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

Abstract

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 l 50 24.5 mag arcsec À2 . 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 À1 ). About 6% of galaxies that satisfy the selection criteria are not observed because they have a companion closer than the 55 00 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.

Improved constraints on the expansion rate of the Universe up to z ∼ 1.1 from the spectroscopic evolution of cosmic chronometers

“…This sample has been taken from the analysis of M11. ETGs have been extracted from the SDSS-DR6 Main Galaxy Sample (MGS, [30]), matching SDSS photometry (u, g, r, i, and z) to 2MASS photometry (J, H, and K), in order to obtain a wider photometric coverage and extract robust mass estimates from the fitting of their photometric SEDs. For each galaxy, the 4000 A break amplitudes have been taken from the MPA-JHU DR7 release of spectral measurements 1 .…”

Occurrence of LINER galaxies within the galaxy group environment

“…The SDSS-DR7 main galaxy sample is essentially a magnitude limited catalog (Petrosian 1976) with r lim < 17.77, consisting of ∼ 700000 galaxies with measured spectra and photometry in five optical bands (u,g,r,i,z), and with a median redshift of 0.1 (Strauss et al 2002). The procedures to estimate the physical properties of galaxies used in this analysis are described by Brinchmann et al (2004), Tremonti et al (2004) and Blanton et al (2005).…”

SDSS data management and photometric quality assessment

“…Individual imaging runs that interleave are prepared for spectroscopy with the following steps: resolve selects a primary detection for objects that fall in an overlap area; the target selection pipeline (target) selects objects for spectroscopic observation (for galaxies see Strauss et al 2002, for quasars Richards et al 2002, and for luminous red galaxies Eisenstein et al 2002); and the plate pipeline (plate) specifies the locations of the plates on the sky and the location of holes to be drilled in each plate (Blanton et al 2003). Spectroscopic data are first extracted and calibrated with the two-dimensional pipeline (spectro2d) and then classified and measured with the one-dimensional pipeline (spectro1d).…”