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This paper describes the Sixth Data Release of the Sloan Digital Sky Survey. With this data release, the imaging of the northern Galactic cap is now complete. The survey contains images and parameters of roughly 287 million objects over 9583 deg2, including scans over a large range of Galactic latitudes and longitudes. The survey also includes 1.27 million spectra of stars, galaxies, quasars, and blank sky (for sky subtraction) selected over 7425 deg2. This release includes much more stellar spectroscopy than was available in previous data releases and also includes detailed estimates of stellar temperatures, gravities, and metallicities. The results of improved photometric calibration are now available, with uncertainties of roughly 1% in g, r, i, and z, and 2% in u, substantially better than the uncertainties in previous data releases. The spectra in this data release have improved wavelength and flux calibration, especially in the extreme blue and extreme red, leading to the qualitatively better determination of stellar types and radial velocities. The spectrophotometric fluxes are now tied to point-spread function magnitudes of stars rather than fiber magnitudes. This gives more robust results in the presence of seeing variations, but also implies a change in the spectrophotometric scale, which is now brighter by roughly 0.35 mag. Systematic errors in the velocity dispersions of galaxies have been fixed, and the results of two independent codes for determining spectral classifications and redshifts are made available. Additional spectral outputs are made available, including calibrated spectra from individual 15 minute exposures and the sky spectrum subtracted from each exposure. We also quantify a recently recognized underestimation of the brightnesses of galaxies of large angular extent due to poor sky subtraction; the bias can exceed 0.2 mag for galaxies brighter than r = 14 mag., Astronomy

We quantify the variability of faint unresolved optical sources using a catalog based on multiple SDSS imaging observations. The catalog covers SDSS stripe 82, which lies along the celestial equator in the southern Galactic hemisphere (22h24m < αJ2000.0 < 04h08m, -1.27° < δJ2000.0 < +1.27°, ~290 deg2), and contains 34 million photometric observations in the SDSS ugriz system for 748,084 unresolved sources at high Galactic latitudes (b < -20°) that were observed at least four times in each of the ugri bands (with a median of 10 observations obtained over ~6 yr). In each photometric bandpass we compute various low-order light-curve statistics, such as rms scatter, χ2 per degree of freedom, skewness, and minimum and maximum magnitude, and use them to select and study variable sources. We find that 2% of unresolved optical sources brighter than g = 20.5 appear variable at the 0.05 mag level (rms) simultaneously in the g and r bands (at high Galactic latitudes). The majority (2 out of 3) of these variable sources are low-redshift (<2) quasars, although they represent only 2% of all sources in the adopted flux-limited sample. We find that at least 90% of quasars are variable at the 0.03 mag level (rms) and confirm that variability is as good a method for finding low-redshift quasars as the UV excess color selection (at high Galactic latitudes). We analyze the distribution of light-curve skewness for quasars and find that it is centered on zero. We find that about one-fourth of the variable stars are RR Lyrae stars, and that only 0.5% of stars from the main stellar locus are variable at the 0.05 mag level. The distribution of light-curve skewness in the g - r versus u - g color-color diagram on the main stellar locus is found to be bimodal (with one mode consistent with Algol-like behavior). Using over 600 RR Lyrae stars, we demonstrate rich halo substructure out to distances of 100 kpc. We extrapolate these results to the expected performance by the Large Synoptic Survey Telescope and estimate that it will obtain well-sampled, 2% accurate, multicolor light curves for ~2 million low-redshift quasars and discover at least 50 million variable stars., Astronomy

We present a study of variable faint optical sources discovered by comparing the Sloan Digital Sky Survey (SDSS) and the Palomar Observatory Sky Survey (POSS) catalogs. We use SDSS measurements to photometrically recalibrate several publicly available POSS catalogs (USNO-A2.0, USNO-B1.0, DPOSS [the Digitized Second Palomar Observatory Sky Survey], and GSC2.2 [the Guide Star Catalog 2.2]). A piecewise recalibration of the POSS data in 100 arcmin2 patches (one SDSS field) generally results in an improvement of photometric accuracy (rms) by nearly a factor of 2, compared to the original data. In addition to the smaller core width of the error distribution, the tails of the distribution become much steeper after the recalibration. These improvements are mostly due to the very dense grid of calibration stars provided by SDSS, which rectifies the intrinsic inhomogeneities of Schmidt plates. We find that the POSS I magnitudes can be improved to ~0.15 mag accuracy, and POSS II magnitudes to ~0.10 mag accuracy. The smallest final errors are obtained with the GSC2.2 catalog, for which they approach 0.07 mag at the bright end. We use the recalibrated catalogs for the ~3300 deg2 of sky in the SDSS Data Release 2 to construct a catalog of ~60,000 sources that are variable on timescales of 10-50 yr, and make it publicly available. Using this catalog, we find that at least 1% of faint optical sources appear variable at the >0.25 mag level, and that about 20% of the variable population are quasars, although they represent only 0.6% of all point sources in the adopted flux-limited sample (g < 19). A series of statistical tests based on the morphology of SDSS color-magnitude and color-color diagrams, as well as visual comparison of images and comparison with repeated SDSS observations, demonstrate the robustness of the selection methods: three out of four candidate variable sources are correctly recognized to vary. We also demonstrate that candidate RR Lyrae stars trace the same halo structures, such as the Sgr dwarf tidal stream, that were discovered using repeated SDSS observations. We utilize the POSS-SDSS selected candidates to constrain the halo structure in the parts of sky for which repeated SDSS observations do not exist. We quantify the distribution of variable sources in the SDSS color-color diagrams and the variability characteristics of quasars. The observed long-term quasar variability (structure function) is smaller than predicted by the extrapolation of the power law measured for short timescales using repeated SDSS imaging (0.35 vs. 0.60 mag for SDSS-POSS I and 0.24 vs. 0.35 mag for SDSS-POSS II, rms). This turnover in structure function suggests that the characteristic timescale for quasar variability is on the order of 1 yr in the rest frame., Astronomy

This paper describes the Third Data Release of the Sloan Digital Sky Survey (SDSS). This release, containing data taken up through 2003 June, includes imaging data in five bands over 5282 deg2, photometric and astrometric catalogs of the 141 million objects detected in these imaging data, and spectra of 528,640 objects selected over 4188 deg2. The pipelines analyzing both images and spectroscopy are unchanged from those used in our Second Data Release., Astronomy

We measure cosmological parameters using the three-dimensional power spectrum P(k) from over 200,000 galaxies in the Sloan Digital Sky Survey (SDSS) in combination with WMAP and other data. Our results are consistent with a ``vanilla'' flat adiabatic Lambda-CDM model without tilt (n=1), running tilt, tensor modes or massive neutrinos. Adding SDSS information more than halves the WMAP-only error bars on some parameters, tightening 1 sigma constraints on the Hubble parameter from h~0.74+0.18-0.07 to h~0.70+0.04-0.03, on the matter density from Omega_m~0.25+/-0.10 to Omega_m~0.30+/-0.04 (1 sigma) and on neutrino masses from <11 eV to <0.6 eV (95%). SDSS helps even more when dropping prior assumptions about curvature, neutrinos, tensor modes and the equation of state. Our results are in substantial agreement with the joint analysis of WMAP and the 2dF Galaxy Redshift Survey, which is an impressive consistency check with independent redshift survey data and analysis techniques. In this paper, we place particular emphasis on clarifying the physical origin of the constraints, i.e., what we do and do not know when using different data sets and prior assumptions. For instance, dropping the assumption that space is perfectly flat, the WMAP-only constraint on the measured age of the Universe tightens from t0~16.3+2.3-1.8 Gyr to t0~14.1+1.0-0.9 Gyr by adding SDSS and SN Ia data. Including tensors, running tilt, neutrino mass and equation of state in the list of free parameters, many constraints are still quite weak, but future cosmological measurements from SDSS and other sources should allow these to be substantially tightened., Astronomy

The Sloan Digital Sky Survey (SDSS) has validated and made publicly available its Second Data Release. This data release consists of 3324 deg2 of five-band (ugriz) imaging data with photometry for over 88 million unique objects, 367,360 spectra of galaxies, quasars, stars, and calibrating blank sky patches selected over 2627 deg2 of this area, and tables of measured parameters from these data. The imaging data reach a depth of r ≈ 22.2 (95% completeness limit for point sources) and are photometrically and astrometrically calibrated to 2% rms and 100 mas rms per coordinate, respectively. The imaging data have all been processed through a new version of the SDSS imaging pipeline, in which the most important improvement since the last data release is fixing an error in the model fits to each object. The result is that model magnitudes are now a good proxy for point-spread function magnitudes for point sources, and Petrosian magnitudes for extended sources. The spectroscopy extends from 3800 to 9200 Å at a resolution of 2000. The spectroscopic software now repairs a systematic error in the radial velocities of certain types of stars and has substantially improved spectrophotometry. All data included in the SDSS Early Data Release and First Data Release are reprocessed with the improved pipelines and included in the Second Data Release. Further characteristics of the data are described, as are the data products themselves and the tools for accessing them., Astronomy

The Sloan Digital Sky Survey (SDSS) mosaic camera and telescope have obtained five-band optical-wavelength imaging near the Galactic plane outside of the nominal survey boundaries. These additional data were obtained during commissioning and subsequent testing of the SDSS observing system, and they provide unique wide-area imaging data in regions of high obscuration and star formation, including numerous young stellar objects, Herbig-Haro objects, and young star clusters. Because these data are outside the survey regions in the Galactic caps, they are not part of the standard SDSS data releases. This paper presents imaging data for 832 square degrees of sky (including repeats), in the star-forming regions of Orion, Taurus, and Cygnus. About 470 deg2 are now released to the public, with the remainder to follow at the time of SDSS Data Release 4. The public data in Orion include the star-forming region NGC 2068/NGC 2071/HH 24 and a large part of Barnard's loop., Astronomy

The Sloan Digital Sky Survey (SDSS) has validated and made publicly available its First Data Release. This consists of 2099 deg2 of five-band (u, g, r, i, z) imaging data, 186,240 spectra of galaxies, quasars, stars and calibrating blank sky patches selected over 1360 deg2 of this area, and tables of measured parameters from these data. The imaging data go to a depth of r ≈ 22.6 and are photometrically and astrometrically calibrated to 2% rms and 100 mas rms per coordinate, respectively. The spectra cover the range 3800–9200 Å, with a resolution of 1800–2100. This paper describes the characteristics of the data with emphasis on improvements since the release of commissioning data (the SDSS Early Data Release) and serves as a pointer to extensive published and on-line documentation of the survey., Astronomy

We discuss the optical and radio properties of ~30,000 FIRST (radio, 20 cm, sensitive to 1 mJy) sources positionally associated within 1farcs5 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* < 18.5 are radio-loud, and this fraction seems independent of redshift and optical luminosity. The radio-loud quasars have a redder median color by 0.08 ± 0.02 mag, and show a 3 times larger fraction of objects with extremely red colors. FIRST galaxies represent 5% of all SDSS galaxies with r* < 17.5, and 1% for r* < 20, and are dominated by red (u*-r* > 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., Astronomy

We have created a variety of composite quasar spectra using a homogeneous data set of over 2200 spectra from the Sloan Digital Sky Survey (SDSS). The quasar sample spans a redshift range of 0.044 ≤ z ≤ 4.789 and an absolute r' magnitude range of -18.0 to -26.5. The input spectra cover an observed wavelength range of 3800–9200 Å at a resolution of 1800. The median composite covers a rest-wavelength range from 800 to 8555 Å and reaches a peak signal-to-noise ratio of over 300 per 1 Å resolution element in the rest frame. We have identified over 80 emission-line features in the spectrum. Emission-line shifts relative to nominal laboratory wavelengths are seen for many of the ionic species. Peak shifts of the broad permitted and semiforbidden lines are strongly correlated with ionization energy, as previously suggested, but we find that the narrow forbidden lines are also shifted by amounts that are strongly correlated with ionization energy. The magnitude of the forbidden line shifts is lesssim100 km s-1, compared with shifts of up to 550 km s-1 for some of the permitted and semiforbidden lines. At wavelengths longer than the Lyα emission, the continuum of the geometric mean composite is well fitted by two power laws, with a break at ≈5000 Å. The frequency power-law index, αν, is -0.44 from ≈1300 to 5000 Å and -2.45 redward of ≈5000 Å. The abrupt change in slope can be accounted for partly by host-galaxy contamination at low redshift. Stellar absorption lines, including higher order Balmer lines, seen in the composites suggest that young or intermediate-age stars make a significant contribution to the light of the host galaxies. Most of the spectrum is populated by blended emission lines, especially in the range 1500–3500 Å, which can make the estimation of quasar continua highly uncertain unless large ranges in wavelength are observed. An electronic table of the median quasar template is available., Astronomy

We present an empirical investigation of the colors of quasars in the Sloan Digital Sky Survey (SDSS) photometric system. The sample studied includes 2625 quasars with SDSS photometry: 1759 quasars found during SDSS spectroscopic commissioning and SDSS follow-up observations on other telescopes, 50 matches to FIRST quasars, 573 matches to quasars from the NASA Extragalactic Database, and 243 quasars from two or more of these sources. The quasars are distributed in a 2fdg5 wide stripe centered on the celestial equator covering ~529 deg2. Positions (accurate to 0farcs2) and SDSS magnitudes are given for the 898 quasars known prior to SDSS spectroscopic commissioning. New SDSS quasars, which range in brightness from i* = 15.39 to the photometric magnitude limit of the survey, represent an increase of over 200% in the number of known quasars in this area of the sky. The ensemble average of the observed colors of quasars in the SDSS passbands are well represented by a power-law continuum with αν = -0.5 (fν ∝ να) and are close to those predicted by previous simulations. However, the contributions of the "small blue (or λ3000) bump" and other strong emission lines have a significant effect upon the colors. The color-redshift relation exhibits considerable structure, which may be of use in determining photometric redshifts for quasars from their colors alone. The range of colors at a given redshift can generally be accounted for by a range in the optical spectral index with a distribution αν = -0.5 ± 0.65 (95% confidence), but there is a red tail in the distribution. This tail may be a sign of internal reddening, especially since fainter objects at a given redshift tend to exhibit redder colors than the average. Finally, we show that there is a continuum of properties between quasars and Seyfert galaxies, and we test the validity of the traditional dividing line (MB = -23) between the two classes of active galactic nuclei., Astronomy