Composite Quasar Spectra from the Sloan Digital Sky Survey (original) (raw)
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Narrow absorption line variability in repeat quasar observations from the Sloan Digital Sky Survey
Monthly Notices of the Royal Astronomical Society, 2013
We present the results from a time domain study of absorption lines detected in quasar spectra with repeat observations from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7). Beginning with over 4500 unique time separation baselines of various absorption line species identified in the SDSS DR7 quasar spectra, we create a catalogue of 2522 quasar absorption line systems with two to eight repeat observations, representing the largest collection of unbiased and homogeneous multi-epoch absorption systems ever published. To investigate these systems for time variability of narrow absorption lines, we refine this sample based on the reliability of the system detection, the proximity of pixels with bright sky contamination to individual absorption lines, and the quality of the continuum fit. Variability measurements of this sub-sample based on the absorption line equivalent widths yield a total of 33 systems with indications of significantly variable absorption strengths on timescales ranging from one day to several years in the rest frame of the absorption system. Of these, at least 10 are from a class known as intervening absorption systems caused by foreground galaxies along the line of sight to the background quasar. This is the first evidence of possible absorption line variability detected in intervening systems, and their short timescale variations suggest that small-scale structures (∼10-100 au) are likely to exist in their host foreground galaxies.
Broad-band spectral energy distribution of 3000 Å break quasars from the Sloan Digital Sky Survey
Astronomy and Astrophysics, 2016
Context. In past decades, huge surveys have confirmed the existence of populations of exotic and hitherto unknown quasar types. The discovery and investigation of these rare peculiar objects is important because they may represent links to special evolutionary stages and hold clues to the evolution of quasars and galaxies. Aims. The Sloan Digital Sky Survey (SDSS) discovered the unusual quasars J010540.75-003313.9 and J220445.27+003141.8 and a small number of similar objects. Their spectra are characterised by a break in the continuum around 3000 Å that neither shows the typical structure of broad absorption line (BAL) troughs nor is explained by typical intrinsic dust reddening. The main aim of the present paper was twofold. First, a new target-oriented search was performed in the spectra database of the SDSS to construct a sizable sample of such 3000 Å break quasars. Second, their broad-band spectral energy distribution (SED) was compared with SEDs of BAL quasars. Methods. We used the method of Kohonen self-organising maps for data mining in the SDSS spectra archive to search for more quasars with properties comparable to the prototypes J010540.75-003313.9 and J220445.27+003141.8. We constructed a sample of 3000 Å break quasars and comparison samples of quasars with similar properties, to some extent, but also showing indications for typical BAL features. Particular attention was payed to a possible contamination by rare stellar spectral types, in particular DQ white dwarfs. We construct ensemble-averaged broad-band SEDs based on archival data from SDSS, GALEX, 2MASS, UKIDSS, WISE, and other surveys. The SEDs were corrected for dust absorption at the systemic redshifts of the quasars by the comparison with the average SED of normal quasars. Results. We compiled a list of 23 quasars classified as 3000 Å break quasars with properties similar to 010540.75-003313.9 and J220445.27+003141.8. Their de-reddened arithmetic median composite SED is indistinguishable from that of the unusual BAL quasars. We conclude that 3000 Å break quasars are most likely extreme versions of BAL quasars. Assuming that the intrinsic SED of the continuum source is represented by the quasar composite SED, the 3000 Å break quasars tend to be intrinsically more luminous than ordinary quasars.
Red and reddened quasars in the Sloan Digital Sky Survey
We investigate the overall continuum and emission line properties of quasars as a function of their optical/UV spectral energy distributions. Our sample consists of 4576 quasars from the Sloan Digital Sky Survey (SDSS) that were chosen using homogeneous selection criteria. Expanding on our previous work which demonstrated that the optical/UV color distribution of quasars is roughly Gaussian, but with a red tail, here we distinguish between 1) quasars that have intrinsically blue (optically flat) power-law continua, 2) quasars that have intrinsically red (optically steep) power-law continua, and 3) quasars whose colors are inconsistent with a single power-law continuum. We find that 273 (6.0%) of the quasars in our sample fall into the latter category and appear to be redder because of SMC-like dust extinction and reddening rather than because of synchrotron emission. Even though the SDSS Quasar Survey is optically -2selected and flux-limited, we demonstrate that it is sensitive to dust reddened quasars with E(B − V ) 0.5 assuming a classical Small Magellanic Cloud (SMC) extinction curve. The color distribution of our SDSS quasar sample suggests that the population of moderately dust reddened, but otherwise normal (i.e., Type 1) quasars is smaller than the population of unobscured quasars: we estimate that a further 10% of the quasar population with M i < −25.61 is missing from the SDSS sample because of extinction, bringing the total fraction of dust reddened quasars to 15% of broad-line quasars.
Quasar absorption lines: an overview
Bulletin of the Astronomical Society of India
Quasar spectra, apart from showing the broad, redshifted emission lines, often show narrow as well as broad absorption lines. The presence of absorption lines has been known since shortly after the discovery of these objects. The spectrum of a single quasar can have more than one group of absorption lines, termed absorption line systems, each system having its characteristic redshift. Most of these systems are believed to be produced by galactic or intergalactic clouds, while those having redshifts close to that of the quasar may be produced by material intrinsic to the quasar. These systems thus offer a sensitive probe to understand the evolutionary history of the Universe over 90% of its age as well as the quasar environment. In recent times, the commissioning of large telescopes, high resolution spectrographs, space telescopes, large sky digital surveys and progress in hydro-simulations have resulted in rapid progress in this field. In this article, I will summarize some of our c...
Unusual broad absorption line quasars from the Sloan Digital Sky Survey
The Sloan Digital Sky Survey has confirmed the existence of populations of broad absorption line (BAL) quasars with various unusual properties. We present and discuss twenty-three such objects and consider the implications of their wide range of properties for models of BAL outflows and quasars in general. We have discovered one BAL quasar with a record number of absorption lines. Two other similarly complex objects with many narrow troughs show broad Mg ii absorption extending longward of their systemic host galaxy redshifts. This can be explained as absorption of an extended continuum source by the rotation-dominated base of a disk wind. Five other objects have absorption which removes an unprecedented ∼90% of all flux shortward of Mg ii. The absorption in one of them has varied across the ultraviolet with an amplitude and rate of change as great as ever seen. This same object may also show broad Hβ absorption. Numerous reddened BAL quasars have been found, including at least one reddened mini-BAL quasar with very strong Fe ii emission. The five reddest objects have continuum reddenings of E(B − V ) ≃ 0.5, and in two of them we find strong evidence that the reddening curve is even steeper than that of the SMC. We have found at least one object with absorption from Fe iii but not Fe ii. This may be due to a high column density of moderately high-ionization gas, but the Fe iii level populations must also be affected by some sort of resonance. Finally, we have found two luminous, probably reddened high-redshift objects which may be BAL quasars whose troughs partially cover different regions of the continuum source as a function of velocity.
Broad Absorption Line Variability in Repeat Quasar Observations from the Sloan Digital Sky Survey
The Astrophysical Journal, 2007
We present a time-variability analysis of 29 broad absorption line quasars (BALQSOs) observed in two epochs by the Sloan Digital Sky Survey. These spectra are selected from a larger sample of BALQSOs with multiple observations by virtue of exhibiting a broad C iv k1549 absorption trough separated from the rest frame of the associated emission peak by more than 3600 km s À1. Detached troughs facilitate higher precision variability measurements, since the measurement of the absorption in these objects is not complicated by variation in the emission-line flux. We have undertaken a statistical analysis of these detached-trough BALQSO spectra to explore the relationships between BAL features that are seen to vary and the dynamics of emission from the quasar central engine. We have measured variability within our sample, which includes three strongly variable BALs. We have also verified that the statistical behavior of the overall sample agrees with current model predictions and previous studies of BAL variability. Specifically, we observe that the strongest BAL variability occurs among the smallest equivalent width features and at velocities exceeding 12,000 km s À1 , as predicted by recent disk-wind modeling.
The Sloan Digital Sky Survey Quasar Catalog. IV. Fifth Data Release
Astronomical Journal, 2007
We present the fourth edition of the Sloan Digital Sky Survey (SDSS) Quasar Catalog. The catalog contains 77,429 objects; this is an increase of over 30,000 entries since the previous edition. The catalog consists of the objects in the SDSS Fifth Data Release that have luminosities larger than M i = −22.0 (in a cosmology with H 0 = 70 km s −1 Mpc −1 , Ω M = 0.3, and Ω Λ = 0.7), have at least one emission line with FWHM larger than 1000 km s −1 or have interesting/complex absorption features, are fainter than i ≈ 15.0, and have highly reliable redshifts. The area covered by the catalog is ≈ 5740 deg 2 . The quasar redshifts range from 0.08 to 5.41, with a median value of 1.48; the catalog includes 891 quasars at redshifts greater than four, of which 36 are at redshifts greater than five. Approximately half of the catalog quasars have i < 19; nearly all have i < 21. For each object the catalog presents positions accurate to better than 0.2 ′′ rms per coordinate, five-band (ugriz) CCD-based photometry with typical accuracy of 0.03 mag, and information on the morphology and selection method. The catalog also contains basic radio, near-infrared, and X-ray emission properties of the quasars, when available, from other large-area surveys. The calibrated digital spectra cover the wavelength region 3800-9200Å at a spectral resolution of ≃ 2000; the spectra can be retrieved from the public database using the information provided in the catalog. The average SDSS colors of quasars as a function of redshift, derived from the catalog entries, are presented in tabular form. Approximately 96% of the objects in the catalog were discovered by the SDSS.
Composite Spectra from the FIRST Bright Quasar Survey
ApJ, 2001
We present a very high signal-to-noise ratio composite spectrum created using 657 radio-selected quasars from the FIRST Bright Quasar Survey. The spectrum spans rest-frame wavelengths 900 -7500Å. Additionally we present composite spectra formed from subsets of the total data set in order to investigate the spectral dependence on radio loudness and the presence of broad absorption. In particular, radio-loud quasars are red compared to radio-quiet quasars, and quasars showing low-ionization broad absorption lines are red compared to other quasars. We compare our composites with those from the Large Bright Quasar Survey. Composite quasar spectra have proven to be valuable tools for a host of applications, and in that spirit we make these publically available via the FIRST survey web page.
A Census of Intrinsic Narrow Absorption Lines in the Spectra of Quasars at z = 2–4
The Astrophysical Journal Supplement Series, 2007
We use Keck/HIRES spectra of 37 optically bright quasars at z = 2-4 to study narrow absorption lines that are intrinsic to the quasars (intrinsic NALs, produced in gas that is physically associated with the quasar central engine). We identify 150 NAL systems, that contain 124 C IV, 12 N V, and 50 Si IV doublets, of which 18 are associated systems (within 5,000 km s −1 of the quasar redshift). We use partial coverage analysis to separate intrinsic NALs from NALs produced in cosmologically intervening structures. We find 39 candidate intrinsic systems, (28 reliable determinations and 11 that are possibly intrinsic). We estimate that 10-17% of C IV systems at blueshifts of 5,000-70,000 km s −1 relative to quasars are intrinsic. At least 32% of quasars contain one or more intrinsic C IV NALs. Considering N V and Si IV doublets showing partial coverage as well, at least 50% of quasars host intrinsic NALs. This result constrains the solid angle subtended by the absorbers to the background source(s). We identify two families of intrinsic NAL systems, those with strong N V absorption, and those with negligible absorption in N V, but with partial coverage in the C IV doublet. We discuss the idea that these two families represent different regions or conditions in accretion disk winds. Of the 26 intrinsic C IV NAL systems, 13 have detectable low-ionization absorption lines at similar velocities, suggesting that these are two-phase structures in the wind rather than absorbers in the host galaxy. We also compare possible models for quasar outflows, including radiatively accelerated disk-driven winds, magnetocentrifugally accelerated winds, and pressure-driven winds, and we discuss ways of distinguishing between these models observationally.
The Astrophysical Journal
We present an analysis of the variability of broad absorption lines (BALs) in a quasar SDSS J141955.26+522741.1 at z = 2.145 with 72 observations from the Sloan Digital Sky Survey Data Release 16 (SDSS DR16). The strong correlation between the equivalent widths of BAL and the continuum luminosity, reveals that the variation of BAL trough is dominated by the photoionization. The photoionization model predicts that when the time interval ∆T between two observations is longer than the recombination timescale t rec , the BAL variations can be detected. This can be characterized as a "sharp rise" in the detection rate of BAL variation at ∆T = t rec. For the first time, we detect such a "sharp rise" signature in the detection rate of BAL variations. As a result, we propose that the t rec can be obtained from the "sharp rise" of the detection rate of BAL variation. It is worth mentioning that the BAL variations are detected at the time-intervals less than the t rec for half an order of magnitude in two individual troughs. This result indicates that there may be multiple components with different t rec but the same velocity in an individual trough.