35,480 research outputs found
The Mississippian fin de siècle in the middle Cumberland region of Tennessee
Bayesian chronological modeling is used to investigate the chronology for a large-scale human depopulation event during the Mississippian period (AD 1000–1600) known as the Vacant Quarter phenomenon. The Middle Cumberland region (MCR) of Tennessee is within the Vacant Quarter area, and six villages from the final phase of Mississippian activity in the MCR have been subjected to radiocarbon dating. Complete radiocarbon datasets from these sites are presented within an interpretative Bayesian statistical framework. The results provide a unique history of each settlement and demonstrate that Mississippian occupations at each site likely terminated in the mid- to late fifteenth and possibly early sixteenth centuries AD, which is 50 to 100 years later than the most recent estimate for the timing of the Vacant Quarter. Mississippian abandonment in the MCR was relatively quick, likely occurring over less than a century. The exact reasons for abandonment are not entirely clear but appear to be linked to climate change. A radiocarbon simulation experiment indicates that future robust radiocarbon dating with well-selected samples could greatly improve the chronological precision for this late Mississippian activity. More broadly, this example demonstrates that model building with radiocarbon simulations can be used to address regional-scale chronological issues within the American Southeast and beyond
Plumbophyllite, a new species from the Blue Bell claims near Baker, San Bernardino County, California
The new mineral plumbophyllite, Pb2Si4O10·H2O, orthorhombic with space group Pbcn and cell parameters a = 13.2083(4), b = 9.7832(3), c = 8.6545(2) Å, V = 1118.33(5) Å^3, and Z = 4. It occurs as colorless to pale blue prismatic crystals to 3 mm, with wedge-shaped terminations at the Blue Bell claims, about 11 km west of Baker, San Bernardino County, California. It is found in narrow veins in a highly siliceous hornfels in association with cerussite, chrysocolla, fluorite, goethite, gypsum, mimetite, opal, plumbotsumite, quartz, sepiolite, and wulfenite. The streak is white, the luster is vitreous, the Mohs hardness is about 5, and there is one perfect cleavage, {100}. The measured density is 3.96(5) g/cm^3 and the calculated density is 3.940 g/cm^3. Optical properties (589 nm): biaxial (+), {alpha} = 1.674(2), β = 1.684(2), {gamma} = 1.708(2), 2V = 66(2)°, dispersion r > v (strong); X = b, Y = c, Z = a. Electron microprobe analysis provided PbO 60.25, CuO 0.23, SiO_2 36.22 wt%, and CHN analysis provided H_2O 3.29 wt% for a total of 99.99 wt%. Powder IR spectroscopy confirmed the presence of H_2O and single-crystal IR spectroscopy indicated the H_2O to be oriented perpendicular to the b axis. Raman spectra were also obtained. The strongest powder X-ray diffraction lines are [d (hkl) I]: 7.88(110)97, 6.63(200)35, 4.90(020)38, 3.623(202)100, 3.166(130)45, 2.938(312/411/222)57, 2.555(132/213)51, and 2.243(521/332)50. The atomic structure (R1 = 2.04%) consists of undulating sheets of silicate tetrahedra between which are located Pb atoms and channels containing H_2O (and Pb^(2+) lone-pair electrons). The silicate sheets can be described as consisting of zigzag pyroxene-like (SiO_3)_n chains joined laterally into sheets with the unshared tetrahedral apices in successive chains pointed alternately up and down, a configuration also found in pentagonite
Design and fabrication of a stringer stiffened discrete-tube actively cooled panel for a hypersonic aircraft
A 0.61 x 1.22 m (2 x 4 ft) test panel was fabricated and delivered to the Langley Research Center for assessment of the thermal and structural features of the optimized panel design. The panel concept incorporated an aluminum alloy surface panel actively cooled by a network of discrete, parallel, redundant, counterflow passage interconnected with appropriate manifolding, and assembled by adhesive bonding. The cooled skin was stiffened with a mechanically fastened conventional substructure of stringers and frames. A 40 water/60 glycol solution was the coolant. Low pressure leak testing, radiography, holography and infrared scanning were applied at various stages of fabrication to assess integrity and uniformity. By nondestructively inspecting selected specimens which were subsequently tested to destruction, it was possible to refine inspection standards as applied to this cooled panel design
Dynamics of merging: Post-merger mixing and relaxation of an Illustris galaxy
During the merger of two galaxies, the resulting system undergoes violent
relaxation and seeks stable equilibrium. However, the details of this evolution
are not fully understood. Using Illustris simulation, we probe two physically
related processes, mixing and relaxation. Though the two are driven by the same
dynamics---global time-varying potential for the energy, and torques caused by
asymmetries for angular momentum---we measure them differently. We define
mixing as the redistribution of energy and angular momentum between particles
of the two merging galaxies. We assess the degree of mixing as the difference
between the shapes of their N(E)s, and their N(L^2)s. We find that the
difference is decreasing with time, indicating mixing. To measure relaxation,
we compare N(E) of the newly merged system to N(E) of a theoretical prediction
for relaxed collisionless systems, DARKexp, and witness the system becoming
more relaxed, in the sense that N(E) approaches DARKexp N(E). Because the
dynamics driving mixing and relaxation are the same, the timescale is similar
for both. We measure two sequential timescales: a rapid, 1 Gyr phase after the
initial merger, during which the difference in N(E) of the two merging halos
decreases by ~80%, followed by a slow phase, when the difference decreases by
~50% over ~8.5 Gyrs. This is a direct measurement of the relaxation timescale.
Our work also draws attention to the fact that when a galaxy has reached Jeans
equilibrium it may not yet have reached a fully relaxed state given by DARKexp,
in that it retains information about its past history. This manifests itself
most strongly in stars being centrally concentrated. We argue that it is
particularly difficult for stars, and other tightly bound particles, to mix
because they have less time to be influenced by the fluctuating potential, even
across multiple merger events.Comment: accepted for publication in JCA
Ubiquity of density slope oscillations in the central regions of galaxy and cluster-sized systems
One usually thinks of a radial density profile as having a monotonically
changing logarithmic slope, such as in NFW or Einasto profiles. However, in two
different classes of commonly used systems, this is often not the case. These
classes exhibit non-monotonic changes in their density profile slopes which we
call oscillations for short. We analyze these two unrelated classes separately.
Class 1 consists of systems that have density oscillations and that are defined
through their distribution function , or differential energy distribution
, such as isothermal spheres, King profiles, or DARKexp, a theoretically
derived model for relaxed collisionless systems. Systems defined through
or generally have density slope oscillations. Class 1 system
oscillations can be found at small, intermediate, or large radii but we focus
on a limited set of Class 1 systems that have oscillations in the central
regions, usually at , where is the largest
radius where . We show that the shape of their
can roughly predict the amplitude of oscillations. Class 2 systems which are a
product of dynamical evolution, consist of observed and simulated galaxies and
clusters, and pure dark matter halos. Oscillations in the density profile slope
seem pervasive in the central regions of Class 2 systems. We argue that in
these systems, slope oscillations are an indication that a system is not fully
relaxed. We show that these oscillations can be reproduced by small
modifications to of DARKexp. These affect a small fraction of systems'
mass and are confined to . The size of these
modifications serves as a potential diagnostic for quantifying how far a system
is from being relaxed.Comment: accepted by the Journal of Cosmology and Astroparticle Physics (JCAP
Evidence that particle acceleration in hotspots of FR II galaxies is not constrained by synchrotron cooling
We study the hotspots of powerful radiogalaxies, where electrons accelerated
at the jet termination shock emit synchrotron radiation. The turnover of the
synchrotron spectrum is typically observed between infrared and optical
frequencies, indicating that the maximum energy of non-thermal electrons
accelerated at the shock is ~TeV for a canonical magnetic field of ~100 micro
Gauss. We show that this maximum energy cannot be constrained by synchrotron
losses as usually assumed, unless the jet density is unreasonably large and
most of the jet upstream energy goes to non-thermal particles. We test this
result by considering a sample of hotspots observed at radio, infrared and
optical wavelengths.Comment: 7 pages, 2 figures. To be appear in the proceedings of the conference
"Cosmic ray origin - beyond the standard models" (San Vito di Cadore, Italy,
September 2016
Particle acceleration and magnetic field amplification in the jets of 4C74.26
We model the multi-wavelength emission in the southern hotspot of the radio
quasar 4C74.26. The synchrotron radio emission is resolved near the shock with
the MERLIN radio-interferometer, and the rapid decay of this emission behind
the shock is interpreted as the decay of the amplified downstream magnetic
field as expected for small scale turbulence. Electrons are accelerated to only
0.3 TeV, consistent with a diffusion coefficient many orders of magnitude
greater than in the Bohm regime. If the same diffusion coefficient applies to
the protons, their maximum energy is only ~100 TeV.Comment: Accepted for publication in ApJ. 6 pages - 2 figures. Minor
correction
Interloper bias in future large-scale structure surveys
Next-generation spectroscopic surveys will map the large-scale structure of
the observable universe, using emission line galaxies as tracers. While each
survey will map the sky with a specific emission line, interloping emission
lines can masquerade as the survey's intended emission line at different
redshifts. Interloping lines from galaxies that are not removed can contaminate
the power spectrum measurement, mixing correlations from various redshifts and
diluting the true signal. We assess the potential for power spectrum
contamination, finding that an interloper fraction worse than 0.2% could bias
power spectrum measurements for future surveys by more than 10% of statistical
errors, while also biasing power spectrum inferences. We also construct a
formalism for predicting cosmological parameter bias, demonstrating that a
0.15%-0.3% interloper fraction could bias the growth rate by more than 10% of
the error, which can affect constraints on gravity upcoming surveys. We use the
COSMOS Mock Catalog (CMC), with the emission lines re-scaled to better
reproduce recent data, to predict potential interloper fractions for the Prime
Focus Spectrograph (PFS) and the Wide-Field InfraRed Survey Telescope (WFIRST).
We find that secondary line identification, or confirming galaxy redshifts by
finding correlated emission lines, can remove interlopers for PFS. For WFIRST,
we use the CMC to predict that the 0.2% target can be reached for the WFIRST
H survey, but sensitive optical and near-infrared photometry will be
required. For the WFIRST [OIII] survey, the predicted interloper fractions
reach several percent and their effects will have to be estimated and removed
statistically (e.g. with deep training samples). (Abridged)Comment: Matches version accepted by PAS
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