Sloan Digital Sky Survey Spectra Research Articles

A Census of Object Types and Redshift Estimates in the SDSS Photometric Catalog from a Trained Decision Tree Classifier

We have applied ClassX, an oblique decision tree classifier optimized for astronomical analysis, to the homogeneous multicolor imaging data base of the Sloan Digital Sky Survey (SDSS), training the software on subsets of SDSS objects whose nature is precisely known via spectroscopy. We find that the software, using photometric data only, correctly classifies a very large fraction of the objects with existing SDSS spectra, both stellar and extragalactic. ClassX also accurately predicts the redshifts of both normal and active galaxies in SDSS. To illustrate ClassX applications in SDSS research, we (a) derive the object content of the SDSS DR2 photometric catalog and (b) provide a sample catalog of resolved SDSS objects that contains a large number of candidate AGN galaxies, 27,000, along with 63,000 candidate normal galaxies at magnitudes substantially fainter than typical magnitudes of SDSS spectroscopic objects. The surface density of AGN selected by ClassX to i~19 is in agreement with that quoted by SDSS. When ClassX is applied to the photometric data fainter than the SDSS spectroscopic limit, the inferred surface density of AGN rises sharply, as expected. The ability of the classifier to accurately constrain the redshifts of huge numbers (ultimately ~ 10^7) of active galaxies in the photometric data base promises new insights into fundamental issues of AGN research, such as the evolution of the AGN luminosity function with cosmic time, the starburst--AGN connection, and AGN--galactic morphology relationships.

Observational constraints on silent quartessence

We derive new constraints set by SNIa experiments (‘gold’ data sample of Riess et al),x-ray galaxy cluster data (Allen et al Chandra measurements of the x-ray gas mass fractionin 26 clusters), large scale structure (Sloan Digital Sky Survey spectrum) andcosmic microwave background (WMAP) on the quartessence Chaplygin model. Weconsider both adiabatic perturbations and intrinsic non-adiabatic perturbations suchthat the effective sound speed vanishes (silent Chaplygin). We show that for theadiabatic case only models with equation of state parameter are allowed: this means that the allowed models are very close toΛCDM.In the silent case, however, the results are consistent with observations in a much broader range,−0.3<α<0.7.

The excess far-infrared emission of active galactic nuclei in the local Universe

We have cross-correlated the Sloan Digital Sky Survey (SDSS) second data release spectroscopic galaxy sample with the IRAS Faint Source Catalogue (FSC). Optical emission line ratios are used to classify the galaxies with reliable IRAS 60- and 100-μm detections into active galactic nuclei (AGN) and normal star-forming galaxies. We then create subsamples of normal galaxies and AGN that are very closely matched in terms of key physical properties such as stellar mass, redshift, size, concentration and mean stellar age (as measured by absorption line indicators in the SDSS spectra). We then quantify whether there are systematic differences between the infrared (IR) luminosities of the galaxies and the AGN in the matched subsamples. We find that the AGN exhibit a significant excess in far-infrared (FIR) emission relative to the star-forming galaxies in our sample. The excesses at 60 and 100 μm are 0.21 ± 0.03 and 0.12 ± 0.035 dex in log L 60 /M * and log L 100 /M * , respectively. We then discuss whether the FIR excess is produced by radiation from the active nucleus that is absorbed by dust or, alternatively, by an extra population of young stars that is not detectable at optical wavelengths.

Peering through the OH forest: a new technique to remove residual sky features from Sloan Digital Sky Survey spectra

The Sloan Digital Sky Survey (SDSS) currently provides by far the largest homogeneous sample of intermediate signal-to-noise (S/N) ratio optical spectra of galaxies and quasars. The fully automated SDSS spectroscopic reduction pipeline has provided spectra of unprecedented quality that cover the wavelength range 3800–9200 A. However, in common with spectra from virtually all multi-object surveys employing fibres, there remain significant systematic residuals in many of the spectra owing to the incomplete subtraction of the strong OH sky emission lines longward of 6700 A. These sky lines affect almost half the wavelength range of the SDSS spectra, and the S/N ratio over substantial wavelength regions in many spectra is reduced by more than a factor of 2 over that expected from counting statistics. We present a method to automatically remove the sky-residual signal, using a principal component analysis which takes advantage of the correlation in the form of the sky-subtraction residuals present in each spectrum. Application of the method results in spectra with essentially no evidence for degradation owing to the incomplete subtraction of OH emission features. A dramatic improvement in the quality of a substantial number of spectra, particularly those of faint objects such as the bulk of the high-redshift quasars, is achieved. We make available Interactive Data Language (IDL) code and documentation to implement the sky-residual subtraction scheme on SDSS spectra included in the public data releases. To ensure that absorption and emission features intrinsic to the object spectra do not affect the subtraction procedure, line masks must be created that depend on the scientific application of interest. We illustrate the power of the sky-residual subtraction scheme using samples of SDSS galaxy and quasar spectra, presenting tests involving the near-infrared Ca ii triplet absorption, metal absorption line features in damped Lyman-α systems and composite spectra of high-redshift quasars.

Systematics of the Ultraviolet Rising Flux in a GALEX /SDSS Sample of Early-Type Galaxies

We present ultraviolet photometry for a sample of morphologically early-type galaxies selected by matching the Sloan Digital Sky Survey (SDSS) First Data Release with the Galaxy Evolution Explorer (GALEX) Medium and All-sky Imaging Surveys. We obtain a working sample of 1032 early-type galaxies with GALEX far-UV (FUV) detections, SDSS spectroscopy, and z < 0.2. Using the SDSS spectra to identify galaxies with even weak star formation or evidence of active galactic nuclei, and further removing galaxies with any evidence of non-early-type morphology, we derive a final sample of 172 red quiescent early-type galaxies. We find that the FUV - r color has a full range of 5 mag. Plotting the FUV - r color against the metallicity sensitive Lick Mg2 and D4000 indices and against the stellar velocity dispersion, we find no correlation between our measurement of UV rising flux and any parameter sensitive to the global metallicity of quiescent early-type galaxies.

Spectral Classification of Quasars in the Sloan Digital Sky Survey: Eigenspectra, Redshift, and Luminosity Effects

We study 16,707 quasar spectra from the Sloan Digital Sky Survey (SDSS) (an early version of the First Data Release; DR1) using the Karhunen-Loeve transform (or principal components analysis). The redshifts of these quasars range from 0.08 to 5.41, the i-band absolute magnitudes from -30 to -22, and the resulting rest-frame wavelengths from 900 to 8000 A. The quasar eigenspectra of the full catalog reveal the following: first order—the mean spectrum; second order—a host-galaxy component; third order—the UV-optical continuum slope; fourth order—the correlations of Balmer emission lines. These four eigenspectra account for 82% of the total sample variance. Broad absorption features are found not to be confined in one particular order but to span a number of higher orders. We find that the spectral classification of quasars is redshift and luminosity dependent; as such there does not exist a compact set (i.e., less than ≈10 modes) of eigenspectra (covering 900–8000 A) that can describe most variations (i.e., greater than ≈95%) of the entire catalog. We therefore construct several sets of eigenspectra in different redshift and luminosity bins. From these eigenspectra we find that quasar spectra can be classified (by the first two eigenspectra) into a sequence that is defined by a simple progression in the steepness of the slope of the continuum. We also find a dependence on redshift and luminosity in the eigencoefficients. The dominant redshift effect is a result of the evolution of the blended Fe II emission (optical) and the Balmer continuum (the small bump, λrest ≈ 2000–4000 A). A luminosity dependence is also present in the eigencoefficients and is related to the Baldwin effect—the decrease of the equivalent width of an emission line with luminosity, which is detected in Lyα, Si IV+O IV], C IV, He II, C III] and Mg II, while the effect in N V seems to be redshift dependent. If we restrict ourselves to the rest-wavelength regions 1150–2000 A and 4000–5500 A, the eigenspectra constructed from the wavelength-selected SDSS spectra are found to agree with the principal components by Francis et al. and the well-known Eigenvector-1 of Boroson & Green, respectively. ASCII formatted tables of the eigenspectra are available.

Rest‐Frame Mid‐Infrared Detection of an Extremely Luminous Lyman Break Galaxy with the Spitzer Infrared Spectrograph (IRS)

We present the first rest-frame ∼4 μm detection of a Lyman break galaxy. The data were obtained using the 16 μm imaging capability of the Spitzer Infrared Spectrograph. The target object, J134026.44+634433.2, is an extremely luminous Lyman break galaxy at z = 2.79, first identified in Sloan Digital Sky Survey (SDSS) spectra (as reported by Bentz et al.). The source is strongly detected with a flux of 0.94 ± 0.02 mJy. Combining Spitzer and SDSS photometry with supporting ground-based J- and K-band data, we show that the spectral energy distribution is consistent with an actively star-forming galaxy. We also detect other objects in the Spitzer field of view, including a very red mid-infrared source. We find no evidence of a strong lens among the mid-infrared sources.

SDSS J115517.35+634622.0: A Newly Discovered Gravitationally Lensed Quasar

We report the discovery of SDSSJ115517.35+634622.0, a previously unknown gravitationally lensed quasar. The lens system exhibits two images of a $z = 2.89$ quasar, with an image separation of $1{\farcs}832 \pm 0.007$ . Near-IR imaging of the system reveals the presence of the lensing galaxy between the two quasar images. Based on absorption features seen in the Sloan Digital Sky Survey (SDSS) spectrum, we determine a lens galaxy redshift of $z = 0.1756$. The lens is rather unusual in that one of the quasar images is only $0{\farcs}22\pm0{\farcs}07$ ($\sim 0.1 R_{\rm eff}$) from the center of the lens galaxy and photometric modeling indicates that this image is significantly brighter than predicted by a SIS model. This system was discovered in the course of an ongoing search for strongly lensed quasars in the dataset from the SDSS.

Bright Lyman Break Galaxy Candidates in the Sloan Digital Sky Survey First Data Release

We report the discovery of six compact starburst galaxy candidates with redshifts 2.3 < z < 2.8 and r-band magnitudes 19.8-20.5 in the Quasar Catalog of the Sloan Digital Sky Survey (SDSS) First Data Release. The SDSS spectra resemble the composite spectrum of Lyman break galaxies (LBGs) at z ? 3 (albeit with some differences and broader spectral lines), but the objects are 4-5 mag brighter than an L* LBG. These objects could be extremely luminous LBGs, normal LBGs amplified by gravitational lensing, or a rare class of broad absorption line (BAL) quasars. In the first case, star formation rates inferred from the UV continuum luminosities, with no correction for dust extinction, are ~300-1000 M? yr-1, similar to those of ultraluminous infrared galaxies, but in these UV-bright objects the star formation is evidently not obscured by high dust column densities. The SDSS images show no evidence of multiple imaging or foreground lensing structures. The spectra have fairly broad absorption features and prominent high-ionization absorption but do not show high-ionization emission lines and do not resemble known BAL quasars. A rough estimate of the luminosity function of the objects is above a Schechter function extrapolation from LBGs at fainter magnitudes. Improved optical spectra and observations at X-ray, IR, and submillimeter wavelengths could help determine the nature of these remarkable objects.

Cataclysmic Variables from the Sloan Digital Sky Survey. II. The Second Year

The first full year of operation following the commissioning year of the Sloan Digital Sky Survey (SDSS) has revealed a wide variety of newly discovered cataclysmic variables (CVs). We show the SDSS spectra of 42 CVs observed in 2002, of which 35 are new classifications, four are known dwarf novae (CT Hya, RZ Leo, T Leo, and BZ UMa), one is a known CV identified from a previous quasar survey (Aqr1), and two are known ROSAT or FIRST discovered CVs (RX J09445+0357, FIRST J102347.6+003841). The SDSS positions, colors, and spectra of all 42 systems are presented. In addition, the results of follow-up studies of several of these objects identify the orbital periods, velocity curves, and polarization that provide the system geometry and accretion properties. While most of the SDSS discovered systems are faint (greater than 18th magnitude) with low accretion rates (as implied from their spectral characteristics), there are also a few bright objects that may have escaped previous surveys due to changes in the mass transfer rate.

Investigating the Sloan Digital Sky Survey Cataclysmic Variable SDSS J132723.39+652854.211Based on observations with the Apache Point Observatory 3.5 m telescope and the Sloan Digital Sky Survey, which are owned and operated by the Astrophysical Research Consortium (ARC), and the Manastash Ridge Observatory 0.76 m telescope owned and operated by the University of Washington. Observations were also done with the Steward

ABSTRACTPhotometric and spectroscopic observations reveal the Sloan Digital Sky Survey cataclysmic variable SDSS J132723.39+652854.2 to be a likely SW Sex star with an orbital period of P = 3.28 hr. The Sloan Digital Sky Survey spectrum shows strong He i, He ii, and Balmer emission lines. Time‐resolved spectra acquired at Apache Point Observatory reveal absorption‐line structure within the emission lines near phase 0.5. Photometric data obtained at Manastash Ridge Observatory using Harris V and B filters reveal a high‐inclination system (i = 80°) with deep (∼2 mag) eclipses. The orbital period and the spectral variations favor an SW Sex interpretation.

Damped Ly Gas Metallicities at Redshifts z =0.9-2.0 from Sloan Digital Sky Survey Spectra

Using Sloan Digital Sky Survey early data release spectra, we have identified 370 Mg II absorption systems with Mg II λ2796 rest equivalent widths ≥1 Å and redshifts z = 0.9-2.2. From our previous and ongoing Hubble Space Telescope UV spectroscopic studies, we estimate that the mean neutral hydrogen column density of a system selected in this manner is N = × 1020 atoms cm-2, which corresponds to the damped Lyα (DLA) regime. We have formed high signal-to-noise ratio composite spectra using 223 of these systems with z = 0.9-2.0 in order to study the strength of the Zn II and Cr II absorption lines corresponding to this mean neutral hydrogen column density. After making a correction for missed DLAs, overall we find that [Zn/H] = -1.13 ± 0.19. We find [Cr/Zn] = -0.45 ± 0.13, which indicates that ≈65% of the Cr is depleted onto grains, but this does not correct for the missed DLAs. We have also derived Zn and Cr abundances in two kinematic regimes, and within each regime we consider two redshift intervals. We find trends that indicate that metallicities are higher in the composites in which the absorption has larger velocity spreads as measured by Mg II λ2796 rest equivalent width. Larger velocity spreads may correspond to deeper gravitational potential wells that represent more massive and chemically evolved structures and/or regions associated with winds from starbursting galaxies, also leading to kinematically broad structures of chemically enriched gas. Within the large velocity spread regime, we find that at lower redshifts the Zn metallicity is larger and more Cr is depleted onto grains.

Average Spectra of Massive Galaxies in the Sloan Digital Sky Survey

We combine Sloan Digital Sky Survey spectra of 22,000 luminous, red, bulge-dominated galaxies to get high signal-to-noise ratio average spectra in the rest-frame optical and ultraviolet (2600-7000 A). The average spectra of these massive, quiescent galaxies are early type with weak emission lines and with absorption lines indicating an apparent excess of α-elements over solar abundance ratios. We make average spectra of subsamples selected by luminosity, environment, and redshift. The average spectra are remarkable in their similarity. What variations do exist in the average spectra as a function of luminosity and environment are found to form a nearly one-parameter family in spectrum space. We present a high signal-to-noise ratio spectrum of the variation. We measure the properties of the variation with a modified version of the Lick index system and compare to model spectra from stellar population syntheses. The variation may be a combination of age and chemical abundance differences, but the conservative conclusion is that the quality of the data considerably exceeds the current state of the models.

Optical and Radio Properties of Extragalactic Sources Observed by the FIRST Survey and the Sloan Digital Sky Survey

We discuss the optical and radio properties of ~30,000 FIRST (radio, 20 cm, sensitive to 1 mJy) sources positionally associated within 15 with a Sloan Digital Sky Survey (SDSS) (optical, sensitive to r* ~ 22.2) source in 1230 deg2 of sky. The matched sample represents ~30% of the 108,000 FIRST sources and 0.1% of the 2.5 ? 107 SDSS sources in the studied region. SDSS spectra are available for 4300 galaxies and 1154 quasars from the matched sample and for a control sample of 140,000 galaxies and 20,000 quasars in 1030 deg2 of sky. Here we analyze only core sources, which dominate the sample; the fraction of SDSS-FIRST sources with complex radio morphology is determined to be less than 10%. This large and unbiased catalog of optical identifications provides much firmer statistical footing for existing results and allows several new findings. The majority (83%) of the FIRST sources identified with an SDSS source brighter than r* = 21 are optically resolved; the fraction of resolved objects among the matched sources is a function of the radio flux, increasing from ~50% at the bright end to ~90% at the FIRST faint limit. Nearly all optically unresolved radio sources have nonstellar colors indicative of quasars. We estimate an upper limit of ~5% for the fraction of quasars with broadband optical colors indistinguishable from those of stars. The distribution of quasars in the radio flux?optical flux plane suggests the existence of the quasar radio dichotomy; 8% ? 1% of all quasars with i* 2.22) galaxies, especially those with r* > 17.5. Magnitude- and redshift-limited samples show that radio galaxies have a different optical luminosity distribution than nonradio galaxies selected by the same criteria; when galaxies are further separated by their colors, this result remains valid for both blue and red galaxies. For a given optical luminosity and redshift, the observed optical colors of radio galaxies are indistinguishable from those of all SDSS galaxies selected by identical criteria. The distributions of radio-to-optical flux ratio are similar for blue and red galaxies in redshift-limited samples; this similarity implies that the difference in their luminosity functions and resulting selection effects are the dominant cause for the preponderance of red radio galaxies in flux-limited samples. The fraction of radio galaxies whose emission-line ratios indicate an AGN (30%), rather than starburst, origin is 6 times larger than the corresponding fraction for all SDSS galaxies (r* < 17.5). We confirm that the AGN-to-starburst galaxy number ratio increases with radio flux and find that radio emission from AGNs is more concentrated than radio emission from starburst galaxies.

Spectroscopic Target Selection in the Sloan Digital Sky Survey: The Main Galaxy Sample

We describe the algorithm that selects the main sample of galaxies for spectroscopy in the Sloan Digital Sky Survey (SDSS) from the photometric data obtained by the imaging survey. Galaxy photometric properties are measured using the Petrosian magnitude system, which measures flux in apertures determined by the shape of the surface brightness profile. The metric aperture used is essentially independent of cosmological surface brightness dimming, foreground extinction, sky brightness, and the galaxy central surface brightness. The main galaxy sample consists of galaxies with r-band Petrosian magnitudes r ≤ 17.77 and r-band Petrosian half-light surface brightnesses μ50 ≤ 24.5 mag arcsec-2. These cuts select about 90 galaxy targets per square degree, with a median redshift of 0.104. We carry out a number of tests to show that (1) our star-galaxy separation criterion is effective at eliminating nearly all stellar contamination while removing almost no genuine galaxies, (2) the fraction of galaxies eliminated by our surface brightness cut is very small (~0.1%), (3) the completeness of the sample is high, exceeding 99%, and (4) the reproducibility of target selection based on repeated imaging scans is consistent with the expected random photometric errors. The main cause of incompleteness is blending with saturated stars, which becomes more significant for brighter, larger galaxies. The SDSS spectra are of high enough signal-to-noise ratio (S/N > 4 per pixel) that essentially all targeted galaxies (99.9%) yield a reliable redshift (i.e., with statistical error less than 30 km s-1). About 6% of galaxies that satisfy the selection criteria are not observed because they have a companion closer than the 55'' minimum separation of spectroscopic fibers, but these galaxies can be accounted for in statistical analyses of clustering or galaxy properties. The uniformity and completeness of the galaxy sample make it ideal for studies of large-scale structure and the characteristics of the galaxy population in the local universe.

The Redshift of a Lensing Galaxy in PMN J0134−0931

The Sloan Digital Sky Survey (SDSS) automatically targeted as a quasar candidate the recently discovered, gravitationally lensed, extremely reddened z = 2.2 quasar PMN 0134-0931. The SDSS spectrum exhibits Ca II absorption at z = 0.76451, which we identify as the redshift of a lensing galaxy. Hubble Space Telescope imaging shows that components C, D, and E of the system are significantly redder than components A or B and detects faint galaxy emission between D and A+B. The redshift of the dust responsible for the reddening remains unconstrained with current data. However, we outline a model wherein lensing and differential reddening by a z = 0.76451 galaxy pair can entirely explain this system.

High-Redshift Quasars Found in Sloan Digital Sky Survey Commissioning Data. VI. Sloan Digital Sky Survey Spectrograph Observations

We present data for over 100 high-redshift quasars found in ?700 deg2 by the Sloan Digital Sky Survey (SDSS), using automated selection algorithms applied to SDSS imaging data and with spectroscopic confirmation obtained during routine spectroscopic observations by the Sloan 2.5 m telescope. The SDSS spectra cover the wavelength range 3900?9200 ? at a spectral resolution of 1800 and have been obtained for 116 quasars with redshifts greater than 3.94; 92 of these objects were previously uncataloged, significantly increasing the current tally of published z > 4 quasars. The paper also reports observations of seven additional new z > 4 quasars; all were selected from the SDSS imaging survey and spectroscopically confirmed with observations by the Apache Point Observatory's 3.5 m telescope or the Hobby-Eberly Telescope. The i' magnitudes of the quasars range from 18.03 to 20.68. Of the 99 new objects reported in this paper, 13 appear to be broad absorption line quasars. Five quasars, including one object at a redshift of 5.11, have 20 cm peak flux densities greater than 1 mJy. Two of the quasars, both at z ? 4.5, have very weak emission lines; one of these objects is a radio source. Nineteen of the newly discovered objects have redshifts above 4.6, and the maximum redshift is z = 5.41; among objects reported to date, the latter is the third highest redshift AGN and is penultimate in redshift among luminous quasars.