We present an atlas of the spectral energy distributions (SEDs) of normal, nonblazar, quasars over the whole available range (radio to 10 keV X-rays) of the electromagnetic spectrum. The primary (UVSX) sample includes 47 quasars for which the spectral energy distributions include X-ray spectral indices and UV data. Of these, 29 are radio quiet, and 18 are radio loud. The SEDs are presented both in figures and in tabular form, with additional tabular material published on CD-ROM. Previously unpublished observational data for a second set of quasars excluded from the primary sample are also tabulated. The effects of host galaxy starlight contamination and foreground extinction on the UVSX sample are considered and the sample is used to investigate the range of SED properties. Of course, the properties we derive are influenced strongly by the selection effects induced by quasar discovery techniques. We derive the mean energy distribution (MED) for radio-loud and radio-quiet objects and present the bolometric corrections derived from it. We note, however, that the dispersion about this mean is large (-one decade for both the infrared and ultraviolet components when the MED is normalized at the near-infrared inflection). At least part of the dispersion in the ultraviolet may be due to time variability, but this is unhkely to be important in the infrared. The existence of such a large dispersion indicates that the MED reflects only some of the properties of quasars and so should be used only with caution. Subject headings: atlases-galaxies: photometry-quasars: general

We present the Ðnal results of a ROSAT PSPC program to study the soft X-ray emission properties of a complete sample of low-z quasars. This sample includes all 23 quasars from the Bright Quasar Survey with z ¹ 0.400 and cm~2. Pointed ROSAT PSPC observations were made for all N H I Gal \ 1.9 ] 1020 quasars, yielding high signal-to-noise (S/N) spectra for most objects, which allowed an accurate determination of the spectral shape. The following main results were obtained :1. The spectra of 22 of the 23 quasars are consistent, to within D30%, with a single power-law model at rest-frame 0.2È2 keV. There is no evidence for signiÐcant soft excess emission with respect to the bestÐt power law. We place a limit (95% conÐdence) of D5 ] 1019 cm~2 on the amount of excess foreground absorption by cold gas for most of our quasars. The limits are D1 ] 1019 cm~2 in the two highest S/N spectra.2. The mean 0.2È2 keV continuum of quasars agrees remarkably well with an extrapolation of the mean 1050È350 continuum recently determined by et al. for z [ 0.33 quasars. This sug-A Zheng (1996) gests that there is no steep soft component below 0.2 keV.3. SigniÐcant X-ray absorption (q [ 0.3) by partially ionized gas ("" warm absorber ÏÏ) in quasars is rather rare, occurring for of the population, which is in sharp contrast to lower luminosity active [5% galactic nuclei (AGNs), where signiÐcant absorption probably occurs for D50% of the population.4. Extensive correlation analysis of the X-ray continuum emission parameters with optical emissionline parameters indicates that the strongest correlation is between the spectral slope and the Hb a x FWHM. A possible explanation for this remarkably strong correlation is a dependence of on a x L /L Edd , as seen in Galactic black hole candidates.5. The strong correlations between and Fe II/Hb, and the peak [O III] to Hb Ñux ratio are a x L *O III+ , veriÐed. The physical origin of these correlations is still not understood.6. There appears to be a distinct class of "" X-rayÈweak ÏÏ quasars, which form D10% of the population (three out of 23), where the X-ray emission is smaller, by a factor of 10È30, than expected based on their luminosity at other bands and on their Hb luminosity. These may be quasars in which the direct X-ray source is obscured and only scattered X-rays are observed. 7. Thin accretion disk models cannot reproduce the observed 0.2È2 keV spectral shape, and they also cannot reproduce the tight correlation between the optical and soft X-ray emission. An as yet unknown physical mechanism must be maintaining a strong correlation between the optical and soft X-ray emission.8. The H I/He I ratio in the high Galactic latitude ISM must be within 20%, and possibly within 5%, of the total H/He ratio of 10, which indicates that He in the di †use H II gas component of the interstellar medium is mostly ionized to He II or He III.We Ðnally note the intriguing possibility that although in radio-loud quasars ([1.15^0.14) is Sax T signiÐcantly Ñatter than in radio-quiet quasars ([1.72^0.09) the X...

We present an empirical template spectrum suitable for fitting and subtracting/studying the Fe ii and Fe iii emission lines in the restframe ultraviolet spectra of quasars and active galatic nuclei, the first empirical ultraviolet iron template to cover the full range of λ1250−3090Å. Iron emission is often a severe contaminant in opticalultraviolet spectra of active galactic nuclei and quasars. Its presence complicates and limits the accuracy of measurements of both strong and weak emission lines and the continuum emission, affecting studies of line and continuum interrelations, the ionization structure, and elemental abundances in active galaxies and quasars. Despite the wealth of work on modeling the quasar Fe ii emission and the need to account for this emission in observed quasar spectra, there is no ultraviolet template electronically available to aid this process. The iron template we present is based on Hubble Space Telescope spectra of the Narrow Line Seyfert 1 galaxy, I Zwicky 1 (I Zw 1, z = 0.061). The intrinsic narrow lines ( > ∼ 900 km s −1 ) of this source and its rich iron spectrum make the template particularly suitable for use with most active galactic nuclei and quasar spectra.The iron emission spectrum, the line identifications, and the measurements of absorption and emission lines are presented and compared with the work of Laor et al. Comments on each individual line feature and the line fitting are available in the Appendix. The methods used to develop and apply the template are also described. We illustrate the application of the derived Fe ii and Fe iii templates by fitting and subtracting iron emission from the spectra of four high redshift quasars and of the nearby quasar, 3C273, confirming their general applicability to active galaxies despite the somewhat unusual properties of I Zw 1. We briefly discuss the small discrepancies between the observed iron emission of these quasars and the ultraviolet template, and compare the template with previously published ones. We discuss the advantages and limitations of the UV Fe ii and Fe iii templates and of the template fitting method. We conclude that the templates work sufficiently well to be a valuable and important tool for eliminating and studying the iron emission in active galaxies, at least until accurate theoretical iron emission models are developed.The Si iv+O iv] λ 1400 feature in I Zw 1 is clearly strong relative to C iv λ 1549, and C iv and C iii] λ 1909 are both relatively weak. This may partially be due to the higher densities and lower ionization parameter prevailing in Narrow Line Seyfert 1 galaxies, and to the big blue bump shifting towards lower energies in more luminous Seyferts, such as I Zw 1. In I Zw 1 the narrow line width reveals that C iii] is heavily blended with Si iii] λ 1892, Al iii λλ 1854,1863, and Fe iii transitions. This suggests that the C iii] line strength and width may be overestimated in many quasar line studies where the lines are broader and deblending is not possible. This affects density estimates of th...

We present a measure of the hard (2Y8 keV ) X-ray luminosity function (XLF) of AGNs up to z $ 5. At high redshifts, the wide area coverage of the Chandra Multiwavength Project is crucial to detect rare and luminous (L X > 10 44 ergs s À1 ) AGNs. The inclusion of samples from deeper published surveys, such as the Chandra Deep Fields, allows us to span the lower L X range of the XLF. Our sample is selected from both the hard (z < 3, f 2Y8 keV > 6:3 ; 10 À16 ergs cm À2 s À1 ) and soft (z > 3, f 0:5Y2:0 keV > 1:0 ; 10 À16 ergs cm À2 s À1 ) energy band detections. Within our optical magnitude limits (r 0 ; i 0 < 24), we achieve an adequate level of completeness (>50%) regarding X-ray source identification (i.e., redshift). We find that the luminosity function is similar to that found in previous X-ray surveys up to z $ 3 with an evolution dependent on both luminosity and redshift. At z > 3, there is a significant decline in the numbers of AGNs with an evolution rate similar to that found by studies of optically selected QSOs. Based on our XLF, we assess the resolved fraction of the cosmic X-ray background, the cumulative mass density of SMBHs, and the comparison of the mean accretion rate onto SMBHs and the star formation history of galaxies as a function of redshift. A coevolution scenario up to z $ 2 is plausible, although at higher redshifts the accretion rate onto SMBHs drops more rapidly. Finally, we highlight the need for better statistics of high-redshift AGNs at z k 3, which is achievable with the upcoming Chandra surveys.

Using the large multiwavelength data set in the Chandra/SWIRE Survey (0.6 deg 2 in the Lockman Hole), we show evidence for the existence of highly obscured (Compton-thick) AGNs, estimate a lower limit to their surface density, and characterize their multiwavelength properties. Two independent selection methods based on the X-ray and infrared spectral properties are presented. The two selected samples contain (1) five X-ray sources with hard X-ray spectra and column densities k10 24 cm À2 and (2) 120 infrared sources with red and AGN-dominated infrared SEDs. We estimate a surface density of at least 25 Compton-thick AGNs deg À2 detected in the infrared in the Chandra/ SWIRE field, of which $40% show distinct AGN signatures in their optical/near-infrared SEDs, the remaining being dominated by the host galaxy emission. Only $33% of all Compton-thick AGNs are detected in the X-rays at our depth [F(0:3 8 keV) > 10 À15 ergs cm À2 s À1 ]. We report the discovery of two sources in our sample of Comptonthick AGNs, SWIRE J104409.95+585224.8 (z ¼ 2:54) and SWIRE J104406.30+583954.1 (z ¼ 2:43), which are the most luminous Compton-thick AGNs at high z currently known. The properties of these two sources are discussed in detail with an analysis of their spectra, SEDs, luminosities, and black hole masses.