334 research outputs found
Hyperstaticity and loops in frictional granular packings
The hyperstatic nature of granular packings of perfectly rigid disks is
analyzed algebraically and through numerical simulation. The elementary loops
of grains emerge as a fundamental element in addressing hyperstaticity. Loops
consisting of an odd number of grains behave differently than those with an
even number. For odd loops, the latent stresses are exterior and are
characterized by the sum of frictional forces around each loop. For even loops,
the latent stresses are interior and are characterized by the alternating sum
of frictional forces around each loop. The statistics of these two types of
loop sums are found to be Gibbsian with a "temperature" that is linear with the
friction coefficient mu when mu<1.Comment: 4 pages; Powders and Grains 2009, Golden, Colorado, US
The SBF Survey of Galaxy Distances. I. Sample Selection, Photometric Calibration, and the Hubble Constant
We describe a program of surface brightness fluctuation (SBF) measurements
for determining galaxy distances. This paper presents the photometric
calibration of our sample and of SBF in general. Basing our zero point on
observations of Cepheid variable stars, we find that the absolute SBF magnitude
in the Kron-Cousins I band correlates well with the mean (V-I)o color of a
galaxy according to
M_Ibar = (-1.74 +/- 0.07) + (4.5 +/- 0.25) [ (V-I)o - 1.15 ]
for 1.0 < (V-I) < 1.3. This agrees well with theoretical estimates from
stellar population models. Comparisons between SBF distances and a variety of
other estimators, including Cepheid variable stars, the Planetary Nebula
Luminosity Function (PNLF), Tully-Fisher (TF), Dn-sigma, SNII, and SNIa,
demonstrate that the calibration of SBF is universally valid and that SBF error
estimates are accurate. The zero point given by Cepheids, PNLF, TF (both
calibrated using Cepheids), and SNII is in units of Mpc; the zero point given
by TF (referenced to a distant frame), Dn-sigma and SNIa is in terms of a
Hubble expansion velocity expressed in km/s. Tying together these two zero
points yields a Hubble constant of H_0 = 81 +/- 6 km/s/Mpc. As part of this
analysis, we present SBF distances to 12 nearby groups of galaxies where
Cepheids, SNII, and SNIa have been observed.Comment: 29 pages plus 8 figures; LaTeX (AASTeX) uses aaspp4.sty (included);
To appear in The Astrophysical Journal, 1997 February 1 issue; Compressed
PostScript available from ftp://mars.tuc.noao.edu/sbf
The SBF Survey of Galaxy Distances. IV. SBF Magnitudes, Colors, and Distances
We report data for band Surface Brightness Fluctuation (SBF) magnitudes,
V-I colors, and distance moduli for 300 galaxies. The Survey contains E, S0 and
early-type spiral galaxies in the proportions of 49:42:9, and is essentially
complete for E galaxies to Hubble velocities of 2000 km/s, with a substantial
sampling of E galaxies out to 4000 km/s. The median error in distance modulus
is 0.22 mag.
We also present two new results from the Survey. (1) We compare the mean
peculiar flow velocity (bulk flow) implied by our distances with predictions of
typical cold dark matter transfer functions as a function of scale, and find
very good agreement with cold, dark matter cosmologies if the transfer function
scale parameter , and the power spectrum normalization are
related by . Derived directly from
velocities, this result is independent of the distribution of galaxies or
models for biasing. The modest bulk flow contradicts reports of large-scale,
large-amplitude flows in the Mpc diameter volume surrounding our
Survey volume. (2) We present a distance-independent measure of absolute galaxy
luminosity, \Nbar, and show how it correlates with galaxy properties such as
color and velocity dispersion, demonstrating its utility for measuring galaxy
distances through large and unknown extinction.Comment: Accepted for publication in ApJ (10 January 2001); 23 page
Technical Research Priorities for Big Data
To drive innovation and competitiveness, organisations need to foster the development and broad adoption of data technologies, value-adding use cases and sustainable business models. Enabling an effective data ecosystem requires overcoming several technical challenges associated with the cost and complexity of management, processing, analysis and utilisation of data. This chapter details a community-driven initiative to identify and characterise the key technical research priorities for research and development in data technologies. The chapter examines the systemic and structured methodology used to gather inputs from over 200 stakeholder organisations. The result of the process identified five key technical research priorities in the areas of data management, data processing, data analytics, data visualisation and user interactions, and data protection, together with 28 sub-level challenges. The process also highlighted the important role of data standardisation, data engineering and DevOps for Big Data
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