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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Galaxy populations in massive galaxy clusters to $z$ = 1.1: Color distribution, concentration, halo occupation number and red sequence fraction

We study the galaxy populations in 74 Sunyaev–Zeldovich effect selected clusters from the South Pole Telescope survey, which have been imaged in the science verification phase of the Dark Energy Survey. The sample extends up to z ~ 1.1 with 4 × 10 14 M⊙ ≤ M200 ≤ 3 × 10 15 M⊙. Using the band containing the 4000 Å break and its redward neighbour, we study the colour–magnitude distributions of cluster galaxies to ~m* + 2, finding that: (1)The intrinsic rest frame g – r colour width of the red sequence (RS) population is ~0.03 out to z ~ 0.85 with a preference for an increase to ~0.07 at z = 1, and (2) the prominence of the RS declines beyond z ~ 0.6. The spatial distribution of cluster galaxies is well described by the NFW profile out to 4R200 with a concentration of c g = 3.59$+0.20\atop{–0.18}$, 5.37$+0.27\atop{-0.24}$ and 1.38$+0.21\atop{-0.19}$ for the full, the RS and the blue non-RS populations, respectively, but with ~40 per cent to 55 per cent cluster to cluster variation and no statistically significant redshift or mass trends. The number of galaxies within the virial region N200 exhibits a mass trend indicating that the number of galaxies per unit total mass is lower in the most massive clusters, and shows no significant redshift trend. The RS fraction within R200 is (68 ± 3) per cent at z = 0.46, varies from ~55 per cent at z = 1 to ~80 per cent at z = 0.1 and exhibits intrinsic variation among clusters of ~14 per cent. Finally, we discuss a model that suggests that the observed redshift trend in RS fraction favours a transformation time-scale for infalling field galaxies to become RS galaxies of 2–3 Gyr.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Energy occupation of waves and structures in 3D compressive MHD turbulence

Abstract Structures and propagating waves are often observed in solar wind turbulence. Their origins and features remain to be uncovered. In this work, we use 3D driven, compressible MHD turbulence simulations to investigate the global signatures of the driven fluctuations in whole spatial and temporal domain. With four-dimensional spatial-temporal (x, y, z, t) Fourier transformations implemented, we have identified two distinct main populations: waves, which satisfy the $\omega -\boldsymbol {k}$ dispersion relations and are propagating; and structures, which satisfy the polarization relations but non-propagating (ω = 0). Whereas the overall turbulent energy spectrum is still consistent with k−5/3, the contributions from waves and structures show very different behaviour in $\boldsymbol {k}$ space, with structures dominating at small k but waves becomes comparable to structures at large k. Overall, the fluctuations in the directions perpendicular to the large-scale mean field $\boldsymbol {B_0}$ are a manifestation of structures, while along the parallel direction, the fluctuations are dominated by waves. Also, a significant portion of the incompressible structures are the Alfvénic nature, and with imbalanced increased, the waves predominantly propagate in one direction and nearly perpendicular to $\boldsymbol {B_0}$. Differentiating the relative contributions from waves and structures could have important implications for understanding the non-linear cascade processes in the inertial range as well as particle-fluctuation interactions at small scales.

Yang, L. P.↗
DOE OSTI2023

Occupational disorder as the origin of flattening of the acoustic phonon branches in the clathrate Ba 8 ⁢Ga 16 ⁢Ge 30

In the search for high-performance thermoelectrics, materials such as clathrates have drawn attention due to having both glasslike low phonon thermal conductivity and crystal-like high electrical conductivity. Ba 8 ⁢Ga 16 ⁢Ge 30 (BGG) has a loosely bound guest Ba atom trapped inside rigid Ga-Ge cage structures. Avoided crossings between acoustic phonons and the flat guest atom branches have been proposed to be the source of the low lattice thermal conductivity of BGG. Ga-Ge site disorder with Ga and Ge exchanging places in different unit cells has also been reported. We used time-of-flight neutron scattering to measure the complete phonon spectrum in a large single crystal of BGG and compared these results with predictions of density functional theory to elucidate the effect of the disorder on heat-carrying phonons. Experimental results agreed much better with the calculation assuming the disorder than with the calculation assuming the ordered configuration. Although the atomic masses of Ga and Ge are nearly identical, we found that disorder strongly reduces phonon group velocities, which significantly reduces thermal conductivity. Finally, our work points to a path towards optimizing thermoelectrics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗