The Seventh Data Release of the Sloan Digital Sky Survey

TL;DR

The Seventh Data Release (DR7) marks the culmination of SDSS-II, delivering a comprehensive, calibrated dataset comprising imaging over 11663 deg$^2$ with hundreds of millions of detections and over a million spectra. The paper documents substantial improvements across imaging and spectroscopy, including ubercalibrated photometry, UCAC2-based astrometry, crowded-field photometry via PSPhot, refined photometric redshifts (neural nets and a new hybrid method), and enhanced SEGUE parameter estimation. DR7 also expands data accessibility through Runs and Stripe 82 databases and coadds, and reports on systematic issues and ongoing data processing refinements, setting the stage for SDSS-III. The work enhances the reliability and depth of SDSS data for Galactic, extragalactic, and transient studies, with broad implications for cosmology and stellar astrophysics.

Abstract

This paper describes the Seventh Data Release of the Sloan Digital Sky Survey (SDSS), marking the completion of the original goals of the SDSS and the end of the phase known as SDSS-II. It includes 11663 deg^2 of imaging data, with most of the roughly 2000 deg^2 increment over the previous data release lying in regions of low Galactic latitude. The catalog contains five-band photometry for 357 million distinct objects. The survey also includes repeat photometry over 250 deg^2 along the Celestial Equator in the Southern Galactic Cap. A coaddition of these data goes roughly two magnitudes fainter than the main survey. The spectroscopy is now complete over a contiguous area of 7500 deg^2 in the Northern Galactic Cap, closing the gap that was present in previous data releases. There are over 1.6 million spectra in total, including 930,000 galaxies, 120,000 quasars, and 460,000 stars. The data release includes improved stellar photometry at low Galactic latitude. The astrometry has all been recalibrated with the second version of the USNO CCD Astrograph Catalog (UCAC-2), reducing the rms statistical errors at the bright end to 45 milli-arcseconds per coordinate. A systematic error in bright galaxy photometr is less severe than previously reported for the majority of galaxies. Finally, we describe a series of improvements to the spectroscopic reductions, including better flat-fielding and improved wavelength calibration at the blue end, better processing of objects with extremely strong narrow emission lines, and an improved determination of stellar metallicities. (Abridged)

Figures (10)

• Figure 1: The distribution on the sky of the data included in DR7 (upper panel: imaging; lower panel: spectra), shown in an Aitoff equal-area projection in J2000 Equatorial Coordinates. The Galactic Plane is the sinuous line that goes through each panel. The center of each panel is at $\alpha = 120^\circ \equiv 8^{\rm h}$, and the plots cut off at $\delta = -25^\circ$, below which the SDSS did not extend. The Legacy imaging survey covers the contiguous area of the Northern Galactic Cap (centered roughly at $\alpha = 200^\circ, \delta = 30^\circ$), as well as three stripes (each of width $2.5^\circ$) in the Southern Galactic Cap. In addition, several stripes (indicated in blue in the imaging data) are auxiliary imaging data, while the SEGUE imaging scans are indicated in red. The green scans are additional runs as described in Finkbeiner et al. (2004). In the spectroscopy panel, the lighter regions indicate that area in the Northern Galactic Cap which is new to DR7; note that the Northern Galactic Cap is now contiguous. Red points indicate SEGUE plates, and blue points indicate other non-Legacy plates (mostly as described in the DR4 paper).
• Figure 2: The distribution on the sky of SEGUE (red) and other non-Legacy (blue) spectroscopic observations, here plotted in Galactic coordinates. The contiguous blue stripe across the bottom is Stripe 82, along the Celestial Equator. As described in the DR4 paper, Stripe 82 includes extensive spectroscopy of a number of different types of targets outside the Legacy survey.
• Figure 3: Stripe 82, the Equatorial stripe in the South Galactic Cap, has been imaged multiple times. The lower pair of curves show the number of scans covering a given right ascension in the North and South strip that are included in the coaddition (mostly data taken through 2005). In addition, Stripe 82 has been covered many more times as part of a comprehensive survey for $0.05 < z < 0.35$ supernovae, although often in conditions of poor seeing, bright moon, and/or clouds; the total numbers of scans at each right ascension in the North and South strip are indicated in the upper pair of curves. All these data have been flux-calibrated, as discussed in the text, and are available (together with the coadd itself) in the stripe82 database.
• Figure 4: The distribution of median ${Q}_{{gri}}$ and ${Q}_{{riz}}$ parameters measuring the position of the stellar locus within each field for the photo (left) and PSPhot (right) photometric pipelines; zero values are indicative of uniform photometry. Within the Galactic plane (lower panels), the PSPhot values are more concentrated, but contain a higher number of systematic departures from the main locus. The PSPhot code in fact gives a tighter locus at high latitudes as well (upper panels). Histogram equalization of the gray-scale was used to emphasize low density regions.
• Figure 5: The template-based estimated redshifts versus the true spectroscopic redshifts for a random sample of 30,000 galaxies with redshifts from SDSS. The estimated values calculated with the old (DR6) method has significantly larger scatter and more outliers than the ones with the new hybrid (DR7) technique. Note that the sample is dominated by red galaxies (whose photometric redshifts are intrinsically easier to measure) at $z > 0.2$.