University of Arizona

About: University of Arizona is a education organization based out in Tucson, Arizona, United States. It is known for research contribution in the topics: Population & Galaxy. The organization has 63805 authors who have published 155998 publications receiving 6854915 citations. The organization is also known as: UA & U of A.

Galaxy Clustering in the Completed SDSS Redshift Survey: The Dependence on Color and Luminosity

Abstract: We measure the luminosity and color dependence of galaxy clustering in the largest-ever galaxy redshift survey, the main galaxy sample of the Sloan Digital Sky Survey (SDSS) Seventh Data Release (DR7). We focus on the projected correlation function wp(rp) of volume-limited samples, extracted from the parent sample of ∼ 700,000 galaxies over 8000 deg 2 , extending up to redshift of 0.25. We interpret our measurements using halo occupation distribution (HOD) modeling assuming aCDM cosmol- ogy (inflationary cold dark matter with a cosmological constant). The amplitude of wp(rp) grows slowly with luminosity for L L) × (�8/0.8) = 1.06 + 0.21(L/L∗) 1.12 , where L is the sample luminosity threshold. At fixed luminosity, redder galaxies exhibit a higher amplitude and steeper correlation function, a steady trend that runs through the "blue cloud" and "green valley" and continues across the "red sequence." The cross-correlation of red and blue galaxies is close to the geometric mean of their auto- correlations, dropping slightly below at rp 4L∗, but the lowest luminosity red galaxies (0.04−0.25L∗) show very strong clustering on small scales (rp < 2h −1 Mpc). Most of the observed trends can be naturally understood within theCDM+HOD framework. The growth of wp(rp) for higher luminosity galaxies reflects an overall shift in the mass scale of their host dark matter halos, in particular an increase in the minimum host halo mass Mmin. The mass at which a halo has, on average, one satellite galaxy brighter than L is M1 ≈ 17Mmin(L) over most of the luminosity range, with a smaller ratio above L∗. The growth and steepening of wp(rp) for redder galaxies reflects the increasing fraction of galaxies that are satellite systems in high mass halos instead of central systems in low mass halos, a trend that is especially marked at low luminosities. Our exten- sive measurements, provided in tabular form, will allow detailed tests of theoretical models of galaxy formation, a firm grounding of semi-empirical models of the galaxy population, and new constraints on cosmological parameters from combining real-space galaxy clustering with mass-sensitive statistics such as redshift-space distortions, cluster mass-to-light ratios, and galaxy-galaxy lensing. Subject headings: cosmology: observations — cosmology: theory — galaxies: distances and redshifts — galaxies: halos — galaxies: statistics — large-scale structure of universe

Matrix-Mediated Synthesis of Nanocrystalline γ-Fe2O3: A New Optically Transparent Magnetic Material

Abstract: A new magnetic material with appreciable optical transmission in the visible region at room temperature has been isolated as a gamma-Fe(2)O(3)/polymer nanocomposite. The synthesis is carried out in an ion-exchange resin at 60 degrees C. Magnetization and susceptibility data demonstrate loading-dependent saturation moments as high as 46 electromagnetic units per gram and superparamagnetism for lower loadings where particle sizes are less than 100 angstroms. Optical absorption studies show that the small-particle form of gamma-Fe(2)O(3) is considerably more transparent to visible light than the single-crystal form. The difference in absorption ranges from nearly an order of magnitude in the "red" spectral region to a factor of 3 at 5400 angstroms. The magnetization of the nanocomposite is greater by more than an order of magnitude than those of the strongest room-temperature transparent magnets, FeBO(3) and FeF(3).

An O ( ND ) difference algorithm and its variations

Abstract: The problems of finding a longest common subsequence of two sequencesA andB and a shortest edit script for transformingA intoB have long been known to be dual problems. In this paper, they are shown to be equivalent to finding a shortest/longest path in an edit graph. Using this perspective, a simpleO(ND) time and space algorithm is developed whereN is the sum of the lengths ofA andB andD is the size of the minimum edit script forA andB. The algorithm performs well when differences are small (sequences are similar) and is consequently fast in typical applications. The algorithm is shown to haveO(N+D 2) expected-time performance under a basic stochastic model. A refinement of the algorithm requires onlyO(N) space, and the use of suffix trees leads to anO(N logN+D 2) time variation.

Source regions and timescales for the delivery of water to the Earth

Abstract: — In the primordial solar system, the most plausible sources of the water accreted by the Earth were in the outer asteroid belt, in the giant planet regions, and in the Kuiper Belt. We investigate the implications on the origin of Earth's water of dynamical models of primordial evolution of solar system bodies and check them with respect to chemical constraints. We find that it is plausible that the Earth accreted water all along its formation, from the early phases when the solar nebula was still present to the late stages of gas-free sweepup of scattered planetesimals. Asteroids and the comets from the Jupiter-Saturn region were the first water deliverers, when the Earth was less than half its present mass. The bulk of the water presently on Earth was carried by a few planetary embryos, originally formed in the outer asteroid belt and accreted by the Earth at the final stage of its formation. Finally, a late veneer, accounting for at most 10% of the present water mass, occurred due to comets from the Uranus-Neptune region and from the Kuiper Belt. The net result of accretion from these several reservoirs is that the water on Earth had essentially the D/H ratio typical of the water condensed in the outer asteroid belt. This is in agreement with the observation that the D/H ratio in the oceans is very close to the mean value of the D/H ratio of the water inclusions in carbonaceous chondrites.

The Internal-collision-induced Magnetic Reconnection and Turbulence (ICMART) Model of Gamma-ray Bursts

Abstract: The recent Fermi observation of GRB 080916C shows that the bright photosphere emission associated with a putative fireball is missing, which suggests that the central engine likely launches a Poynting-flux-dominated (PFD) outflow. We propose a model of gamma-ray burst (GRB) prompt emission in the PFD regime, namely, the InternalCollision-induced MAgnetic Reconnection and Turbulence (ICMART) model. It is envisaged that the GRB central engine launches an intermittent, magnetically dominated wind, and that in the GRB emission region, the ejecta is still moderately magnetized (e.g., 1 σ 100). Similar to the internal shock (IS) model, the mini-shells interact internally at the radius RIS ∼ Γ 2 cΔt. Most of these early collisions, however, have little energy dissipation, but serve to distort the ordered magnetic field lines entrained in the ejecta. At a certain point, the distortion of magnetic field configuration reaches the critical condition to allow fast reconnection seeds to occur, which induce relativistic MHD turbulence in the interaction regions. The turbulence further distorts field lines easing additional magnetic reconnections, resulting in a runway release of the stored magnetic field energy (an ICMART event). Particles are accelerated either directly in the reconnection zone, or stochastically in the turbulent regions, which radiate synchrotron photons that power the observed gamma rays. Each ICMART event corresponds to a broad pulse in the GRB light curve, and a GRB is composed of multiple ICMART events. This model retains the merits of IS and other models, but may overcome several difficulties/issues faced by the IS model (e.g., low efficiency, fast cooling, electron number excess, Amati/Yonetoku relation inconsistency, and missing bright photosphere). Within this model, the observed GRB variability timescales could have two components, one slow component associated with the central engine time history, and another fast component associated with relativistic magnetic turbulence in the emission region. The model predicts a decrease of gamma-ray polarization degree and Ep in each ICMART event (broad pulse) during the prompt GRB phase, as well as a moderately magnetized external reverse shock. The model may be applied to the GRBs that have time-resolved, featureless Band-function spectra, such as GRB 080916C and most GRBs detected by Fermi LAT.