Quasar Templates Research Articles

ALMA FIR View of Ultra-high-redshift Galaxy Candidates at z ∼ 11–17: Blue Monsters or Low-z Red Interlopers?

We present Atacama Large Millimeter/submillimeter Array (ALMA) Band 7 observations of a remarkably bright galaxy candidate at (M UV = −21.6), S5-z17-1, identified in James Webb Space Telescope (JWST) Early Release Observation data of Stephen’s Quintet. We do not detect the dust continuum at 866 μm, ruling out the possibility that S5-z17-1 is a low-z dusty starburst with a star formation rate of ≳30 M ⊙ yr−1. We detect a 5.1σ line feature at 338.726 ± 0.007 GHz exactly coinciding with the JWST source position, with a 2% likelihood of the signal being spurious. The most likely line identification would be [O iii]52 μm at z = 16.01 or [C ii]158 μm at z = 4.61, whose line luminosities do not violate the nondetection of the dust continuum in both cases. Together with three other z ≳ 11–13 candidate galaxies recently observed with ALMA, we conduct a joint ALMA and JWST spectral energy distribution (SED) analysis and find that the high-z solution at z ∼ 11–17 is favored in every candidate as a very blue (UV continuum slope of ≃−2.3) and luminous (M UV ≃ [ − 24:−21]) system. Still, we find in several candidates that reasonable SED fits (Δχ 2 ≲ 4) are reproduced by type II quasar and/or quiescent galaxy templates with strong emission lines at z ∼ 3–5, where such populations predicted from their luminosity functions and EW([O iii]+Hβ) distributions are abundant in survey volumes used for the identification of the z ∼ 11–17 candidates. While these recent ALMA observation results have strengthened the likelihood of the high-z solutions, lower-z possibilities are not completely ruled out in several of the z ∼ 11–17 candidates, indicating the need to consider the relative surface densities of the lower-z contaminants in the ultra-high-z galaxy search.

Performance of the Quasar Spectral Templates for the Dark Energy Spectroscopic Instrument

Millions of quasar spectra will be collected by the Dark Energy Spectroscopic Instrument (DESI), leading to a fourfold increase in the number of known quasars. High-accuracy quasar classification is essential to tighten constraints on cosmological parameters measured at the highest redshifts DESI observes (z > 2.0). We present spectral templates for identification and redshift estimation of quasars in the DESI Year 1 data release. The quasar templates are comprised of two quasar eigenspectra sets, trained on spectra from the Sloan Digital Sky Survey. The sets are specialized to reconstruct quasar spectral variation observed over separate yet overlapping redshift ranges and, together, are capable of identifying DESI quasars from 0.05 < z < 7.0. The new quasar templates show significant improvement over the previous DESI quasar templates regarding catastrophic failure rates, redshift precision and accuracy, quasar completeness, and the contamination fraction in the final quasar sample.

Extreme ultraviolet quasar colours from GALEX observations of the SDSS DR14Q catalogue

ABSTRACT The rest-frame far to extreme ultraviolet (UV) colour–redshift relationship has been constructed from data on over $480\, 000$ quasars carefully cross-matched between SDSS Data Release 14 and the final GALEX photometric catalogue. UV matching and detection probabilities are given for all the quasars, including dependencies on separation, optical brightness, and redshift. Detection limits are also provided for all objects. The UV colour distributions are skewed redward at virtually all redshifts, especially when detection limits are accounted for. The median GALEX far-UV minus near-UV (FUV − NUV) colour–redshift relation is reliably determined up to z ≈ 2.8, corresponding to rest-frame wavelengths as short as 400 Å. Extreme UV (EUV) colours are substantially redder than found previously, when detection limits are properly accounted for. Quasar template spectra were forward modelled through the GALEX bandpasses, accounting for intergalactic opacity, intrinsic reddening, and continuum slope variations. Intergalactic absorption by itself cannot account for the very red EUV colours. The colour–redshift relation is consistent with no intrinsic reddening, at least for SMC-like extinction. The best model fit has a FUV continuum power-law slope αν, FUV = −0.34 ± 0.03 consistent with previous results, but an EUV slope αν, EUV = −2.90 ± 0.04 that is much redder and inconsistent with any previous composite value (all ≳ −2.0). The EUV slope difference can be attributed in part to the tendency of previous studies to preferentially select UV brighter and bluer objects. The weak EUV flux suggests quasar accretion disc models that include outflows such as disc winds.

The beamed jet and quasar core of the distant blazar 4C 71.07

ABSTRACT The object 4C 71.07 is a high-redshift blazar whose spectral energy distribution shows a prominent big blue bump and a strong Compton dominance. We present the results of a 2-yr multiwavelength campaign led by the Whole Earth Blazar Telescope (WEBT) to study both the quasar core and the beamed jet of this source. The WEBT data are complemented by ultraviolet and X-ray data from Swift, and by γ-ray data by Fermi. The big blue bump is modelled by using optical and near-infrared mean spectra obtained during the campaign, together with optical and ultraviolet quasar templates. We give prescriptions to correct the source photometry in the various bands for the thermal contribution, in order to derive the non-thermal jet flux. The role of the intergalactic medium absorption is analysed in both the ultraviolet and X-ray bands. We provide opacity values to deabsorb ultraviolet data, and derive a best-guess value for the hydrogen column density of $N_{\rm H}^{\rm best}=6.3 \times 10^{20} \rm \, cm^{-2}$ through the analysis of X-ray spectra. We estimate the disc and jet bolometric luminosities, accretion rate, and black hole mass. Light curves do not show persistent correlations among flux changes at different frequencies. We study the polarimetric behaviour and find no correlation between polarization degree and flux, even when correcting for the dilution effect of the big blue bump. Similarly, wide rotations of the electric vector polarization angle do not seem to be connected with the source activity.

Polar Dust, Nuclear Obscuration, and IR SED Diversity in Type-1 AGNs*

Abstract Despite the hypothesized similar face-on viewing angles, the infrared emission of type-1 active galactic nuclei (AGNs) has diverse spectral energy distribution (SED) shapes that deviate substantially from the well-characterized quasar templates. Motivated by the commonly seen UV-optical obscuration and the discovery of parsec-scale mid-IR polar dust emission in some nearby AGNs, we develop semi-empirical SED libraries for reddened type-1 AGNs built on quasar intrinsic templates, assuming low-level extinction caused by an extended distribution of large dust grains. We demonstrate that this model can reproduce the nuclear UV to IR SED and the strong mid-IR polar dust emission of NGC 3783, the type-1 AGN with the most relevant and robust observational constraints. In addition, we compile 64 low-z Seyfert-1 nuclei with negligible mid-IR star formation contamination and satisfactorily fit the individual IR SEDs as well as the composite UV to mid-IR composite SEDs. Given the success of these fits, we characterize the possible infrared SED of AGN polar dust emission and utilize a simple but effective strategy to infer its prevalence among type-1 AGNs. The SEDs of high-z peculiar AGNs, including the extremely red quasars, mid-IR warm-excess AGNs, and hot dust-obscured galaxies, can be also reproduced by our model. These results indicate that the IR SEDs of most AGNs, regardless of redshift or luminosity, arise from similar circumnuclear torus properties but differ mainly due to the optical depths of extended obscuring dust components.

Composite Spectral Energy Distributions and Infrared–Optical Colors of Type 1 and Type 2 Quasars

Abstract We present observed mid-infrared and optical colors and composite spectral energy distributions (SEDs) of type 1 (broad-line) and 2 (narrow-line) quasars selected from Sloan Digital Sky Survey (SDSS) spectroscopy. A significant fraction of powerful quasars are obscured by dust and are difficult to detect in optical photometric or spectroscopic surveys. However, these may be more easily identified on the basis of mid-infrared (MIR) colors and SEDs. Using samples of SDSS type 1 and 2 matched in redshift and [O iii] luminosity, we produce composite rest-frame 0.2–15 μm SEDs based on SDSS, UKIDSS, and Wide-field Infrared Survey Explorer photometry and perform model fits using simple galaxy and quasar SED templates. The SEDs of type 1 and 2 quasars are remarkably similar, with the differences explained primarily by the extinction of the quasar component in the type 2 systems. For both types of quasar, the flux of the active galactic nucleus (AGN) relative to the host galaxy increases with AGN luminosity ( L [ O III ] ) and redder observed MIR color, but we find only weak dependencies of the composite SEDs on mechanical jet power as determined through radio luminosity. We conclude that luminous quasars can be effectively selected using simple MIR color criteria similar to those identified previously ( W 1 – W 2 &gt; 0.7 ; Vega), although these criteria miss many heavily obscured objects. Obscured quasars can be further identified based on optical–IR colors (for example, ( u – W 3 [ AB ] ) &gt; 1.4 ( W 1 – W 2 [ Vega ] ) + 3.2 ). These results illustrate the power of large statistical studies of obscured quasars selected on the basis of MIR and optical photometry.

The near-to-mid infrared spectrum of quasars

We analyse a sample of 85 luminous (log(nuLnu(3\um)/erg s-1)>45.5) quasars with restframe ~2-11 \um spectroscopy from AKARI and Spitzer. Their high luminosity allows a direct determination of the near-infrared quasar spectrum free from host galaxy emission. A semi-empirical model consisting of a single template for the accretion disk and two blackbodies for the dust emission successfully reproduces the 0.1-10 \um spectral energy distributions (SEDs). Excess emission at 1-2 \um over the best-fitting model suggests that hotter dust is necessary in addition to the ~1200 K blackbody and the disk to reproduce the entire near-infrared spectrum. Variation in the extinction affecting the disk and in the relative strength of the disk and dust components accounts for the diversity of individual SEDs. Quasars with higher dust-to-disk luminosity ratios show slightly redder infrared continua and less prominent silicate emission. We find no luminosity dependence in the shape of the average infrared quasar spectrum. We generate a new quasar template that covers the restframe range 0.1-11 \um, and separate templates for the disk and dust components. Comparison with other infrared quasar composites suggests that previous ones are less reliable in the 2-4 \um range. Our template is the first one to provide a detailed view of the infrared emission on both sides of the 4 \um bump.

Extinction curve template for intrinsically reddened quasars

We analyzed the near-infrared to UV data of 16 quasars with redshifts ranging from 0.71 <z< 2.13 to investigate dust extinction properties. The sample presented in this work was obtained from the High AV Quasar (HAQ) survey. The quasar candidates were selected from the Sloan Digital Sky Survey (SDSS) and the UKIRT Infrared Deep Sky Survey (UKIDSS), and follow-up spectroscopy was carried out at the Nordic Optical Telescope (NOT) and the New Technology Telescope (NTT). To study dust extinction curves intrinsic to the quasars, we selected 16 cases from the HAQ survey for which the Small Magellanic Cloud (SMC) law could not provide a good solution to the spectral energy distributions (SEDs). We derived the extinction curves using the Fitzpatrick & Massa (1986, ApJ, 307, 286, FM) law by comparing the observed SEDs to a combined previously published quasar template. The derived extinction, AV, ranges from 0.2–1.0 mag. All the individual extinction curves of our quasars are steeper (RV = 2.2–2.7) than that of the SMC, with a weighted mean value of RV = 2.4. We derived an average quasar extinction curve for our sample by simultaneously fitting SEDs by using the weighted mean values of the FM law parameters and a varying RV. The entire sample is well fit with a single best-fit value of RV = 2.2 ± 0.2. The average quasar extinction curve deviates from the steepest Milky Way and SMC extinction curves at a confidence level ≳95%. Such steep extinction curves suggest that a significant population of silicates is involved in producing small dust grains. Another possibility might be that the large dust grains may have been destroyed by the activity of the nearby active galactic nuclei (AGN), resulting in steep extinction curves.

A quasar reddened by a sub-parsec-sized, metal-rich and dusty cloud in a damped Lyman α absorber atz= 2.13

We present a detailed analysis of a red quasar at z=2.32 with an intervening damped Lyman-alpha absorber (DLA) at z=2.13. Using high quality data from the X-shooter spectrograph at ESO Very Large Telescope we find that the absorber has a metallicity consistent with Solar. We observe strong C I and H$_2$ absorption indicating a cold, dense absorbing medium. Partial coverage effects are observed in the C I lines, from which we infer a covering fraction of $27 \pm 6$ % and a physical diameter of the cloud of 0.1 pc. From the covering fraction and size, we estimate the size of the background quasar's broad line region. We search for emission from the DLA counterpart in optical and near-infrared imaging. No emission is observed in the optical data. However, we see tentative evidence for a counterpart in the H and K' band images. The DLA shows high depletion (as probed by [Fe/Zn]=-1.22) indicating that significant amounts of dust must be present in the DLA. By fitting the spectrum with various dust reddened quasar templates we find a best-fitting amount of dust in the DLA of $A(V)_{\rm DLA}=0.28 \pm 0.01|_{\rm stat} \pm 0.07|_{\rm sys}$. We conclude that dust in the DLA is causing the colours of this intrinsically very luminous background quasar to appear much redder than average quasars, thereby not fulfilling the criteria for quasar identification in the Sloan Digital Sky Survey. Such chemically enriched and dusty absorbers are thus underrepresented in current samples of DLAs.

AHERSCHELSTUDY OF 24μm-SELECTED AGNs AND THEIR HOST GALAXIES

We present a sample of 290 24-micron-selected active galactic nuclei (AGNs) mostly at z ~ 0.3 -- 2.5, within 5.2 square degrees distributed as 25' X 25' fields around each of 30 galaxy clusters in the Local Cluster Substructure Survey (LoCuSS). The sample is nearly complete to 1 mJy at 24 microns, and has a rich multi-wavelength set of ancillary data; 162 are detected by Herschel. We use spectral templates for AGNs, stellar populations, and infrared emission by star forming galaxies to decompose the spectral energy distributions (SEDs) of these AGNs and their host galaxies, and estimate their star formation rates (SFRs), AGN luminosities, and host galaxy stellar masses. The set of templates is relatively simple: a standard Type-1 quasar template; another for the photospheric output of the stellar population; and a far infrared star-forming template. For the Type-2 AGN SEDs, we substitute templates including internal obscuration, and some Type-1 objects require a warm component (T > 50 K). The individually Herschel- detected Type-1 AGNs and a subset of 17 Type-2 ones typically have luminosities > 10^{45} ergs/s, and supermassive black holes of ~ 3 X 10^8 Msun emitting at ~ 10% of the Eddington rate. We find them in about twice the numbers of AGN identified in SDSS data in the same fields, i.e., they represent typical high luminosity AGN, not an infrared-selected minority. These AGNs and their host galaxies are studied further in an accompanying paper.

NuSTARANDXMM-NEWTONOBSERVATIONS OF LUMINOUS, HEAVILY OBSCURED,WISE-SELECTED QUASARS ATZ∼ 2

We report on a NuSTAR and XMM-Newton program that has observed a sample of three extremely luminous, heavily obscured WISE-selected active galactic nuclei (AGNs) at z ∼ 2 across a broad X-ray band (0.1 − 79 keV). The parent sample, selected to be faint or undetected in the WISE 3.4 μm (W1) and 4.6 μm (W2) bands but bright at 12 μm (W3) and 22 μm (W4), are extremely rare, with only ∼1000 so-called "W1W2-dropouts" across the extragalactic sky. Optical spectroscopy reveals typical redshifts of z ∼ 2 for this population, implying rest-frame mid-IR luminosities of νLν(6 μm) ∼ 6 × 1046 erg s−1 and bolometric luminosities that can exceed Lbol ∼ 1014 L☉. The corresponding intrinsic, unobscured hard X-ray luminosities are L(2–10 keV) ∼ 4 × 1045 erg s−1 for typical quasar templates. These are among the most AGNs known, though the optical spectra rarely show evidence of a broad-line region and the selection criteria imply heavy obscuration even at rest-frame 1.5 μm. We designed our X-ray observations to obtain robust detections for gas column densities NH ⩽ 1024 cm−2. In fact, the sources prove to be fainter than these predictions. Two of the sources were observed by both NuSTAR and XMM-Newton, with neither being detected by NuSTAR (f3–24 keV ≲ 10−13 erg cm−2 s−1), and one being faintly detected by XMM-Newton (f0.5–10 keV ∼ 5 × 10−15 erg cm−2 s−1). A third source was observed only with XMM-Newton, yielding a faint detection (f0.5–10 keV ∼ 7 × 10−15 erg cm−2 s−1). The X-ray data imply these sources are either X-ray weak, or are heavily obscured by column densities NH ≳ 1024 cm−2. The combined X-ray and mid-IR analysis seems to favor this second possibility, implying the sources are extremely obscured, consistent with Compton-thick, luminous quasars. The discovery of a significant population of heavily obscured, extremely luminous AGNs would not conform to the standard paradigm of a receding torus, in which more luminous quasars are less likely to be obscured, and instead suggests that an additional source of obscuration is present in these extreme sources.

FAR-INFRARED PROPERTIES OF TYPE 1 QUASARS

We use the Spitzer Space Telescope Enhanced Imaging Products (SEIP) and the Spitzer Archival Far-InfraRed Extragalactic Survey (SAFIRES) to study the spectral energy distributions of spectroscopically confirmed type 1 quasars selected from the Sloan Digital Sky Survey (SDSS). By combining the Spitzer and SDSS data with the 2-Micron All Sky Survey (2MASS) we are able to construct a statistically robust rest-frame 0.1-100 micron type 1 quasar template. We find the quasar population is well-described by a single power-law SED at wavelengths less than 20 microns, in good agreement with previous work. However, at longer wavelengths we find a significant excess in infrared luminosity above an extrapolated power-law, along with signifiant object-to-object dispersion in the SED. The mean excess reaches a maximum of 0.8 dex at rest-frame wavelengths near 100 microns.

SPECTRAL CLASSIFICATION AND REDSHIFT MEASUREMENT FOR THE SDSS-III BARYON OSCILLATION SPECTROSCOPIC SURVEY

(abridged) We describe the automated spectral classification, redshift determination, and parameter measurement pipeline in use for the Baryon Oscillation Spectroscopic Survey (BOSS) of the Sloan Digital Sky Survey III (SDSS-III) as of Data Release 9, encompassing 831,000 moderate-resolution optical spectra. We give a review of the algorithms employed, and describe the changes to the pipeline that have been implemented for BOSS relative to previous SDSS-I/II versions, including new sets of stellar, galaxy, and quasar redshift templates. For the color-selected CMASS sample of massive galaxies at redshift 0.4 <~ z <~ 0.8 targeted by BOSS for the purposes of large-scale cosmological measurements, the pipeline achieves an automated classification success rate of 98.7% and confirms 95.4% of unique CMASS targets as galaxies (with the balance being mostly M stars). Based on visual inspections of a subset of BOSS galaxies, we find that ~0.2% of confidently reported CMASS sample classifications and redshifts are incorrect, and ~0.4% of all CMASS spectra are objects unclassified by the current algorithm which are potentially recoverable. The BOSS pipeline confirms that ~51.5% of the quasar targets have quasar spectra, with the balance mainly consisting of stars. Statistical (as opposed to systematic) redshift errors propagated from photon noise are typically a few tens of km/s for both galaxies and quasars, with a significant tail to a few hundreds of km/s for quasars. We test the accuracy of these statistical redshift error estimates using repeat observations, finding them underestimated by a factor of 1.19 to 1.34 for galaxies, and by a factor of 2 for quasars. We assess the impact of sky-subtraction quality, S/N, and other factors on galaxy redshift success. Finally, we document known issues, and describe directions of ongoing development.

Spectral energy distribution template of redshift-zero AGN and the comparison with that of quasars

Spectral energy distributions (SEDs) of the central few tens of parsecs of some of the nearest active galactic nuclei [AGN] have been used to construct a genuine redshift-zero SED template [1]. This paper presents the comparison between this redshift-zero template and those of the radio loud and radio quiet quasar templates by [2]. Despite the four orders of magnitude difference in power between the redshift-zero AGNs, mostly Seyfert galaxies, and that of quasars, they all show a remarkable similarity. The distinction relays on the importance of the so-called blue bump component – emission from the accretion disk - which is absent in Seyfert type 2, mildly present in Seyfert type 1, seen in full realm in quasars. Conversely, the characteristic red bump component - dust reprocessed emission in the IR - is present in all cases. The difference between the three AGN classes can be ascribed in terms of central dust obscuration and progressive view angle of the nuclear core.

Spectroscopic follow-up of 70-μm sources in Spitzer Wide-area Infrared Extragalactic Legacy Survey

We present spectroscopic follow-up observations of 70-μm selected galaxies from the Spitzer Wide-area InfraRed Extragalactic Survey XMM-LSS and Lockman Hole fields. We have measured spectroscopic redshifts for 293 new sources down to a 70 μm flux limit of 9 mJy and r < 22 mag. The redshift distribution peaks at z ∼ 0.3 and has a high redshift tail out to z = 3.5. We perform emission line diagnostics for 91 sources where [O iii], Hβ, [N ii], Hα and [S ii] emission lines are available to determine their power source. We find in our sample 13 quasi-stellar objects, one type 2 Seyfert galaxy, 33 star-forming galaxies, 30 composite galaxies, five LINERs and 21 ambiguous galaxies. We fit single temperature dust spectral energy distributions (SEDs) to 81 70-μm sources with 160-μm photometry to estimate dust temperatures and masses. Assuming the dust emissivity factor (β) as 1.5, we determine dust temperatures in the range ∼20–60 K and dust masses with a range of 106–109 M⊙. Plotting these objects in the luminosity–temperature diagram suggests that these objects have lower dust temperatures than local IR luminous galaxies. The Herschel Space Observatory will be crucial in understanding the nature of these sources and to accurately determine the shape of the Rayleigh–Jeans tail of the dust SED. We then model SEDs from optical to far-IR (FIR) for each source using a set of galaxy and quasar templates in the optical and near-IR and with a set of dust emission templates [cirrus, M82 starburst, Arp 220 starburst and active galactic nucleus (AGN) dust torus] in the mid-IR to FIR. The numbers of objects fitted with each dust template are 57 Arp 220, 127 M82, nine cirrus, one AGN dust torus, 70 M82 and cirrus, 26 M82 and AGN dust torus, and three Arp 220 and AGN dust torus. We determine the total IR luminosity (LIR) in range 108–1015 L⊙ by integrating the SED models from 8 to 1000 μm.

THE MID-INFRARED LUMINOSITY FUNCTION ATz&lt; 0.3 FROM 5MUSES: UNDERSTANDING THE STAR FORMATION/ACTIVE GALACTIC NUCLEUS BALANCE FROM A SPECTROSCOPIC VIEW

We present rest-frame 15 and 24 μm luminosity functions (LFs) and the corresponding star-forming LFs at z < 0.3 derived from the 5MUSES sample. Spectroscopic redshifts have been obtained for ∼98% of the objects and the median redshift is ∼0.12. The 5–35 μm Infrared Spectrograph spectra allow us to estimate accurately the luminosities and build the LFs. Using a combination of starburst and quasar templates, we quantify the star formation (SF) and active galactic nucleus (AGN) contributions in the mid-IR spectral energy distribution. We then compute the SF LFs at 15 and 24 μm, and compare with the total 15 and 24 μm LFs. When we remove the contribution of AGNs, the bright end of the LF exhibits a strong decline, consistent with the exponential cutoff of a Schechter function. Integrating the differential LF, we find that the fractional contribution by SF to the energy density is 58% at 15 μm and 78% at 24 μm, while it goes up to ∼86% when we extrapolate our mid-IR results to the total IR luminosity density. We confirm that the AGNs play more important roles energetically at high luminosities. Finally, we compare our results with work at z ∼ 0.7 and confirm that evolution on both luminosity and density is required to explain the difference in the LFs at different redshifts.

The extinction law at high redshift and its implications

We analyze the optical-near infrared spectra of 33 quasars with redshifts 3.9<z<6.4 to investigate the properties of dust extinction at these cosmic epochs. The SMC extinction curve has been shown to reproduce the dust reddening of most quasars at z<2.2; we investigate whether this curve also provides a good description of dust extinction at higher redshifts. We fit the observed spectra with synthetic absorbed quasar templates obtained by varying the intrinsic slope (alpha), the absolute extinction (A3000), and by using a grid of empirical and theoretical extinction curves. We find that seven quasars in our sample are affected by substantial extinction (A3000 >0.8), and characterized by very steep intrinsic slopes (alpha<-2.3). All of the individual quasars require extinction curve deviating from that of the SMC, with a tendency to flatten at lambda<2000 A (in the rest frame of the source). We obtain a mean extinction curve at z>4, both by performing a simultaneous fit of all quasars and by averaging the extinction curves inferred for individual quasars. In the case of broad absorption line quasars the mean extinction curve deviates from the SMC at a confidence level >95%. The difference between extinction curves in quasars at z>4 and in quasars at lower redshift is indicative of either a different dust production mechanism at high redshift, or a different mechanism for processing dust into the ISM. We suggest that the same transitions may also apply to normal, star-forming galaxies at z>4. In particular, the observed change in the average spectral slope of galaxies at z>4 may be partially ascribed to a variation in the extinction curve, rather than a lower dust content at high redshift. In this scenario, the extinction curve inferred at z>4 would imply a cosmic star-formation corrected for dust attenuation a factor of ~2 higher than estimated in the past.

Improved redshifts for SDSS quasar spectra

A systematic investigation of the relationship between different redshift estimation schemes for more than 91000 quasars in the Sloan Digital Sky Survey (SDSS) Data Release 6 (DR6) is presented. The publicly available SDSS quasar redshifts are shown to possess systematic biases of Dz/(1+z)>=0.002 (600km/s) over both small (dz~0.1) and large (dz~1) redshift intervals. Empirical relationships between redshifts based on i) CaII H & K host galaxy absorption, ii) quasar [OII] 3728, iii) [OIII] 4960,5008 emission, and iv) cross-correlation (with a master quasar template) that includes, at increasing quasar redshift, the prominent MgII 2799, CIII] 1908 and CIV 1549 emission lines, are established as a function of quasar redshift and luminosity. New redshifts in the resulting catalogue possess systematic biases a factor of ~20 lower compared to the SDSS redshift values; systematic effects are reduced to the level of Dz/(1+z)<10^-4 (30km/s) per unit redshift, or <2.5x10^-5 per unit absolute magnitude. Redshift errors, including components due both to internal reproducibility and the intrinsic quasar-to-quasar variation among the population, are available for all quasars in the catalogue. The improved redshifts and their associated errors have wide applicability in areas such as quasar absorption outflows, quasar clustering, quasar-galaxy clustering and proximity-effect determinations.

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.

Identifications of FIRST radio sources in the NOAO Deep Wide‐Field Survey

AbstractIn this paper we present the results of an optical and near infrared identification of 514 radio sources from the FIRST survey (Faint Images of the Radio Sky Survey at Twenty centimetres) with a flux‐density limit of 1 mJy in the NOAO Deep Wide‐Field Survey (NDWFS) Boötes field. Using optical (Bw, R, I) and K band data with approximate limits of Bw ∼ 25.5 mag, R ∼ 25.8 mag, I ∼ 25.5 mag and K ∼ 19.4 mag, optical counterparts have been identified for 378 of 514 FIRST radio sources. This corresponds to an identification rate of 34% in four bands (BwRIK), 60% in optical bands (BwRI) and 74% in the I band. Photometric redshifts for these sources have been computed using the hyperz code. The inclusion of quasar template spectra in hyperz is investigated. We note that the photometric data are, in many cases, best matched to templates with very short star‐formation timescales and the inferred ages of identified galaxies depend strongly on the assumptions about the star‐formation timescale. The redshifts obtained are fairly consistent with those expected from the K ‐z relation for brighter radio sources but there is more scatter in the K ‐z diagram at z &lt; 1. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)