The Eighth Data Release of the Sloan Digital Sky Survey: First Data from SDSS-III

Abstract

The Sloan Digital Sky Survey (SDSS) started a new phase in August 2008, with new instrumentation and new surveys focused on Galactic structure and chemical evolution, measurements of the baryon oscillation feature in the clustering of galaxies and the quasar Ly alpha forest, and a radial velocity search for planets around ~8000 stars. This paper describes the first data release of SDSS-III (and the eighth counting from the beginning of the SDSS). The release includes five-band imaging of roughly 5200 deg^2 in the Southern Galactic Cap, bringing the total footprint of the SDSS imaging to 14,555 deg^2, or over a third of the Celestial Sphere. All the imaging data have been reprocessed with an improved sky-subtraction algorithm and a final, self-consistent photometric recalibration and flat-field determination. This release also includes all data from the second phase of the Sloan Extension for Galactic Understanding and Evolution (SEGUE-2), consisting of spectroscopy of approximately 118,000 stars at both high and low Galactic latitudes. All the more than half a million stellar spectra obtained with the SDSS spectrograph have been reprocessed through an improved stellar parameters pipeline, which has better determination of metallicity for high metallicity stars.

Figures (5)

• Figure 1: The sky coverage of DR8 in J2000 Equatorial coordinates, in imaging (upper) and spectroscopy (lower). Right ascension $\alpha = 120^\circ$ is at the center of these plots. The Galactic plane is the solid curve that snakes through the figure. Note the contiguous imaging coverage of the Southern Galactic Cap (centered roughly at $\alpha = 0^\circ, \delta = +10^\circ$); in DR7, this region of sky was covered by a few disjoint stripes. The red regions in the lower panel show the coverage of the SEGUE-2 plates. The BOSS survey will obtain spectra over 10,000 deg$^2$, including the contiguous areas in the Northern and Southern Galactic caps.
• Figure 2: The grayscale and contours show the distribution of galaxies in SDSS in apparent magnitude and Petrosian half-light radius. The red dots show the distribution of artificial galaxies added to the imaging frames to explore the ability of the pipeline to photometer large galaxies. We have deliberately biased the sample of artificial galaxies to larger objects at a given magnitude.
• Figure 3: Differences between the true and measured $r$-band half-light radii and magnitudes as a function of $r_{50}\times (b/a)^{1/2}$ (whose square is proportional to the area of the galaxy; here $b/a$ is the axis ratio of the galaxy from the model fit), for a sample of simulated galaxies. The sample has Sérsic profiles, with a range of magnitudes and sizes (and therefore surface brightnesses), designed to sample the observed distribution of large bright galaxies. The measured magnitudes are the combined "cmodel" magnitudes using the exponential and de Vaucouleurs fits, and the measured sizes are the effective radii from the better of those two fits for each galaxy. Top panel shows the logarithmic difference between the measured half-light radius and the true one ($\Delta \log_{10} r_{50} = \log_{10} r_{50,meas} - \log_{10} r_{50,true}$). Bottom panel shows the magnitude difference ($\Delta m = m_{meas}-m_{true}$). Results are shown both for the version of photo used in DR7 (red) and DR8 (blue). The running median values as function of radius are shown as the solid lines. The new code reduces the bias at large area, but only incrementally.
• Figure 4: Top left, bottom left, top right: Number density of source galaxies as a function of distance from bright foreground galaxies. Each panel is a separate foreground magnitude bin as labeled on the plot. The black solid and red dashed lines show the results for DR7 and DR8, respectively. Bottom right: Same as other panels but for DR8 only, where separate line colors and styles indicate different foreground magnitude bins. In this case, unlike for the other panels, source galaxy photometric redshifts were used to exclude sources that are in front of, or are physically associated with, the foreground object.
• Figure 5: Ratio of the spectro1d and idlspec2d emission line flux measurements with those of the MPA-JHU pipeline, as a function of rest-frame equivalent width, for galaxies in DR8 with emission line measurements with greater than $3\sigma$ significance. In performing this comparison, we have put all measurements on a common scale by removing the Milky Way reddening and and spectrophotometric zeropoint corrections from the MPA-JHU line measurements. The remaining differences are due to the different methods of modeling the stellar continuum. The dotted lines show the deviation expected due to random error.