Search

International audience; A sample of 379 extragalactic sources is presented that has mid-infrared, high-resolution spectroscopy from the Spitzer Infrared Spectrograph (IRS) and also spectroscopy of the [C II] 158 μm line from the Herschel Photodetector Array Camera and Spectrometer (PACS). The emission line profiles of [Ne II] 12.81 μm, [Ne III] 15.55 μm, and [C II] 158 μm are presented, and intrinsic line widths are determined (full width half maximum of Gaussian profiles after instrumental correction). All line profiles, together with overlays comparing the positions of PACS and IRS observations, are made available in the Cornell Atlas of Spitzer IRS Sources. Sources are classified from active galactic nucleus (AGN) to starburst based on equivalent widths of the 6.2 μm polycyclic aromatic hydrocarbon feature. It is found that intrinsic line widths do not change among classifications for [C II], with median widths of 207 km s-1 for AGNs, 248 km s-1 for composites, and 233 km s-1 for starbursts. The [Ne II] line widths also do not change with classification, but [Ne III] lines are progressively broader from starburst to AGN. A few objects with unusually broad lines or unusual redshift differences in any feature are identified.

A population of quasars at z ~ 2 is determined based on dust luminosities vLv(7.8 um) that includes unobscured, partially obscured, and obscured quasars. Quasars are classified by the ratio vLv(0.25 um)/vLv(7.8 um) = UV/IR, assumed to measure obscuration of UV luminosity by the dust which produces IR luminosity. Quasar counts at rest frame 7.8 um are determined for quasars in the Bootes field of the NOAO Deep Wide Field Survey using 24 um sources with optical redshifts from the AGN and Galaxy Evolution Survey (AGES) or infrared redshifts from the Spitzer Infrared Spectrograph. Spectral energy distributions are extended to far infrared wavelengths using observations from the Herschel Space Observatory Spectral and Photometric Imaging Receiver (SPIRE), and new SPIRE photometry is presented for 77 high redshift quasars from the Sloan Digital Sky Survey. It is found that unobscured and obscured quasars have similar space densities at rest frame 7.8 um, but the ratio Lv(100 um)/Lv(7.8 um) is about three times higher for obscured quasars compared to unobscured, so that far infrared or submm discoveries are dominated by obscured quasars. Quasar source counts for these samples are determined for comparison to the number of submm sources that have been discovered with the SCUBA-2 camera at z ~ 2 using the Lv(100 um)/Lv(7.8 um) results together with the Bootes 7.8 um counts, and we find that only ~ 5% of high redshift submm sources are quasars, including even the most obscured quasars. Illustrative source counts are predicted to z = 10, and we show that existing SCUBA-2 850 um surveys or 2 mm surveys with the Goddard-IRAM Superconducting 2 Millimeter Observer (GISMO) survey camera should already have detected sources at z ~ 10 if quasar and starburst luminosity functions remain the same from z = 2 until z = 10., Accepted for publication in The Astrophysical Journal

James Richard Houck, the leading figure in developing IR spectroscopy for astrophysics, died in Ithaca, New York, on 18 September 2015 from complications of Alzheimer’s disease.

Star formation rates (SFR) are compared as determined from mid-infrared 7.7 um PAH luminosity [SFR(PAH)], from 1.4 GHz radio luminosity [SFR(radio)], and from far ultraviolet luminosity [SFR(UV)] for a sample of 287 starburst galaxies with z < 0.5 having Spitzer IRS observations. The previously adopted relation log [SFR(PAH)] = log [vLv(7.7 um)] - 42.57+-0.2, for SFR in solar masses per year and vLv(7.7 um) the luminosity at the peak of the 7.7 um PAH feature in ergs per s, is found to agree with SFR(radio). Comparing with SFR(UV) determined independently from ultraviolet observations of the same sources with the GALEX mission (not corrected for dust extinction), the median log [SFR(PAH)/SFR(UV)] = 1.67, indicating that only 2% of the ultraviolet continuum typically escapes extinction by dust within a starburst. The ratio SFR(PAH)/SFR(UV) depends on infrared luminosity, with form log [SFR(PAH)/SFR(UV)]= (0.53+-0.05)log Lir - 4.11+-0.18, for Lir in solar luminosities. We also find that the ratio SFR(PAH)/SFR(UV) is similar to that in infrared-selected starbursts for a sample of Markarian starburst galaxies originally selected using optical classification, which implies that there is no significant selection effect in SFR(PAH)/SFR(UV) using starburst galaxies discovered by Spitzer. These results indicate that SFRs determined with ultraviolet luminosities require dust corrections by a factor of ~ 10 for typical local starbursts but this factor increases to > 700 for the most luminous starbursts at z ~ 2.5. With this amount of extinction, the optical magnitude of a starburst having fv(7.7 um) of 1 mJy should be V ~ 25.6., Final version as published, including minor corrections made in proof

Using the Infrared Spectrograph on board the Spitzer Space Telescope, we present low-resolution (64 0.5 mJy; 2) R-[24] > 14 Vega mag; and 3) a prominent rest-frame 1.6 micron stellar photospheric feature redshifted into Spitzer's 3-8 micron IRAC bands. Of these, 20 show emission from polycyclic aromatic hydrocarbons (PAHs), usually interpreted as signatures of star formation. The PAH features indicate redshifts in the range 1.5 =1.96 and a dispersion of 0.30. Based on local templates, these sources have extremely large infrared luminosities, comparable to that of submillimeter galaxies. Our results confirm previous indications that the rest-frame 1.6 micron stellar bump can be efficiently used to select highly obscured starforming galaxies at z~2, and that the fraction of starburst-dominated ULIRGs increases to faint 24 micron flux densities. Using local templates, we find that the observed narrow redshift distribution is due to the fact that the 24 micron detectability of PAH-rich sources peaks sharply at z = 1.9. We can analogously explain the broader redshift distribution of Spitzer-detected AGN-dominated ULIRGs based on the shapes of their SEDs. Finally, we conclude that z~2 sources with a detectable 1.6 micron stellar opacity feature lack sufficient AGN emission to veil the 7.7 micron PAH band.

A summary of starburst luminosities based on PAH features is given for 243 starburst galaxies with 0 < z < 2.5, observed with the Spitzer Infrared Spectrograph. Luminosity vLv(7.7um) for the peak luminosity of the 7.7um PAH emission feature is found to scale as log[vLv(7.7um)] = 44.63(+-0.09) + 2.48(+-0.28)log(1+z) for the most luminous starbursts observed. Empirical calibrations of vLv(7.7um) are used to determine bolometric luminosity Lir and the star formation rate (SFR) for these starbursts. The most luminous starbursts found in this sample have log Lir = 45.4(+-0.3) + 2.5(+-0.3)log(1+z), in ergs per s, and the maximum star formation rates for starbursts in units of solar masses per yr are log(SFR) = 2.1(+-0.3) + 2.5(+-0.3)log(1+z), up to z = 2.5. The exponent for pure luminosity evolution agrees with optical and radio studies of starbursts but is flatter than previous results based in infrared source counts. The maximum star formation rates are similar to the maxima determined for submillimeter galaxies; the most luminous individual starburst included within the sample has log Lir = 46.9, which gives a SFR = 3400 solar masses per yr., Accepted for publication in The Astrophysical Journal

Observations with Spitzer Space Telescope have recently revealed a significant population of high-redshift z~2 dust-obscured galaxies (DOGs) with large mid-IR to UV luminosity ratios. These galaxies have been missed in traditional optical studies of the distant universe. We present a simple method for selecting this high-z population based solely on the ratio of the observed mid-IR 24um to optical R-band flux density. In the 8.6 sq.deg Bootes NDWFS Field, we uncover ~2,600 DOG candidates (= 0.089/sq.arcmin) with 24um flux densities F24>0.3mJy and (R-[24])>14 (i.e., F[24]/F[R] > 1000). These galaxies have no counterparts in the local universe, and become a larger fraction of the population at fainter F24, representing 13% of the sources at 0.3~mJy. DOGs exhibit evidence of both star-formation and AGN activity, with the brighter 24um sources being more AGN- dominated. We have measured spectroscopic redshifts for 86 DOGs, and find a broad z distribution centered at ~2.0. Their space density is 2.82E-5 per cubic Mpc, similar to that of bright sub-mm-selected galaxies at z~2. These redshifts imply very large luminosities LIR>~1E12-14 Lsun. DOGs contribute ~45-100% of the IR luminosity density contributed by all z~2 ULIRGs, suggesting that our simple selection criterion identifies the bulk of z~2 ULIRGs. DOGs may be the progenitors of ~4L* present-day galaxies seen undergoing a luminous,short- lived phase of bulge and black hole growth. They may represent a brief evolution phase between SMGs and less obscured quasars or galaxies. [Abridged], Accepted for publication in the Astrophysical Journal

The Infrared Spectrograph (IRS) on board the Spitzer Space Telescope observed about 15,000 objects during the cryogenic mission lifetime. Observations provided low-resolution (R~60-127) spectra over ~5-38um and high-resolution (R~600) spectra over ~10-37um. The Cornell Atlas of Spitzer/IRS Sources (CASSIS) was created to provide publishable quality spectra to the community. Low-resolution spectra have been available in CASSIS since 2011, and we present here the addition of the high-resolution spectra. The high-resolution observations represent approximately one third of all staring observations performed with the IRS instrument. While low-resolution observations are adapted to faint objects and/or broad spectral features (e.g., dust continuum, molecular bands), high-resolution observations allow more accurate measurements of narrow features (e.g., ionic emission lines) as well as a better sampling of the spectral profile of various features. Given the narrow aperture of the two high-resolution modules, cosmic ray hits and spurious features usually plague the spectra. Our pipeline is designed to minimize these effects through various improvements. A super sampled point-spread function was created in order to enable the optimal extraction in addition to the full aperture extraction. The pipeline selects the best extraction method based on the spatial extent of the object. For unresolved sources, the optimal extraction provides a significant improvement in signal-to-noise ratio over a full aperture extraction. We have developed several techniques for optimal extraction, including a differential method that eliminates low-level rogue pixels (even when no dedicated background observation was performed). The updated CASSIS repository now includes all the spectra ever taken by the IRS, with the exception of mapping observations., Accepted on May 11th 2015 in ApJS

We report the spectroscopic detection of silicate emission at 10 and 18 microns in five PG quasars, the first detection of these two features in galaxies outside the Local Group. This finding is consistent with the unification model for Active Galactic Nuclei (AGNs), which predicts that an AGN torus seen pole-on should show a silicate emission feature in the mid-infrared. The strengths of the detected silicate emission features range from 0.12 to 1.25 times the continuum at 10 microns and from 0.20 to 0.79 times the continuum at 18 microns. The silicate grain temperatures inferred from the ratio of 18-to-10 micron silicate features under the assumption of optically thin emission range from 140 to 220K., Comment: 4 pages, including 2 tables and 3 figures. Accepted by ApJL

Infrared luminosities vLv(7.8 um) arising from dust reradiation are determined for Sloan Digital Sky Survey (SDSS) quasars with 1.4 < z < 5 using detections at 22 um by the Wide-Field Infrared Survey Explorer. Infrared luminosity does not show a maximum at any redshift z < 5, reaching a plateau for z >~ 3 with maximum luminosity vLv(7.8 um) >~ 10^{47} erg per s; luminosity functions show one quasar per cubic Gpc having vLv(7.8 um) > 10^{46.6} erg per s for all 2 < z < 5. We conclude that the epoch when quasars first reached their maximum luminosity has not yet been identified at any redshift below 5. The most ultraviolet luminous quasars, defined by rest frame vLv(0.25 um), have the largest values of the ratio vLv(0.25 um)/vLv(7.8 um) with a maximum ratio at z = 2.9. From these results, we conclude that the quasars most luminous in the ultraviolet have the smallest dust content and appear luminous primarily because of lessened extinction. Observed ultraviolet/infrared luminosity ratios are used to define "obscured" quasars as those having > 5 magnitudes of ultraviolet extinction. We present a new summary of obscured quasars discovered with the Spitzer Infrared Spectrograph and determine the infrared luminosity function of these obscured quasars at z ~ 2.1. This is compared with infrared luminosity functions of optically discovered, unobscured quasars in the SDSS and in the AGN and Galaxy Evolution Survey. The comparison indicates comparable numbers of obscured and unobscured quasars at z ~ 2.1 with a possible excess of obscured quasars at fainter luminosities., Accepted 2014 June 7 for publication in The Astrophysical Journal; received 2013 August 22. v2 contains reference updates added in proof

The galaxy Markarian 231 is reviewed as a prototype that includes all physical processes associated with active galaxies. It contains both a starburst and an AGN which are highly obscured by dust. Its luminosity at various wavelengths is used to summarize how such a galaxy would appear at high redshift, as observed with missions such as HST, AXAF, and SIRTF. The conclusion is that Markarian 231 provides an excellent example for comparing low redshift and high redshift phenomena.

Observations in the rest frame ultraviolet from various space missions are used to define the nearby starburst regions having the highest surface brightness on scales of several hundred pc. The bright limit is found to be 6x10^-16 ergs/cm^2-s-A-arcsec^2 for rest frame wavelength of 1830 A. Surface brightness in the brightest pixel is measured for 18 galaxies in the Hubble Deep Field having z > 2.2. After correcting for cosmological dimming, we find that the high redshift starbursts have intrinsic ultraviolet surface brightness that is typically four times brighter than low redshift starbursts. It is not possible to conclude whether this difference is caused by decreased dust obscuration in the high redshift starburst regions or by intrinsically more intense star formation. Surface brightness enhancement of starburst regions may be the primary factor for explaining the observed increase with redshift of the ultraviolet luminosity arising from star formation., accepted for publication in AJ; 11 pages text, 3 tables, 3 figures (embedded)

Mid-infrared spectroscopic measurements from the Infrared Spectrometer (IRS) on Spitzer are given for 125 hard X-ray active galactic nuclei (AGNs; 14-195?keV) from the Swift Burst Alert Telescope (BAT) sample and for 32 AGNs with black hole masses (BHMs) from reverberation mapping. The 9.7 ?m silicate feature in emission or absorption defines an infrared AGN classification describing whether AGNs are observed through dust clouds, indicating that 55% of the BAT AGNs are observed through dust. The mid-infrared dust continuum luminosity is shown to be an excellent indicator of intrinsic AGN luminosity, scaling closely with the hard X-ray luminosity, log ?L ?(7.8 ?m)/L(X) = ?0.31 ? 0.35, and independent of classification determined from silicate emission or absorption. Dust luminosity scales closely with BHM, log ?L ?(7.8 ?m) = (37.2 ? 0.5) + 0.87 log BHM for luminosity in erg?s?1 and BHM in M ?. The 100 most luminous type 1 quasars as measured in ?L ?(7.8 ?m) are found by comparing Sloan Digital Sky Survey (SDSS) optically discovered quasars with photometry at 22 ?m from the Wide-Field Infrared Survey Explorer (WISE), scaled to rest frame 7.8 ?m using an empirical template determined from IRS spectra. The most luminous SDSS/WISE quasars have the same maximum infrared luminosities for all 1.5?< z?< 5, reaching total infrared luminosity L IR = 1014.4 L ?. Comparing with dust-obscured galaxies from Spitzer and WISE surveys, we find no evidence of hyperluminous obscured quasars whose maximum infrared luminosities exceed the maximum infrared luminosities of optically discovered quasars. Bolometric luminosities L bol estimated from rest-frame optical or ultraviolet luminosities are compared to L IR. For the local AGN, the median log L IR/L bol = ?0.35, consistent with a covering factor of 45% for the absorbing dust clouds. For the SDSS/WISE quasars, the median log L IR/L bol?= 0.1, with extremes indicating that ultraviolet-derived L bol can be seriously underestimated even for type 1 quasars.

We present spectroscopic results for all galaxies observed with the Spitzer Infrared Spectrograph (IRS) that also have total infrared fluxes f IR measured with the Infrared Astronomical Satellite (IRAS), also using AKARI photometry when available. Infrared luminosities and spectral energy distributions (SEDs) from 8 μm to 160 μm are compared to polycyclic aromatic hydrocarbon emission from starburst galaxies or mid-infrared dust continuum from active galactic nuclei (AGNs) at rest-frame wavelengths ~8 μm. A total of 301 spectra are analyzed for which IRS and IRAS include the same unresolved source, as measured by the ratio f ν(IRAS 25 μm)/f ν(IRS 25 μm). Sources have 0.004 < z < 0.34 and 42.5 < log L IR < 46.8 (erg s–1) and cover the full range of starburst galaxy and AGN classifications. Individual spectra are provided electronically, but averages and dispersions are presented. We find that log [L IR/νL ν(7.7 μm)] = 0.74 ± 0.18 in starbursts, log [L IR/νL ν(7.7 μm)] = 0.96 ± 0.26 in composite sources (starburst plus AGN), log [L IR/νL ν(7.9 μm)] = 0.80 ± 0.25 in AGNs with silicate absorption, and that log [L IR/νL ν(7.9 μm)] = 0.51 ± 0.21 in AGNs with silicate emission. L IR for the most luminous absorption and emission AGNs are similar and 2.5 times larger than for the most luminous starbursts. AGNs have systematically flatter SEDs than starbursts or composites, but their dispersion in SEDs overlaps starbursts. Sources with the strongest far-infrared luminosity from cool dust components are composite sources, indicating that these sources may contain the most obscured starbursts.

We present samples of starburst galaxies that represent the extremes discovered with infrared and ultraviolet observations, including 25 Markarian galaxies, 23 ultraviolet luminous galaxies discovered with GALEX, and the 50 starburst galaxies having the largest infrared/ultraviolet ratios. These sources have z < 0.5 and cover a luminosity range of ~ 10^4. Comparisons between infrared luminosities determined with the 7.7 um PAH feature and ultraviolet luminosities from the stellar continuum at 153 nm are used to determine obscuration in starbursts and dependence of this obscuration on infrared or ultraviolet luminosity. A strong selection effect arises for the ultraviolet-selected samples: the brightest sources appear bright because they have the least obscuration. Obscuration correction for the ultraviolet-selected Markarian+GALEX sample has the form log[UV(intrinsic)/UV(observed)] = 0.07(+-0.04)M(UV)+2.09+-0.69 but for the full infrared-selected Spitzer sample is log[UV(intrinsic)/UV(observed)] = 0.17(+-0.02)M(UV)+4.55+-0.4. The relation of total bolometric luminosity L_{ir} to M(UV) is also determined for infrared-selected and ultraviolet-selected samples. For ultraviolet-selected galaxies, log L_{ir} = -(0.33+-0.04)M(UV)+4.52+-0.69. For the full infrared-selected sample, log L_{ir} = -(0.23+-0.02)M(UV)+6.99+-0.41, all for L_{ir} in solar luminosities and M(UV) the AB magnitude at rest frame 153 nm. These results imply that obscuration corrections by factors of two to three determined from reddening of the ultraviolet continuum for Lyman Break Galaxies with z > 2 are insufficient, and should be at least a factor of 10 for M(UV) about -17, with decreasing correction for more luminous sources., accepted for publication in The Astrophysical Journal

We present an atlas of Spitzer/IRS high resolution (R~600) 10-37um spectra for 24 well known starburst galaxies. The spectra are dominated by fine-structure lines, molecular hydrogen lines, and emission bands of polycyclic aromatic hydrocarbons. Six out of the eight objects with a known AGN component show emission of the high excitation [NeV] line. This line is also seen in one other object (NGC4194) with, a priori, no known AGN component. In addition to strong polycyclic aromatic hydrocarbon emission features in this wavelength range (11.3, 12.7, 16.4um), the spectra reveal other weak hydrocarbon features at 10.6, 13.5, 14.2um, and a previously unreported emission feature at 10.75um. An unidentified absorption feature at 13.7um is detected in many of the starbursts. We use the fine-structure lines to derive the abundance of neon and sulfur for 14 objects where the HI 7-6 line is detected. We further use the molecular hydrogen lines to sample the properties of the warm molecular gas. Several basic diagrams characterizing the properties of the sample are also shown. We have combined the spectra of all the pure starburst objects to create a high S/N template, which is available to the community., 25 pages (emulate apj), 6 tables, 14 figures, Accepted for publication in ApJS

A summary of mid-infrared continuum luminosities arising from dust is given for very luminous galaxies, Lir > 10^12 solar luminosities, with 0.005 < z < 3.2 containing active galactic nuclei (AGN), including 115 obscured AGN and 60 unobscured (type 1) AGN. All sources have been observed with the Spitzer Infrared Spectrograph. Obscured AGN are defined as having optical depth > 0.7 in the 9.7 um silicate absorption feature and unobscured AGN show silicate in emission. Luminosity vLv(8 um) is found to scale as (1+z)^2.6 to z = 2.8, and luminosities vLv(8 um) are approximately 3 times greater for the most luminous unobscured AGN. Total infrared luminosities for the most luminous obscured AGN, Lir(AGN_obscured) in solar luminosities, scale as log Lir(AGN_obscured) = 12.3+-0.25 + 2.6(+-0.3)log(1+z), and for the most luminous unobscured AGN, scale as log Lir(AGN1) = 12.6+-0.15 + 2.6(+-0.3)log(1+z), indicating that the most luminous AGN are about 10 times more luminous than the most luminous starbursts. Results are consistent with obscured and unobscured AGN having the same total luminosities with differences arising only from orientation, such that the obscured AGN are observed through very dusty clouds which extinct about 50% of the intrinsic luminosity at 8 um. Both obscured and unobscured AGN should be detected to z ~ 6 by Spitzer surveys with fv(24 um) > 0.3 mJy, even without luminosity evolution for z > 2.5. By contrast, the most luminous starbursts cannot be detected for z > 3, even if luminosity evolution continues beyond z = 2.5., Includes corrected Figure 3, as published

The mid-infrared spectroscopic analysis of a flux-limited sample of galaxies with fv(24um) > 10 mJy is presented. Sources observed are taken from the Spitzer First Look Survey (FLS) catalog and from the NOAO Deep Wide-Field Survey region in Bootes (NDWFS). The spectroscopic sample includes 60 of the 100 sources in these combined catalogs having fv(24um) > 10 mJy. New spectra from the Spitzer Infrared Spectrograph are presented for 25 FLS sources and for 11 Bootes AGN; these are combined with 24 Bootes starburst galaxies previously published to determine the distribution of mid-infrared spectral characteristics for the total 10 mJy sample. Sources have 0.01 < z < 2.4 and 41.8 < log vLv(15um) < 46.2 (ergs/s). Average spectra are determined as a function of luminosity; lower luminosity sources (log vLv(15um) < 44.0) are dominated by PAH features and higher luminosity sources (log vLv(15um) > 44.0) are dominated by silicate absorption or emission. We find that a rest frame equivalent width of 0.4um for the 6.2um PAH emission feature provides a well defined division between lower luminosity, "pure" starbursts and higher luminosity AGN or composite sources. Using the average spectra, fluxes fv(24um) which would be observed with the Spitzer MIPS are predicted as a function of redshift for sources with luminosities that correspond to the average spectra. AGN identical to those in this 10 mJy sample could be seen to z = 3 with fv(24um) > 1 mJy, but starbursts fall to fv(24um) < 1 mJy by z ~ 0.5. This indicates that substantial luminosity evolution of starbursts is required to explain the numerous starbursts found in other IRS results having fv(24um) > 1 mJy and z ~ 2., Accepted for publication in The Astrophysical Journal

Extragalactic sources from the IRAS Faint Source Catalog (FSC) which have the optically faintest magnitudes (E > 18) were selected by spatial coincidence with a source in the FIRST radio survey, and 28 of these sources have been observed with the Infrared Spectrograph on Spitzer (IRS). While an infrared source is always detected with the IRS at the FIRST position, only ~ 50% of the infrared sources are real FSC detections, as estimated from the number of sources for which the fv(25um) determined with the IRS is fainter than the sensitivity limit for the FSC. Sources have 0.12 < z < 1.0 and luminosities (ergs per s) 43.3 < log[vLv(5.5um)] < 46.7, encompassing the range from local ULIRGs to the most luminous sources discovered by Spitzer at z ~ 2. Detectable PAH features are found in 15 of the sources (54%), and measurable silicate absorption is found in 19 sources (68%); both PAH emission and silicate absorption are present in 11 sources. PAH luminosities are used to determine the starburst fraction of bolometric luminosity, and model predictions for a dusty torus are used to determine the AGN fraction of luminosity in all sources based on log[vLv(5.5um)]. Approximately half of the sources have luminosity dominated by an AGN and approximately half by a starburst. The ratio of infrared to radio flux, defined as q = log[fv(25um)/fv(1.4 GHz)], does not distinguish between AGN and starburst for these sources., Accepted for publication in The Astrophysical Journal

We present mid-infrared spectra of thirty two high redshift ultraluminous infrared galaxies, selected via the stellar photospheric feature at rest-frame 1.6um, and an observed-frame 24um flux of >500muJy. Nearly all the sample reside in a redshift range of =1.71+/-0.15, and have rest-frame 1-1000um luminosities of 10^12.9 - 10^13.8 Lsun. Most of the spectra exhibit prominent polycyclic aromatic hydrocarbon emission features, and weak silicate absorption, consistent with a starburst origin for the IR emission. Our selection method appears to be a straightforward and efficient way of finding distant, IR-luminous, star-forming galaxies in narrow redshift ranges. There is however evidence that the mid-IR spectra of our sample differ systematically from those of local ULIRGs; our sample have comparable PAH equivalent widths but weaker apparent silicate absorption, and (possibly) enhanced PAH 6.2um/7.7um and 6.2um/11.2um flux ratios. Furthermore, the composite mid-IR spectrum of our sample is almost identical to that of local starbursts with IR luminosities of 10^10-10^11 Lsun rather than that of local ULIRGs. These differences are consistent with a reduced dust column, which can plausibly be obtained via some combination of (1) star formation that is extended over spatial scales of 1-4Kpc, and (2) star formation in unusually gas-rich regions., Comment: ApJ accepted. Higher quality figures available on request

A complete flux-limited sample of 50 galaxies is presented having f_{\nu}(24um) > 10mJy, chosen from a survey with the Multiband Imaging Photometer on Spitzer (MIPS) of 8.2 deg^{2} within the NOAO Deep Wide-Field Survey region in Bootes (NDWFS). Spectra obtained with the low-resolution modules of the Infrared Spectrograph on Spitzer (IRS) are described for 36 galaxies within this sample; 25 show strong PAH emission features characteristic of starbursts, and 11 show silicate absorption or emission, emission lines, or featureless spectra characteristic of AGN. Infrared or optical spectral classifications are available for 48 of the entire sample of 50; 33 galaxies are classified as starbursts and 15 as AGN. (There are an additional 19 Galactic stars with f_{\nu}(24um) > 10mJy in the survey area.) Using a relation between 7.7um PAH luminosity and star formation rate derived from previous IRS observations of starbursts, the star formation rate per unit volume of the local universe (SFRD) is determined from the complete sample and is found to be 0.008 \mdot Mpc^{-3}. Individual sources in the sample have star formation rates from 0.14 to 160 \mdot. The derived value for the local SFRD is about half that of the local SFRD deduced from bolometric luminosities of the IRAS 60um Bright Galaxy Sample, with the deficiency being at lower luminosities and arising primarily from the small number of low luminosity sources in the 10 mJy sample. The agreement for higher luminosities confirms the validity of using the 7.7um PAH feature as a measure of SFRD in the high redshift universe, where this is often the only indicator available for faint sources., Comment: accepted to publish in ApJ

A sample of 196 AGNs and ULIRGs observed by the Infrared Spectrograph (IRS) on Spitzer is analyzed to study the distribution of the strength of the 9.7 micron silicate feature. Average spectra are derived for quasars, Seyfert 1 and Seyfert 2 AGNs, and ULIRGs. We find that quasars are characterized by silicate features in emission and Seyfert 1s equally by emission or weak absorption. Seyfert 2s are dominated by weak silicate absorption, and ULIRGs are characterized by strong silicate absorption (mean apparent optical depth about 1.5). Luminosity distributions show that luminosities at rest frame 5.5 micron are similar for the most luminous quasars and ULIRGs and are almost 10^5 times more luminous than the least luminous AGN in the sample. The distributions of spectral characteristics and luminosities are compared to those of optically faint infrared sources at z~2 being discovered by the IRS, which are also characterized by strong silicate absorption. It is found that local ULIRGs are a similar population, although they have lower luminosities and somewhat stronger absorption compared to the high redshift sources., Comment: Accepted for publication on ApJL

The unprecedented sensitivity of the Infrared Spectrograph on the Spitzer Space Telescope allows for the first time the measurement of mid-infrared spectra from 14 to 38 microns of faint high-z galaxies. This unique capability is demonstrated with observations of sources having 16 micron fluxes of 3.6 mJy (CFRS 14.1157) and 0.35 mJy (CFRS 14.9025). A spectral-fitting technique is illustrated which determines the redshift by fitting emission and absorption features characteristic of nearby galaxies to the spectrum of an unknown source. For CFRS 14.1157, the measured redshift is z = 1.00+/-0.20 in agreement with the published result of z = 1.15. The spectrum is dominated by emission from an AGN, similar to the nucleus of NGC 1068, rather than a typical starburst with strong PAH emission like M82. Such spectra will be crucial in characterizing the nature of newly discovered distant galaxies, which are too faint for optical follow-up., Accepted in ApJ Sup. Spitzer Special Issue, 4 pages, 5 figures

This Conference was highlighted by exceptional observational results for redshifts above two. These results are summarized in context of three major issues that arose for these high redshifts: 1. Why are there no faint quasars?, 2. How does star birth compare to that in the local universe?, 3. How much of what is there is obscured by dust?

Observations at various wavelengths are considered for extragalactic nuclei and are compared to how our galaxy would appear at comparable distances. The starlight from our nucleus is similar to that from the spirals in the Virgo Cluster. Our nucleus would show no sign of activity to a distant observer, neither unusual color, nor emission lines, nor excess infrared radiation. For example, the luminosity in Hβ emission is about 1038 ergs s−1, which is 100 times fainter than that in the faintest Seyfert galaxy or emission line galaxy. It is also emphasized that there is no evidence from X-ray data for a massive, condensed object in the Galactic nucleus.

Nous présentons les diagrammes magnitude-décalage spectral et diamètre angulaire-décalage spectral pour les galaxies de Sey-fert. Tous deux montrent des corrélations indiquant que les décalages vers le rouge sont cosmologiques. Les luminosités des noyaux des Seyfert de classe I sont comparées â celles des QSO en utilisant des nouvelles mesures de flux de raies en émission. La continuité des luminosités, bien établie, avec des galaxies de Seyfert couvrant un intervalle de luminosité plus grand que les QSO est une indication que les décalages vers le rouge des QSO sont aussi de nature cosmologique.

This book utilises the author's twenty years of research experience to describe conclusions on the nature of quasars, their distribution, and their evolution in the universe. Concentrating on observational astrophysics rather than theory, Quasar Astronomy serves as both a summary of what is known about quasars and a guide to research methods and unanswered questions. After reviewing the cosmological framework and necessary equations, the book discusses all aspects of observed quasar properties. Techniques are summarised for analysing quasar data obtained with various kinds of telescopes and extensive references are given to recent publications. The major original contributions in chapters 5 and 6 deal with the distribution of quasars in spacetime, a subject extensively discussed in the current research literature. The book not only summarises existing results from various techniques but refers to the potentialities of new instruments that are presently being developed, resulting in a unified, up-to-date and practical account of the astronomy of quasars.

To elucidate the nature of infrared-luminous galaxies discovered with the IRAS satellite, the optical and infrared luminosities of 1161 Markarian galaxies and 2146 normal galaxies from the CfA redshift survey are compared. Survival analysis statistical methods that take upper limits fully into account are used. It is found that L(IR)/L(B) is statistically correlated with L(60) in both samples, though they differ in the distribution at low luminosities. The derived correlation shows that L(IR)/L(B) provides an indicator for L(60). Since galaxies selected in unbiased IRAS surveys will have higher L(IR)/L(B) than optically selected galaxies, they are therefore also selected for high L(60). 22 references.