Quasar Luminosity Research Articles

A Flat Photoionization Rate at 2 ≤ z ≤ 4.2: Evidence for a Stellar-Dominated UV Background and against a Decline of Cosmic Star Formation beyond z ~ 3

We investigate the implications of our measurement of the Lyα forest opacity at redshifts -->2 ≤ z ≤ 4.2 from a sample of 86 high-resolution quasar spectra for the evolution of the cosmic ultraviolet luminosity density and its sources. The derived hydrogen photoionization rate Γ is remarkably flat over this redshift range, implying an increasing comoving ionizing emissivity with redshift. Because the quasar luminosity function is strongly peaked near -->z ~ 2, star-forming galaxies likely dominate the ionizing emissivity at -->z 3. Our measurement argues against a star formation rate density declining beyond -->z ~ 3, in contrast to existing state-of-the-art determinations of the cosmic star formation history from direct galaxy counts. Stellar emission from galaxies therefore likely reionized the universe.

Deep optical spectroscopy of extended Lyα emission around three radio-quiet z = 4.5 quasars

We report the first results of a spectroscopic search for Ly alpha envelopes around three z similar to 4.5 radio-quiet quasars. Our observational strategy adopts the FORS2 spectrograph mounted to the UT1 of the Very Large Telescope (VLT) in the multi-slit mode. This allows us to observe simultaneously the quasars and several PSF stars. The spectra of the latter are used to remove the point-like quasar from the data, and to unveil the faint underlying Lya envelopes associated with the quasars to unprecedented depth. We clearly detect an envelope around two of the three quasars. The sizes of these envelopes are 10 '' and 13 '' (i. e. 67 kpc and 87 kpc). This is 5 to 10 times larger than predicted by the models of Haiman & Rees (2001, ApJ, 556, 87) and up to 100 times fainter. Our observations are in more robust agreement with models involving a clumpy envelope such as Alam & Miralda-Escuda (2002, ApJ, 568, 576) or Chelouche et al. (2007, ApJ, submitted). We find that the brighter quasars also have the brighter envelopes but that the extent of the envelopes does not depend on the quasar luminosity. Although our results are based on only two objects with a detected Lya envelope, the quality of the spatial deblending of the spectra lends considerable for hope to estimating the luminosity function and surface brightness profiles of high redshift Ly alpha envelopes down to F similar to 2-3 x 10(-21) erg s(-1) cm(-2) angstrom(-1). We conclude that the most efficient strategy for studying high redshift Lya quasar envelopes is to acquire both narrow-band images and deep slit-spectra.

Optical Selection of Faint Active Galactic Nuclei in the COSMOS Field

We outline a strategy to select faint (i_(AB)<24:5) type 1 active galactic nucleus (AGN) candidates down to the Seyfert/QSO boundary for spectroscopic targeting in the COSMOS field. Our selection process picks candidates by their nonstellar colors in uBVRizK broadband photometry from the Subaru and CFH Telescopes and morphological properties extracted from Hubble Space Telescope (HST ) ACS i-band data. Although the COSMOS field has been used extensively to survey the faint galaxy population out to z ~ 6, AGN optical color selection has not been applied to so faint a level in such a large continuous part of the sky. Hot stars are known to be the dominant contaminant for bright AGN candidate selection at z < 2, but we anticipate the highest color contamination rate at all redshifts to be from faint starburst and compact galaxies. Morphological selection via the Gini Coefficient separates most potential AGNs from these faint blue galaxies. Recent models of the quasar luminosity function (QLF) from Hopkins et al. are used to estimate quasar surface densities, and a recent study of stellar populations in the COSMOS field is applied to infer stellar surface densities and contamination.We use 292 spectroscopically confirmed type 1 broad-line AGN and quasar templates to predict AGN colors as a function of redshift, and then contrast those predictions with the colors of known contaminating populations. Since the number of galaxy contaminants cannot be reliably identified with respect to stellar and predicted QLF numbers, the completeness and efficiency of the selection cannot be calculated before gathering confirming spectroscopic observations. Instead we offer an upper limit estimate to selection efficiency (about 50% for low-z and 20%-40% for intermediate-z and high-z) as well as the completeness and efficiency with respect to an X-ray point source population (from the COSMOS AGN Survey), in the range 20%-50%. The motivation of this study and subsequent spectroscopic follow-up is to populate and refine the faint end of the QLF, at both low and high redshifts, where the population of type 1 AGNs is presently not well known. The anticipated AGN observations will add to the ~300 already known AGNs in the COSMOS field, making COSMOS a densely packed field of quasars to be used to understand supermassive black holes and probe the structure of the intergalactic medium in the intervening volume.

The History and Morphology of Helium Reionization

A variety of observations now indicate that intergalactic helium was fully ionized by z ~ 3. The most recent measurements of the high-redshift quasar luminosity function imply that these sources had produced at least ~2.5 ionizing photons per helium atom by that time, consistent with a picture in which the known quasar population drives He II reionization. Here we describe the distribution of ionized and neutral helium gas during this era. Because the sources were rare and bright (with the photon budget dominated by quasars with luminosities L L), random fluctuations in the quasar population determined the morphology of ionized gas when the global ionized fraction i was small, with the typical radius Rc of a He III bubble ~15-20 comoving Mpc. Only when i 0.5 did the large-scale clustering of the quasars drive the characteristic size of ionized regions above this value. Still later, when i 0.75, most ionizing photons were consumed by dense, recombining systems before they reached the edge of their source's ionized surroundings, halting the bubble growth when Rc ~ 35-40 Mpc. These phases are qualitatively similar to those in hydrogen reionization, but the rarity of the sources makes the early stochastic phase much more important. Moreover, the well-known characteristics of the z = 3 intergalactic medium allow a much more robust description of the late phase in which recombinations dominate.

Biases in Virial Black Hole Masses: An SDSS Perspective

We compile black hole (BH) masses for ~60,000 quasars in the redshift range 0.1 z 4.5 included in the Fifth Data Release of the Sloan Digital Sky Survey, using virial BH mass estimators based on the Hβ, Mg II, and C IV emission lines. Within our sample, the widths of the three lines follow lognormal distributions, with means and dispersions that do not depend strongly on luminosity or redshift. The Mg II- and Hβ-estimated BH masses are consistent with one another, but there is a positive bias between the C IV- and Mg II-estimated BH masses correlated with the C IV-Mg II blueshift, suggesting that the C IV estimator is more severely affected by a disk wind. If the underlying BH mass distribution decreases with mass and the Eddington ratio distribution at fixed true BH mass has nonzero width, we show that the measured virial BH mass and Eddington ratio distributions within finite luminosity bins are subject to Malmquist bias. We present a model that reproduces the observed virial mass distribution, quasar luminosity function, and line width distribution of our sample; it has an underlying BH mass distribution dN/dlog M ∝ M−2.6 and a lognormal true Eddington ratio distribution at fixed true mass with dispersion 0.4 dex and mean dependent on BH mass. In this model, the observed virial mass (Eddington ratio) distribution for the SDSS sample is biased high (low) by ~0.6 dex within finite luminosity bins. Finally, we compare virial BH masses of radio and broad absorption line quasars with ordinary quasars matched in redshift and luminosity.

Measuring the Fraction of Obscured Quasars by the Infrared Luminosity of Unobscured Quasars

Recent work has suggested that the fraction of obscured AGNs declines with increasing luminosity, but it has been difficult to quantify this trend. Here we attempt to measure this fraction as a function of luminosity by studying the ratio of mid-infrared to intrinsic nuclear bolometric luminosity in unobscured AGNs. Because the mid-infrared is created by dust reprocessing of shorter wavelength nuclear light, this ratio is a diagnostic of -->fobsc, the fraction of solid angle around the nucleus covered by obscuring matter. In order to eliminate possible redshift dependences while also achieving a large dynamic range in luminosity, we have collected archival 24 μm MIPS photometry from objects with -->z ~ 1 in the Sloan Digital Sky Survey, the Great Observatories Origins Deep Survey, and the Cosmic Evolution Survey. To measure the bolometric luminosity for each object, we used archival optical data supplemented by GALEX data. We find that the mean ratio of 24 μm to bolometric luminosity decreases by a factor of ~3 in the -->Lbol = 1044–3 × 1047 ergs s -->−1 range, but there is also a large scatter at constant -->Lbol. Using radiation transfer solutions for model geometries, we show how the IR/bolometric ratio relates to -->fobsc and compare these values with those obtained from samples of X-ray-selected AGNs. Although we find approximate agreement, our method indicates somewhat higher values of -->fobsc, particularly in the middle range of luminosities, suggesting that there may be a significant number of heavily obscured AGNs missed by X-ray surveys.

Redshifted 21-cm observations of high-redshift quasar proximity zones

The introduction of low-frequency radio arrays is expected to revolutionize the study of the reionization epoch. Observation of the contrast in redshifted 21-cm emission between a large H ii region and the surrounding neutral intergalactic medium (IGM) will be the simplest and most easily interpreted signature. However, the highest redshift quasars known are thought to reside in an ionized IGM. Using a semi-analytic model we describe the redshifted 21-cm signal from the IGM surrounding quasars discovered using the i-drop-out technique (i.e. quasars at z ∼ 6). We argue that while quasars at z < 6.5 seem to reside in the post-overlap IGM, they will still provide valuable probes of the late stages of the overlap era because the light-travel time across a quasar proximity zone should be comparable to the duration of overlap. For redshifted 21-cm observations within a 32-MHz bandpass, we find that the subtraction of a spectrally smooth foreground will not remove spectral features due to the proximity zone. These features could be used to measure the neutral hydrogen content of the IGM during the late stages of reionization. The density of quasars at z ∼ 6 is now well constrained. We use the measured quasar luminosity function to estimate the prospects for discovery of high-redshift quasars in fields that will be observed by the Murchison Widefield Array. © 2008 RAS.

A Cosmological Framework for the Co‐evolution of Quasars, Supermassive Black Holes, and Elliptical Galaxies. I. Galaxy Mergers and Quasar Activity

We develop a model for the cosmological role of mergers in the evolution of starbursts, quasars, and spheroidal galaxies. By combining theoretically well-constrained halo and subhalo mass functions as a function of redshift and environment with empirical halo occupation models, we can estimate where galaxies of given properties live at a particular epoch. This allows us to calculate, in an a priori cosmological manner, where major galaxy-galaxy mergers occur and what kinds of galaxies merge, at all redshifts. We compare this with the observed mass functions, clustering, fractions as a function of halo and galaxy mass, and small-scale environments of mergers, and we show that this approach yields robust estimates in good agreement with observations and can be extended to predict detailed properties of mergers. Making the simple Ansatz that major, gas-rich mergers cause quasar activity (but not strictly assuming they are the only triggering mechanism), we demonstrate that this model naturally reproduces the observed rise and fall of the quasar luminosity density at -->z = 0–6, as well as quasar luminosity functions, fractions, host galaxy colors, and clustering as a function of redshift and luminosity. The recent observed excess of quasar clustering on small scales at -->z ~ 0.2–2.5 is a natural prediction of our model, as mergers will preferentially occur in regions with excess small-scale galaxy overdensities. In fact, we demonstrate that quasar environments at all observed redshifts correspond closely to the empirically determined small group scale, where major mergers of ~L* gas-rich galaxies will be most efficient. We contrast this with a secular model in which quasar activity is driven by bars or other disk instabilities, and we show that, while these modes of fueling probably dominate the high Eddington ratio population at Seyfert luminosities (significant at -->z = 0), the constraints from quasar clustering, observed pseudobulge populations, and disk mass functions suggest that they are a small contributor to the -->z 1 quasar luminosity density, which is dominated by massive BHs in predominantly classical spheroids formed in mergers. Similarly, low-luminosity Seyferts do not show a clustering excess on small scales, in agreement with the natural prediction of secular models, but bright quasars at all redshifts do so. We also compare recent observations of the colors of quasar host galaxies and show that these correspond to the colors of recent merger remnants, in the transition region between the blue cloud and the red sequence, and are distinct from the colors of systems with observed bars or strong disk instabilities. Even the most extreme secular models, in which all bulge (and therefore BH) formation proceeds via disk instability, are forced to assume that this instability acts before the (dynamically inevitable) mergers, and therefore predict a history for the quasar luminosity density that is shifted to earlier times, in disagreement with observations. Our model provides a powerful means to predict the abundance and nature of mergers and to contrast cosmologically motivated predictions of merger products such as starbursts and active galactic nuclei.

Dark matter accretion into supermassive black holes

The relativistic accretion rate of dark matter by a black hole is revisited. Under the assumption that the phase space density indicator, Q={rho}{sub {infinity}}/{sigma}{sub {infinity}}{sup 3}, remains constant during the inflow, the derived accretion rate can be higher up to 5 orders of magnitude than the classical accretion formula, valid for nonrelativistic and noninteracting particles, when typical dark halo conditions are considered. For these typical conditions, the critical point of the flow is located at distances of about 30-150 times the horizon radius. Application of our results to black hole seeds hosted by halos issued from cosmological simulations indicate that dark matter contributes to no more than {approx}10% of the total accreted mass, confirming that the bolometric quasar luminosity is related to the baryonic accretion history of the black hole.

On the Search for Quasar Light Echoes

The UV radiation from a quasar leaves a characteristic pattern in the distribution of ionized hydrogen throughout the surrounding space. This pattern or light echo propagates through the intergalactic medium at the speed of light, and can be observed by its imprint on the Lyα forest spectra of background sources. As the echo persists after the quasar has switched off, it offers the possibility of searching for dead quasars, and constraining their luminosities and lifetimes. We outline a technique to search for and characterize these light echoes. To test the method, we create artificial Lyα forest spectra from cosmological simulations at z = 3, apply light echoes and search for them. We show how the simulations can also be used to quantify the significance level of any detection. We find that light echoes from the brightest quasars could be found in observational data. With absorption-line spectra of 100 redshift z ~ 3–3.5 quasars or galaxies in a 1 deg2 area, we expect that ~10 echoes from quasars with B-band luminosities LB = 3 × 1045 ergs−1 exist that could be found at 95% confidence, assuming a quasar lifetime of ~107 yr. Even a null result from such a search would have interesting implications for our understanding of quasar luminosities and lifetimes.

A SURVEY OFz∼ 6 QUASARS IN THE SLOAN DIGITAL SKY SURVEY DEEP STRIPE. I. A FLUX-LIMITED SAMPLE ATzAB&lt; 21

W e present the discovery ofve quasars at z � 6 selected from 260 deg 2 ofthe Sloan Digital Sky Survey (SDSS)southern survey,a deep im aging survey obtained by repeatedly scanning a stripe along the CelestialEquator. Theve quasarswith 20 < zA B < 21 are 1{2 m agnitudesfainterthan the lum inous z � 6 quasarsdiscovered in the SDSS m ain survey. One ofthem was independently discovered by the UKIRT Infrared Deep Sky Survey. These quasars,com bined with another z � 6 quasarknown in thisregion,m ake a com plete ux-lim ited quasarsam ple at zA B < 21. The sam ple spanstheredshiftrange5:85 � z � 6:12 and the lum inosity range 26:5 � M 1450 � 25:4 (H0 = 70 km s 1 M pc 1 ,m = 0:3,and � = 0:7).W eusethe 1=Va m ethod to determ ine thatthe com oving quasarspatialdensity at hzi = 6:0 and hM 1450i = 25:8 is (5:0 � 2:1) � 10 9 M pc 3 m ag 1 . W e m odelthe bright-end quasarlum inosity function (QLF)at z � 6 asa powerlaw �(L1450) / L � . The slopecalculated from a com bination ofoursam ple and the lum inousSDSS quasarsam ple is 3:1 � 0:4,signicantly steeper than the slope ofthe QLF at z � 4. Based on the derived QLF, wend thatthe quasar/AGN population cannotprovide enough photonsto ionize the intergalactic m edium (IGM )at z � 6 unlessthe IGM isvery hom ogeneousand the lum inosity (L � )atwhich theQLF powerlaw breaksisvery low. Subjectheadings: galaxies:active| quasars:em ission lines| quasars:general

Evidence for Quasar Activity Triggered by Galaxy Mergers inHSTObservations of Dust‐reddened Quasars

We present Hubble Space Telescope ACS images of 13 dust-reddened type 1 quasars selected from the FIRST/2MASS Red Quasar Survey. These quasars have high intrinsic luminosities after correction for dust obscuration (–23.5 ≥ M_B ≥ − 26.2 from K-magnitude). The images show strong evidence of recent or ongoing interaction in 11 of the 13 cases, even before the quasar nucleus is subtracted. None of the host galaxies are well fit by a simple elliptical profile. The fraction of quasars showing interaction is significantly higher than the 30% seen in samples of host galaxies of normal, unobscured quasars. There is a weak correlation between the amount of dust reddening and the magnitude of interaction in the host galaxy, measured using the Gini coefficient and the concentration index. Although few host galaxy studies of normal quasars are matched to ours in intrinsic quasar luminosity, no evidence has been found for a strong dependence of merger activity on host luminosity in samples of the host galaxies of normal quasars. We thus believe that the high merger fraction in our sample is related to their obscured nature, with a significant amount of reddening occurring in the host galaxy. The red quasar phenomenon seems to have an evolutionary explanation, with the young quasar spending the early part of its lifetime enshrouded in an interacting galaxy. This might be further indication of a link between AGNs and starburst galaxies.

Mass Functions of the Active Black Holes in Distant Quasars from the Sloan Digital Sky Survey Data Release 3

We present the mass functions of actively accreting supermassive black holes over the redshift range 0.3 <= z <= 5 for a well-defined, homogeneous sample of 15,180 quasars from the Sloan Digital Sky Survey Data Release 3 (SDSS DR3) within an effective area of 1644 square degrees. This sample is the most uniform statistically significant subset available for the DR3 quasar sample. It was used for the DR3 quasar luminosity function, presented by Richards et al., and is the only sample suitable for the determination of the SDSS quasar black hole mass function. The sample extends from i = 15 to i = 19.1 at z less than about 3 and to i = 20.2 for z greater than about 3. The mass functions display a rise and fall in the space density distribution of active black holes at all epochs. Within the uncertainties the high-mass decline is consistent with a constant slope of beta of about -3.3 at all epochs. This slope is similar to the bright end slope of the luminosity function for epochs below z = 4. Our tests suggest that the down-turn toward lower mass values is due to incompleteness of the quasar sample with respect to black hole mass. Further details and analysis of these mass functions will be presented in forthcoming papers.

Is there a quad problem among optical gravitational lenses?

Most of the optical gravitational lenses recently discovered in the Sloan Digital Sky Survey Quasar Lens Search (SQLS) have two-images rather than four-images, in marked contrast to radio lenses for which the fraction of four-image lenses (quad fraction) is quite high. We revisit the quad fraction among optical lenses by taking the selection function of the SQLS into account. We find that the current observed quad fraction in the SQLS is indeed lower than, but consistent with, the prediction of our theoretical model. The low quad fraction among optical lenses, together with the high quad fraction among radio lenses, implies that the quasar optical luminosity function has a relatively shallow faint end slope.

Four Quasars above Redshift 6 Discovered by the Canada-France High-zQuasar Survey

The Canada-France High-z Quasar Survey (CFHQS) is an optical survey designed to locate quasars during the epoch of reionization. In this paper we present the discovery of the first four CFHQS quasars at redshifts greater than 6, including the most distant known quasar, CFHQS J2329-0301 at z = 6.43. We describe the observational method used to identify the quasars and present optical, infrared, and millimeter photometry and optical and near-infrared spectroscopy. We investigate the dust properties of these quasars, finding an unusual dust extinction curve for one quasar and a high far-infrared luminosity due to dust emission for another. The mean millimeter continuum flux for CFHQS quasars is substantially lower than that for SDSS quasars at the same redshift, likely due to a correlation with quasar UV luminosity. For two quasars with sufficiently high signal-to-noise ratio optical spectra, we use the spectra to investigate the ionization state of hydrogen at z > 5. For CFHQS J1509-1749 at z = 6.12 we find significant evolution (beyond a simple extrapolation of lower redshift data) in the Gunn-Peterson optical depth at z > 5.4. The line of sight to this quasar has one of the highest known optical depths at z ≈ 5.8. An analysis of the sizes of the highly ionized near-zones in the spectra of two quasars at z = 6.12 and 6.43 suggest that the intergalactic medium surrounding these quasars was substantially ionized before these quasars turned on. Together, these observations point toward an extended reionization process, but we caution that cosmic variance is still a major limitation in z > 6 quasar observations.

THE ACCRETION GEOMETRY IN RADIO-LOUD ACTIVE GALAXIES

We review the latest attempts to determine the accretion geometry in radio-loud active galactic nuclei (AGN). These objects, which comprise ~ 10–20% of the AGN population, produce powerful collimated radio jets that can extend thousands of parsecs from the center of the host galaxy. Recent multiwavelength surveys have shown that radio-loudness is more common in low-luminosity AGN than in higher luminosity Seyfert galaxies or quasars. These low-luminosity AGN have small enough accretion rates that they are most likely accreting via a geometrically thick and radiatively inefficient accretion flow. In contrast, X-ray spectroscopic observations of three higher luminosity broad-line radio galaxies (3C 120, 4C+74.26 and PG 1425+267) have found evidence for an untruncated thin disk extending very close to the black hole. These tentative detections indicate that, for this class of radio-loud AGN, the accretion geometry is very similar to their radio-quiet counterparts. These observations suggest that there are three conditions to jet formation that must be satisfied: the presence of a rapidly spinning black hole, an accretion flow with a large H/r ratio, and a favorable magnetic field geometry.

Distribution of the Very First Population III Stars and Their Relation to Brightz ≈ 6 Quasars

We discuss the link between dark matter halos hosting the first Population III stars and the rare, massive halos that are generally considered to host bright quasars at high redshift (z ≈ 6). The main question that we intend to answer is whether the supermassive black holes powering these QSOs grew out from the seeds planted by the first intermediate-mass black holes created in the universe. This question involves a dynamical range of 1013 in mass, and we address it by combining N-body simulations of structure formation to identify the most massive halos at z ≈ 6 with a Monte Carlo method based on linear theory to obtain the location and formation times of the first-light halos within the whole simulation box. We show that the descendants of the first ≈106 M☉ virialized halos do not, on average, end in the most massive halos at z ≈ 6, but rather live in a large variety of environments. The oldest Population III progenitors of the most massive halos at z ≈ 6 form instead from density peaks that are on average 1.5 σ more common than the first Population III star formed in the volume occupied by one bright high-z QSO. The intermediate-mass black hole seeds planted by the very first Population III stars at z 40 can easily grow to masses mBH > 109.5 M☉ by z = 6 assuming Eddington accretion with radiative efficiency 0.1. Quenching of the black hole accretion is therefore crucial to avoid an overabundance of supermassive black holes at lower redshift. This can be obtained if the mass accretion is limited to a fraction η ≈ 6 × 10-3 of the total baryon mass of the halo hosting the black hole. The resulting high-end slope of the black hole mass function at z = 6 is α ≈ -3.7, a value within the 1 σ error bar for the bright-end slope of the observed quasar luminosity function at z = 6.

The Co‐Formation of Spheroids and Quasars Traced in their Clustering

We compare observed clustering of quasars and galaxies as a function of redshift, mass, luminosity, & color/morphology, to constrain models of quasar fueling and spheroid-BH co-evolution. High redshift quasars are shown to be drawn from progenitors of local early-type galaxies, with the characteristic quasar luminosity L* reflecting a characteristic mass of 'active' BH/host populations at each redshift. Evolving observed high-z quasar clustering to z=0 predicts a trend of clustering in 'quasar remnants' as a function of stellar mass identical to that observed for early-types. However, quasar clustering does not simply reflect observed early (or late)-type populations; at each redshift, quasars cluster as an 'intermediate' population. Comparing with the age of elliptical stellar populations reveals that this 'intermediate' population represents those ellipticals undergoing or terminating their final significant star formation at each epoch. Assuming that quasar triggering is associated with the formation/termination epoch of ellipticals predicts quasar clustering at all observed redshifts without any model dependence or assumptions about quasar light curves, lifetimes, or accretion rates. This is not true for disks or quasar halos: i.e. quasars do not generically trace star formation, disks, or halo assembly. Quasar clustering at all z is consistent with a constant halo mass ~4x10^{12} M_sun, similar to local 'group scales.' The observations support a scenario in which major mergers trigger quasar activity and dominate bright, high-z quasar populations. We show that measurements of quasar clustering versus luminosity at z~1 can be used to constrain different lower-luminosity AGN fueling mechanisms, and that high-z clustering is sensitive to whether or not systems 'shut down' growth at z>3.

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.

Observational Evidence for the Coevolution of Galaxy Mergers, Quasars, and the Blue/Red Galaxy Transition

We compile a number of observations to estimate the time-averaged rate of formation or buildup of red sequence galaxies, as a function of mass and redshift. Comparing this with the mass functions of mergers and quasar hosts, and independently comparing their clustering properties as a function of redshift, we find that these populations trace the same mass distribution, with similar evolution, at redshifts 0 < z ≲ 1.5. Knowing one of the quasar, merger, or elliptical mass/luminosity functions, it is possible to predict the others. Allowing for greater model dependence, we compare the rate of early-type buildup with the implied merger and quasar triggering rates as a function of mass and redshift and find agreement. Over this redshift range, observed merger fractions can account for the entire bright quasar luminosity function and buildup of the red sequence at all but the highest masses at low redshift (≳1011 M☉ at z ≲ 0.3) where "dry" mergers appear to dominate. This supports a necessary prediction of theories where mergers between gas-rich galaxies produce ellipticals with an associated phase of quasar activity, after which the remnant becomes red. These populations trace a similar characteristic transition mass, possibly reflecting the mass above which the elliptical population is mostly (≳50%) assembled at a given redshift, which increases with redshift over the observed range in a manner consistent with suggestions that cosmic downsizing may apply to red galaxy assembly as well as star formation. These mass distributions as a function of redshift do not uniformly trace the all/red/blue galaxy population, ruling out models in which quasar activity is generically associated with star formation or is long lived in "old" systems.