Spectral Energy Distributions Of Quasars Research Articles

Herschel-PACS far-infrared photometry of twoz > 4 quasars

We present Herschel far-infrared (FIR) observations of two sub-mm bright quasars at high redshift: SDSS J1148+5251 (z=6.42) and BR 1202-0725 (z=4.69) obtained with the PACS instrument. Both objects are detected in the PACS photometric bands. The Herschel measurements provide additional data points that constrain the FIR spectral energy distributions (SEDs) of both sources, and they emphasise a broad range of dust temperatures in these objects. For lambda_rest ~< 20mu, the two SEDs are very similar to the average SEDs of quasars at low redshift. In the FIR, however, both quasars show excess emission compared to low-z QSO templates, most likely from cold dust powered by vigorous star formation in the QSO host galaxies. For SDSS J1148+5251 we detect another object at 160mu with a distance of ~10 arcseconds from the QSO. Although no physical connection between the quasar and this object can be shown with the available data, it could potentially confuse low-resolution measurements, thus resulting in an overestimate of the FIR luminosity of the z=6.42 quasar.

The Quasar Continuum

AbstractThe powerful compact continuum emission from quasars is understood only in outline. New surveys allow investigation of the quasar continuum over a wide range of parameters (z, L, L/LEdd) and wavelengths (radio to X-ray). I review the spectral energy distributions of quasars and how new scaling relations with physical parameters promise to take us to a deeper understanding of the quasar continuum.

The Host Galaxy and Central Engine of the Dwarf Active Galactic Nucleus POX 52

We present new multi-wavelength observations of the dwarf Seyfert 1 galaxy POX 52 in order to investigate the properties of the host galaxy and the active nucleus, and to examine the mass of its black hole, previously estimated to be ~ 10^5 M_sun. Hubble Space Telescope ACS/HRC images show that the host galaxy has a dwarf elliptical morphology (M_I = -18.4 mag, Sersic index n = 4.3) with no detected disk component or spiral structure, confirming previous results from ground-based imaging. X-ray observations from both Chandra and XMM show strong (factor of 2) variability over timescales as short as 500 s, as well as a dramatic decrease in the absorbing column density over a 9 month period. We attribute this change to a partial covering absorber, with a 94% covering fraction and N_H = 58^{+8.4}_{-9.2} * 10^21 cm^-2, that moved out of the line of sight in between the XMM and Chandra observations. Combining these data with observations from the VLA, Spitzer, and archival data from 2MASS and GALEX, we examine the spectral energy distribution (SED) of the active nucleus. Its shape is broadly similar to typical radio-quiet quasar SEDs, despite the very low bolometric luminosity of L_bol = 1.3 * 10^43 ergs/s. Finally, we compare black hole mass estimators including methods based on X-ray variability, and optical scaling relations using the broad H-beta line width and AGN continuum luminosity, finding a range of black hole mass from all methods to be M_bh = (2.2-4.2) * 10^5 M_sun, with an Eddington ratio of L_bol/L_edd = 0.2-0.5.

EUV spectral energy distribution of quasars restored from associated absorbers

(abridged) Aims. To reconstruct the spectral shape of the quasar ionizing radiation in the extreme-UV range (1Ryd <= E < 10Ryd) from the analysis of narrow absorption lines (NAL) of the associated systems. Methods. Computational technique for inverse spectroscopic problems - Monte Carlo Inversion augmented by procedure of the spectral shape recovering and modified to account for the incomplete coverage of the light source. Results. The ionizing spectra responsible for the ionization structure of the NAL systems require an intensity depression at E > 4Ryd which is attributed to the HeII Lyman continuum opacity (tau^HeII_c ~ 1). A most likely source of this opacity is a quasar accretion disk wind. The corresponding column density of HI in the wind is estimated as a few times 10^16 cm^-2. This amount of neutral hydrogen should cause a weak continuum depression at lamb <= 912A (rest-frame), and a broad and shallow absorption in HI Ly-alpha. If metallicity of the wind is high enough, other resonance lines of OVI, NeVI-NeVIII, etc. are expected. In the analyzed QSO spectra we do observe broad (stretching over 1000s km/s) and shallow (tau << 1) absorption troughs of HI Ly-alpha and OVI 1031,1037A...

Radio through X‐Ray Spectral Energy Distributions of 38 Broad Absorption Line Quasars

We have compiled the largest sample of multiwavelength spectral energy distributions (SEDs) of broad absorption line (BAL) quasars to date, from the radio to the X-ray. We present new Spitzer MIPS (24, 70, and 160 ?m) observations of 38 BAL quasars in addition to data from the literature and public archives. In general, the mid-infrared properties of BAL quasars are consistent with those of non-BAL quasars of comparable luminosity. In particular, the optical to mid-infrared luminosity ratios of the two populations are indistinguishable. We also measure or place upper limits on the contribution of star formation to the far-infrared power. Of 22 (57%) upper limits, seven quasars have sufficiently sensitive constraints to conclude that star formation likely contributes little (<20%) to their far-infrared power. The 17 BAL quasars (45%) with detected excess far-infrared emission likely host hyperluminous starbursts with LFIR, SF = 1013-1014 L?. Mid-infrared through X-ray composite BAL quasar SEDs are presented, incorporating all of the available photometry. Overall, we find no compelling evidence for inherent differences between the SEDs of BAL versus non-BAL quasars of comparable luminosity. Therefore a cocoon picture of a typical BAL quasar outflow whereby the wind covers a large fraction of the sky is not supported by the mid-infrared SED comparison with normal quasars, and the disk-wind paradigm with a typical radio-quiet quasar hosting a BAL region remains viable.

Energy Distribution of Individual Quasars from Far‐Ultraviolet to X‐Rays. I. Intrinsic Ultraviolet Hardness and Dust Opacities

Using Chandra and HST archival data, we have studied the individual Spectral Energy Distribution (SED) of 11 quasars. All UV spectra show a spectral break around 1100A. 5 X-ray spectra show the presence of a ``soft excess'' and 7 spectra showed an intrinsic absorption. We found that for most quasars a simple extrapolation of the far-UV powerlaw into the X-ray domain generally lies below the X-ray data and that the big blue bump and the soft X-ray excess do not share a common physical origin. We explore the issue of whether the observed SED might be dust absorbed in the far and near-UV. We fit the UV break, assuming a powerlaw that is absorbed by cubic nanodiamond dust grains. We then explore the possibility of a universal SED (with a unique spectral index) by including further absorption from SMC-like extinction. Using this approach, satisfactory fits to the spectra can be obtained. The hydrogen column densities required by either nanodiamonds or amorphous dust models are all consistent, except for one object, with the columns deduced by our X-ray analysis, provided that the C depletion is ~0.6. Because dust absorption implies a flux recovery in the extreme UV (<700A), our modeling opens the possibility that the intrinsic quasar SED is much harder and more luminous in the extreme UV than inferred from the near-UV data, as required by photoionization models of the broad emission line region. We conclude that the intrinsic UV SED must undergo a sharp turn-over before the X-ray domain.

Towards a new standard model for black hole accretion

We briefly review recent developments in black hole accretion disk theory, emphasizing the vital role played by magnetohydrodynamic (MHD) stresses in transporting angular momentum. The apparent universality of accretion-related outflow phenomena is a strong indicator that large-scale MHD torques facilitate vertical transport of angular momentum. This leads to an enhanced overall rate of angular momentum transport and allows accretion of matter to proceed at an interesting rate. Furthermore, we argue that when vertical transport is important, the radial structure of the accretion disk is modified at small radii and this affects the disk emission spectrum. We present a simple model demonstrating how energetic, magnetically-driven outflows modify the emergent disk emission spectrum with respect to that predicted by standard accretion disk theory. A comparison of the predicted spectra against observations of quasar spectral energy distributions suggests that mass accretion rates inferred using the standard disk model may be severely underestimated.

An Investigation into the Effects of Luminosity on the Mid‐Infrared Spectral Energy Distributions of Radio‐quiet Quasars

We present an analysis of the effects of luminosity on the shape of the mid-infrared spectral energy distributions (SEDs) of 234 radio-quiet quasars originally presented by Richards et al. In quasars without evident dust extinction, the spectrally integrated optical and infrared luminosities are linearly correlated over nearly three decades in luminosity. We find a significant (≳99.99% confidence) correlation between the 1.8-8.0 μm spectral index and infrared luminosity that indicates an enhancement of the mid-infrared continuum with increasing luminosity. Coupled with strong evidence for spectral curvature in more luminous quasars, we conclude that this trend is likely a manifestation of the near-infrared (3-5 μm) bump noted in earlier quasar SED surveys. The strength of this feature is indicative of the contribution of emission from the hottest (≳1000 K) dust to the mid-infrared spectrum; higher luminosity quasars tend to show more hot dust emission. Finally, the comparable distribution of bolometric corrections from the monochromatic 3 μm luminosity as well as its lack of sensitivity to dust extinction as compared to the standard bolometric correction from νL_(5100 A) suggest that the former may be a more robust indicator of bolometric quasar luminosity. The close link between the power in the mid-infrared and optical and the effect of luminosity on the shape of the mid-infrared continuum indicate that considering mid-infrared emission independent of the properties of the quasar itself is inadequate for understanding the parsec-scale quasar environment.

Spectral Energy Distributions and Multiwavelength Selection of Type 1 Quasars

We present an analysis of the mid-infrared (MIR) and optical properties of type 1 (broad-line) quasars detected by the Spitzer Space Telescope. The MIR color-redshift relation is characterized to z ~ 3, with predictions to z = 7. We demonstrate how combining MIR and optical colors can yield even more efficient selection of active galactic nuclei (AGNs) than MIR or optical colors alone. Composite spectral energy distributions (SEDs) are constructed for 259 quasars with both Sloan Digital Sky Survey and Spitzer photometry, supplemented by near-IR, GALEX, VLA, and ROSAT data, where available. We discuss how the spectral diversity of quasars influences the determination of bolometric luminosities and accretion rates; assuming the mean SED can lead to errors as large as 50% for individual quasars when inferring a bolometric luminosity from an optical luminosity. Finally, we show that careful consideration of the shape of the mean quasar SED and its redshift dependence leads to a lower estimate of the fraction of reddened/obscured AGNs missed by optical surveys as compared to estimates derived from a single mean MIR to optical flux ratio.

Nanodiamond dust and the energy distribution of quasars

AbstractThe spectral energy distribution of quasars shows a sharp steepening of the continuum shortward of ≃1100 Å. The steepening could be a result of dust absorption. We present a dust extinction model which considers crystalline carbon grains and compare it with SMC‐like dust extinction consisting of a mixture of silicate grains with graphite or amorphous carbon grains. We show that the sharp break seen in individual quasar spectra of intermediate redshift ∼1–2 can be reproduced by dust absorption provided the extinction curve consists of nanodiamonds, composed of terrestial cubic diamonds or of diamonds similar to the presolar nanodiamonds found in primitive meteorites. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The associated system of HE 2347-4342

We present an analysis of the complex associated system of the high-redshift QSO HE 2347-4342. Absorption features of H I, C III, C IV, N V, and O Vi with up to 16 components occur in the optical spectral range located up to 1500 km s -1 redwards from the emission line. Apparently, C IVand Nv show the line locking effect. A quantivative analysis of the line distribution comparing simulated spectra with randomly distributed doublets reveals, however, no statistical evidence for its physical reality. Using photoionization calculations to emulate the observed ion column densities we constrain the quasar's spectral energy distribution. Absorbers in the velocity range of 200-600 km s -1 can be modelled successfully with a spectral index of α ∼ -3 at energies higher than 3-4 Ryd, which is an energy distribution similar to the QSO continuum suggested by Mathews & Ferland (1987). The analysis of a group of high velocity absorbers (v > 1300 km s -1 ) leads to a harder energy distribution. The large amount of helium (log N He II > 16.3) associated with these absorbers implies that they are responsible for the observed absence of the proximity effect (Reimers et al. 1997). Clouds located more distant from the quasar may be shielded from the high energy part of the quasar continuum due to optically thick absorption shortward of 228 A by the high velocity absorbers. A group of absorbers with 900 < v < 1200 km s -1 , in particular a cloud at 1033 km s -1 , which has the most reliable column density measurements, can be modelled neither with photoionzation nor under the assumption of collisionally ionized gas. Possible explanations are a multiphase medium with a mixture of photo and collisionally ionized gas and/or gas in non-equilibrium.

Infrared SEDs of quasars and radio galaxies: Unification and dust evolution seen by ISO, SCUBA and MAMBO

Sensitive Infrared Spectral Energy Distributions (SEDs) drawn from the ISO data archive and supplemented with SCUBA and MAMBO observations, provide evidence for the geometric unification of powerful 3CR radio galaxies as “edge-on” quasars. Furthermore, detailed SEDs of 64 Palomar-Green quasars show a diversity of shapes, consistent with the physical evolution of the heating sources and the dust distribution around them.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Technique for Detecting Warm‐Hot Intergalactic Gas in Quasar Ultraviolet Spectra

The ionizing spectral energy distribution of quasars exhibits a steepening of the distribution shortward of ~1200 ?. The change of the power-law index from approximately -1 (near-UV) to -2 (far-UV) has so far been interpreted as being intrinsic to quasars. We consider the possibility that the steepening may result from a tenuous absorption component that is anticorrelated with large mass overdensities. UV-sensitive satellites, whose detectors can extend down to 1000 ?, can set a useful limit to such an absorption component through the search for a flux increase in the window 1050-1190 ? (observer frame) with respect to an extrapolation of the continuum above 1230 ?. Since the recent Far Ultraviolet Spectroscopic Explorer and Hubble Space Telescope Space Telescope Imaging Spectrograph data do not show any obvious discontinuity in this region, this effectively rules out the possibility that intergalactic H I absorption is very important, and we conclude that most if not all of the steepening is intrinsic to quasars. A smaller flux discontinuity of order 1% cannot, however, be ruled out yet and would still be consistent with the warm-hot intergalactic component if it amounts to 30% of the baryonic mass, as predicted by some models of large scale structure formation, provided its temperature lies around 105.3 K.

Emission Line Properties of Active Galactic Nuclei from a Pre‐COSTAR Faint Object Spectrograph Hubble Space Telescope Spectral Atlas

UV/optical emission lines offer some of the most detailed information obtainable about the intrinsic properties of quasars. Studies of the density, ionization and metal abundance of gas near the accreting black hole are probed through an intriguing but poorly understood complex of correlations between emission lines and overall quasar spectral energy distributions that has long suffered from a lack of large, consistently measured samples. As part of a broader effort to expand and systematize the data upon which these studies are built, we present measurements of the UV/optical emission line parameters in a sample of 158 active galactic nuclei observed with the Faint Object Spectrograph on the Hubble Space Telescope (HST), prior to the installation of COSTAR. We use an automated technique that accounts for galactic reddening, includes iron emission blends, galactic and intrinsic absorption lines, and performs multicomponent fits to the emission line profiles. We present measured line parameters (equivalent width and FWHM) for a large number (28) of different UV/optical lines, including upper limits for undetected lines. We also study the relations between the emission line equivalent widths and luminosity (the Baldwin effect), as well as redshift (evolution). We compare results from this HST FOS sample with our previous measurements of 993 QSOs in the Large Bright Quasar Survey using the same analysis technique and sum the samples to achieve better coverage of the luminosity-redshift plane. We confirm a significant Baldwin effect for UV iron emission from Green et al. and find that evolution dominates the effect for iron and for Si IV emission. The values of the Baldwin effect slopes for all UV emission lines and the dependence of the slopes on the sample's luminosity range point to a change of the SED as the cause of the Baldwin effect in the FOS sample.

A Survey of [CLC][ITAL]z[/ITAL][/CLC] &gt; 5.8 Quasars in the Sloan Digital Sky Survey. I. Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at [CLC][ITAL]z[/ITAL][/CLC] ∼ 6

We present the results from a survey of i-dropout objects selected from ~1550 deg2 of multicolor imaging data from the Sloan Digital Sky Survey to search for luminous quasars at z 5.8. Objects with i*-z* > 2.2 and z* 0.90. The ARC 3.5 m spectrum of SDSSp J103027.10+052455.0 shows that over a range of ~300 A immediately blueward of the Lyα emission, the average transmitted flux is only 0.003 ± 0.020 times that of the continuum level, consistent with zero flux over a ~300 A range of the Lyα forest region and suggesting a tentative detection of the complete Gunn-Peterson trough. The existence of strong metal lines in the quasar spectra suggests early metal enrichment in the quasar environment. The three new objects, together with the previously published z = 5.8 quasar SDSSp J104433.04-012502.2, form a complete color-selected flux-limited sample at z 5.8. We estimate the selection function of this sample, taking into account the estimated variations in the quasar spectral energy distribution, as well as observational photometric errors. We find that at z = 6, the comoving density of luminous quasars at M1450 < -26.8 (H0 = 50 km s-1 Mpc-1, Ω = 1) is 1.1 × 10-9 Mpc-3. This is a factor of ~2 lower than that at z ~ 5 and is consistent with an extrapolation of the observed quasar evolution at z < 5. Using the current sample, we discuss the constraint on the shape of the quasar luminosity function and the implications for the contribution of quasars to the ionizing background at z ~ 6. The luminous quasars discussed in the paper have central black hole masses of several times 109 M⊙ by the Eddington argument, with likely dark halo masses on the order of 1013 M⊙. Their observed space density provides a sensitive test of models of quasar and galaxy formation at high redshift.

The KX method for producing K-band flux-limited samples of quasars

The longstanding question of the extent to which the quasar population is affected by dust extinction, within host galaxies or galaxies along the line of sight, remains open. More generally, the spectral energy distributions of quasars vary significantly and flux-limited samples defined at different wavelengths include different quasars. Surveys employing flux measurements at widely separated wavelengths are necessary to characterise fully the spectral properties of the quasar population. The availability of panoramic near-infrared detectors on large telescopes provides the opportunity to undertake surveys capable of establishing the importance of extinction by dust on the observed population of quasars. We introduce an efficient method for selecting K-band, flux-limited samples of quasars, termed ``KX'' by analogy with the UVX method. This method exploits the difference between the power-law nature of quasar spectra and the convex spectra of stars: quasars are relatively brighter than stars at both short wavelengths (the UVX method) and long wavelengths (the KX method). We consider the feasibility of undertaking a large-area KX survey for damped Ly-alpha galaxies and gravitational lenses using the planned UKIRT wide-field near-infrared camera.

Spectroscopy of PKS 0528−250: New Limits on CO Absorption and Emission

We have obtained a moderate-resolution spectrum of the quasar PKS 0528-250 with the Red Channel Spectrograph on the Multiple Mirror Telescope in order to study a damped Lyα absorption-line system at z = 2.8115. We obtain a new upper limit for the CO column density for the z = 2.8108 velocity component in the z = 2.8115 damped Lyα system. The ionization of different species in this component rules out a quasar spectral energy distribution as the ionization field and implies an ultraviolet radiation field intensity a few times that of the Milky Way value. The estimated total number density is n(H) ~ 20 cm-3. The physical size for the z = 2.8108 component implied by these models is about 40 pc. The ionization of different species also suggests a structure with a hot intercloud medium associated with a H I cloud in this component, that is, most low ionized ions are from the cold medium where photoionization and photodissociation dominates. The highly ionized species may be from the intercloud medium where collisional ionization dominates. We also present newly identified Ni II absorption lines in the z = 2.1408 and z = 2.8115 damped Lyα systems. The derived depletion of nickel by dust confirms previous results that the dust-to-gas ratio in these two damped Lyα systems is about 10% of the Milky Way ratio. Millimeter-wavelength observations obtained at the NRAO 12 m telescope provide new upper limits on CO (3-2) emission in the z = 2.8115 damped Lyα system.

Emission Lines and the Spectral Energy Distributions of Quasars

AbstractMany years of study have failed to establish conclusively relationships between a quasar’s spectral energy distribution (SED) and the emission lines it is thought to produce. This is at least partially due to the lack of well-observed SEDs. We present initial results from a line–SED study for a sample of 43 quasars and active galaxies for which we have optical and ultraviolet spectra and far-infrared–X-ray SEDs. We present the results of tests for correlations between line equivalent widths and SED luminosity and slope parameters and compare these results to those from earlier studies. We find that the Baldwin effect is weaker when the luminosity is defined close to the ionizing continuum of that line and conclude that the detailed SED is likely to be important in making further progress.

On the observational evidence for accreting black holes in quasars

We review (mainly high energy) observations which could lend support to (or dismiss) the hypothesis of an accreting central black hole in quasars. Direct imaging is not powerful (1). We focus on three main topics: 2) variability as a constraint of the quasar compactness; 3) X-ray continuum; 4) X-ray spectral features, expected from reprocessing of the X-ray radiation from matter near the X-ray source. We argue that the above observations provide a weaker evidence than once thought for a black hole as the engine of quasars. New tests will come from (5) the study of the evolution of the quasar Spectral Energy Distribution (SED) in the framework of models of quasar physical evolution. We present some new results, obtained comparing ROSAT X-ray observations of z=2–3 quasars with previous monitoring of low-z quasars, which represent a first steep in this direction.

QUASAR SURVEYS

ABSTRACT In this review we discuss the problems encountered in the design, implementation, and analysis of modern surveys aimed at discovering quasars. These fall into two broad categories: (1) the practical difficulties arising from the limited signal-to-noise ratio of the source material and the finite amount of telescope time available for follow-up observations, and (2) the challenges presented by the large ranges in the intrinsic properties of quasars, notably the spread in redshift, luminosity and spectral energy distributions. Three types of surveys are described, in order of increasing difficulty of implementation, the most challenging of which would find both quasars consistently and provide a prediction of those objects that would not_ be found. To achieve the scientific goals underlying such a survey, an accurate assessment of P(M,z,SED), the probability of detection of a quasar as a function of it absolute magnitude, redshift, and spectral energy distribution, must be made. We argue that the analyses of surveys at x-ray and radio wavelengths, as well as those based upon zero proper motion and photometric variability, should also contain such a calculation. If P(M,z,SED) is known, and is non-zero over a substantial range of one or more of the independent variables, then improved constraints on the form of the luminosity function can be obtained compared to those from many surveys which are "complete" in the traditional sense of the term. We highlight the need for accurate, broad-baseline measurement of quasar spectral energy distributions, which are necessary for a quantitative improvement in our knowledge of the evolution of the luminosity function.