Galaxy Digital

S. Alam, M. Ata, S. Bailey, F. Beutler, D. Bizyaev, D. Bizyaev, J. Blazek, A. Bolton, J. Brownstein, A. Burden, C. Chuang, J. Comparat, A. Cuesta, K. Dawson, D. Eisenstein, S. Escoffier, H. Gil-Marín, H. Gil-Marín, J. Grieb, N. Hand, S. Ho, K. Kinemuchi, D. Kirkby, F. Kitaura, F. Kitaura, E. Malanushenko, V. Malanushenko, C. Maraston, C. McBride, R. Nichol, M. Olmstead, D. Oravetz, N. Padmanabhan, N. Palanque-Delabrouille, K. Pan, M. Pellejero-Ibáñez, M. Pellejero-Ibáñez, W. Percival, P. Petitjean, F. Prada, F. Prada, A. Price-Whelan, B. Reid, B. Reid, Sergio A. Rodríguez-Torres, Sergio A. Rodríguez-Torres, N. Roe, A. Ross, A. Ross, N. Ross, G. Rossi, J. Rubiño-Martín, J. Rubiño-Martín, S. Saito, Salvador Salazar-Albornoz, L. Samushia, Ariel G. S'anchez, Siddharth Satpathy, D. Schlegel, D. Schneider, C. Scóccola, C. Scóccola, C. Scóccola, H. Seo, E. Sheldon, A. Simmons, A. Slosar, M. Strauss, M. Swanson, D. Thomas, J. Tinker, R. Tojeiro, M. V. Magaña, M. V. Magaña, J. Vázquez, L. Verde, D. Wake, D. Wake, Yuting Wang, Yuting Wang, D. Weinberg, M. White, M. White, W. M. Wood-Vasey, C. Yéche, I. Zehavi, Zhongxu Zhai, Gong-Bo Zhao, Gong-Bo Zhao

We present cosmological results from the final galaxy clustering data set of the Baryon Oscillation Spectroscopic Survey, part of the Sloan Digital Sky Survey III. Our combined galaxy sample comprises 1.2 million massive galaxies over an effective area of 9329 deg^2 and volume of 18.7 Gpc^3, divided into three partially overlapping redshift slices centred at effective redshifts 0.38, 0.51 and 0.61. We measure the angular diameter distance and Hubble parameter H from the baryon acoustic oscillation (BAO) method, in combination with a cosmic microwave background prior on the sound horizon scale, after applying reconstruction to reduce non-linear effects on the BAO feature. Using the anisotropic clustering of the pre-reconstruction density field, we measure the product D_MH from the Alcock–Paczynski (AP) effect and the growth of structure, quantified by fσ_8(z), from redshift-space distortions (RSD). We combine individual measurements presented in seven companion papers into a set of consensus values and likelihoods, obtaining constraints that are tighter and more robust than those from any one method; in particular, the AP measurement from sub-BAO scales sharpens constraints from post-reconstruction BAOs by breaking degeneracy between D_M and H. Combined with Planck 2016 cosmic microwave background measurements, our distance scale measurements simultaneously imply curvature Ω_K = 0.0003 ± 0.0026 and a dark energy equation-of-state parameter w = −1.01 ± 0.06, in strong affirmation of the spatially flat cold dark matter (CDM) model with a cosmological constant (ΛCDM). Our RSD measurements of fσ_8, at 6 per cent precision, are similarly consistent with this model. When combined with supernova Ia data, we find H_0 = 67.3 ± 1.0 km s^−1 Mpc^−1 even for our most general dark energy model, in tension with some direct measurements. Adding extra relativistic species as a degree of freedom loosens the constraint only slightly, to H_0 = 67.8 ± 1.2 km s^−1 Mpc^−1. Assuming flat ΛCDM, we find Ω_m = 0.310 ± 0.005 and H_0 = 67.6 ± 0.5 km s^−1 Mpc^−1, and we find a 95 per cent upper limit of 0.16 eV c^−2 on the neutrino mass sum.

W. Percival, B. Reid, D. Eisenstein, N. Bahcall, T. Budavári, J. Frieman, M. Fukugita, J. Gunn, Ž. Ivezić, G. Knapp, R. Kron, J. Loveday, R. Lupton, T. Mckay, A. Meiksin, R. Nichol, A. Pope, D. Schlegel, D. Schneider, D. Spergel, C. Stoughton, M. Strauss, A. Szalay, Max Tegmark, M. Vogeley, D. Weinberg, D. York, I. Zehavi

The spectroscopic Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) galaxy sample represents the final set of galaxies observed using the original SDSS target selection criteria. We analyse the clustering of galaxies within this sample, including both the luminous red galaxy and main samples, and also include the 2-degree Field Galaxy Redshift Survey data. In total, this sample comprises 893 319 galaxies over 9100 deg(2). Baryon acoustic oscillations (BAO) are observed in power spectra measured for different slices in redshift; this allows us to constrain the distance-redshift relation at multiple epochs. We achieve a distance measure at redshift z = 0.275, of r(s)(z(d))/D-V(0.275) = 0.1390 +/- 0.0037 (2.7 per cent accuracy), where r(s)(z(d)) is the comoving sound horizon at the baryon-drag epoch, D-V(z) equivalent to [(1 + z)(2)D(A)(2)cz/H(z)](1/3), D-A(z) is the angular diameter distance and H(z) is the Hubble parameter. We find an almost independent constraint on the ratio of distances D-V(0.35)/D-V(0.2) = 1.736 +/- 0.065, which is consistent at the 1.1 sigma level with the best-fitting Lambda cold dark matter model obtained when combining our z = 0.275 distance constraint with the Wilkinson Microwave Anisotropy Probe 5-year (WMAP5) data. The offset is similar to that found in previous analyses of the SDSS DR5 sample, but the discrepancy is now of lower significance, a change caused by a revised error analysis and a change in the methodology adopted, as well as the addition of more data. Using WMAP5 constraints on Omega(b)h(2) and Omega(c) h(2), and combining our BAO distance measurements with those from the Union supernova sample, places a tight constraint on Omega(m) = 0.286 +/- 0.018 and H-0 = 68.2 +/- 2.2 km s(-1) Mpc(-1) that is robust to allowing Omega(k) not equal 0 and omega not equal -1. This result is independent of the behaviour of dark energy at redshifts greater than those probed by the BAO and supernova measurements. Combining these data sets with the full WMAP5 likelihood constraints provides tight constraints on both Omega(k) = -0.006 +/- 0.008 and omega = -0.97 +/- 0.10 for a constant dark energy equation of state.

M. Strauss, D. Weinberg, R. Lupton, V. N. E. al.

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.

C. Lintott, K. Schawinski, A. Slosar, K. Land, S. Bamford, D. Thomas, M. Raddick, R. Nichol, A. Szalay, D. Andreescu, P. Murray, J. V. D. Berg

In order to understand the formation and subsequent evolution of galaxies one must first distinguish between the two main morphological classes of massive systems: spirals and early-type systems. This paper introduces a project, Galaxy Zoo, which provides visual morphological classifications for nearly one million galaxies, extracted from the Sloan Digital Sky Survey (SDSS). This achievement was made possible by inviting the general public to visually inspect and classify these galaxies via the internet. The project has obtained more than 4 × 107 individual classifications made by ∼105 participants. We discuss the motivation and strategy for this project, and detail how the classifications were performed and processed. We find that Galaxy Zoo results are consistent with those for subsets of SDSS galaxies classified by professional astronomers, thus demonstrating that our data provide a robust morphological catalogue. Obtaining morphologies by direct visual inspection avoids introducing biases associated with proxies for morphology such as colour, concentration or structural parameters. In addition, this catalogue can be used to directly compare SDSS morphologies with older data sets. The colour–magnitude diagrams for each morphological class are shown, and we illustrate how these distributions differ from those inferred using colour alone as a proxy for morphology.

D. Patton, Farid D. Qamar, Farid D. Qamar, S. Ellison, A. Bluck, A. Bluck, L. Simard, J. Mendel, J. Moreno, J. Moreno, J. Moreno, P. Torrey, P. Torrey

We describe a statistical approach for measuring the influence that a galaxy's closest companion has on the galaxy's properties out to arbitrarily wide separations. We begin by identifying the closest companion for every galaxy in a large spectroscopic sample of Sloan Digital Sky Survey galaxies. We then characterize the local environment of each galaxy by using the number of galaxies within 2 Mpc and by determining the isolation of the galaxy pair from other neighbouring galaxies. We introduce a sophisticated algorithm for creating a statistical control sample for each galaxy, matching on stellar mass, redshift, local density and isolation. Unlike traditional studies of close galaxy pairs, this approach is effective in a wide range of environments, regardless of how faraway the closest companion is (although a very distant closest companion is unlikely to have a measurable influence on the galaxy in question). We apply this methodology to measurements of galaxy asymmetry, and find that the presence of nearby companions drives a clear enhancement in galaxy asymmetries. The asymmetry excess peaks at the smallest projected separations (<10 kpc), where the mean asymmetry is enhanced by a factor of 2.0 ± 0.2. Enhancements in mean asymmetry decline as pair separation increases, but remain statistically significant (1σ–2σ) out to projected separations of at least 50 kpc.