starburst spectra to absorption-dominated spectra of deeply obscured galactic nuclei. We find that galaxies are systematically distributed along two distinct branches: one of AGN and starburst-dominated spectra and one of deeply obscured nuclei and starburst-dominated spectra. The separation into two branches likely reflects a fundamental difference in the dust geometry in the two sets of sources: clumpy versus nonclumpy obscuration. Spectra of ULIRGs are found along the full length of both branches, reflecting the diverse nature of the ULIRG family.

The Sloan Digital Sky Survey has validated and made publicly available its First Data Release. This consists of 2099 square degrees of five-band (u, g, r, i, z) imaging data, 186,240 spectra of galaxies, quasars, stars and calibrating blank sky patches selected over 1360 square degrees of this area, and tables of measured parameters from these data. The imaging data go to a depth of r ~ 22.6 and are photometrically and astrometrically calibrated to 2% rms and 100 milli-arcsec rms per coordinate, respectively. The spectra cover the range 3800--9200 A, with a resolution of 1800--2100. Further characteristics of the data are described, as are the data products themselves.Comment: Submitted to The Astronomical Journal. 16 pages. For associated documentation, see http://www.sdss.org/dr

Type II quasars are the long-sought luminous analogs of type II (narrow emission line) Seyfert galaxies, suggested by unification models of active galactic nuclei (AGN) and postulated to account for an appreciable fraction of the cosmic hard X-ray background. We present a sample of 291 type II AGN at redshifts 0.3 < Z < 0.83 from the spectroscopic data of the Sloan Digital Sky Survey. These objects have narrow (FWHM< 2000 km s −1 ), high equivalent width emission lines with high-ionization line ratios. We describe the selection procedure and discuss the optical properties of the sample. About 50% of the objects have [OIII] λ air 5007 line luminosities in the range 3 × 10 8 − 10 10 L ⊙ , comparable to those of luminous (−27 < M B < −23) quasars; this, along with other evidence, suggests that the objects in the luminous subsample are type II quasars.

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.

Full low resolution (65

We present a new sample of 116 double-peaked Balmer line Active Galactic Nuclei (AGNs) selected from the Sloan Digital Sky Survey. Double-peaked emission lines are believed to originate in the accretion disks of AGN, a few hundred gravitational radii (R G ) from the supermassive black hole. We investigate the properties of the candidate disk emitters with respect to the full sample of AGN over the same redshifts, focusing on optical, radio and X-ray flux, broad line shapes and narrow line equivalent widths and line flux-ratios. We find that the disk-emitters have medium luminosities (∼10 44 erg s −1 ) and FWHM on average six times broader than the AGN in the parent sample. The double-peaked AGN are 1.6 times more likely to be radio-sources and are predominantly (76%) radio quiet, with about 12% of the objects classified as LINERs. Statistical comparison of the observed double-peaked line profiles with those produced by axisymmetric and non-axisymmetric accretion disk models allows us to impose constraints on accretion disk parameters. The observed Hα line profiles are consistent with accretion disks with inclinations smaller than 50 • , surface emissivity slopes of 1.0-2.5, outer radii larger than ∼2000R G , inner radii between 200-800R G , and local turbulent broadening of 780-1800 km s −1 . The comparison suggests that 60% of accretion disks require some form of asymmetry (e.g., elliptical disks, warps, spiral shocks or hot spots).