4,382 research outputs found
Cosmological Evolution of Supergiant Star-Forming Clouds
In an exploration of the birthplaces of globular clusters, we present a
careful examination of the formation of self-gravitating gas clouds within
assembling dark matter haloes in a hierarchical cosmological model. Our
high-resolution smoothed particle hydrodynamical simulations are designed to
determine whether or not hypothesized supergiant molecular clouds (SGMCs) form
and, if they do, to determine their physical properties and mass spectra. It
was suggested in earlier work that clouds with a median mass of several 10^8
M_sun are expected to assemble during the formation of a galaxy, and that
globular clusters form within these SGMCs. Our simulations show that clouds
with the predicted properties are indeed produced as smaller clouds collide and
agglomerate within the merging dark matter haloes of our cosmological model. We
find that the mass spectrum of these clouds obeys the same power-law form
observed for globular clusters, molecular clouds, and their internal clumps in
galaxies, and predicted for the supergiant clouds in which globular clusters
may form. We follow the evolution and physical properties of gas clouds within
small dark matter haloes up to z = 1, after which prolific star formation is
expected to occur. Finally, we discuss how our results may lead to more
physically motivated "rules" for star formation in cosmological simulations of
galaxy formation.Comment: Accepted to The Astrophysical Journal; 17 pages, 8 figure
SL(2,C) Chern-Simons theory and the asymptotic behavior of the colored Jones polynomial
We clarify and refine the relation between the asymptotic behavior of the
colored Jones polynomial and Chern-Simons gauge theory with complex gauge group
SL(2,C). The precise comparison requires a careful understanding of some
delicate issues, such as normalization of the colored Jones polynomial and the
choice of polarization in Chern-Simons theory. Addressing these issues allows
us to go beyond the volume conjecture and to verify some predictions for the
behavior of the subleading terms in the asymptotic expansion of the colored
Jones polynomial.Comment: 15 pages, 7 figure
Star Formation and Feedback in Dwarf Galaxies
We examine the star formation history and stellar feedback effects of dwarf
galaxies under the influence of extragalactic ultraviolet radiation. We
consider the dynamical evolution of gas in dwarf galaxies using a
one-dimensional, spherically symmetric, Lagrangian numerical scheme to compute
the effects of radiative transfer and photoionization. We include a
physically-motivated star formation recipe and consider the effects of
feedback. Our results indicate that star formation in the severe environment of
dwarf galaxies is a difficult and inefficient process. For intermediate mass
systems, such as the dSphs around the Galaxy, star formation can proceed with
in early cosmic epochs despite the intense background UV flux. Triggering
processes such as merger events, collisions, and tidal disturbance can lead to
density enhancements, reducing the recombination timescale, allowing gas to
cool and star formation to proceed. However, the star formation and gas
retention efficiency may vary widely in galaxies with similar dark matter
potentials, because they depend on many factors, such as the baryonic fraction,
external perturbation, IMF, and background UV intensity. We suggest that the
presence of very old stars in these dwarf galaxies indicates that their initial
baryonic to dark matter content was comparable to the cosmic value. This
constraint suggests that the initial density fluctuation of baryonic matter may
be correlated with that of the dark matter. For the more massive dwarf
elliptical galaxies, the star formation efficiency and gas retention rate is
much higher. Their mass to light ratio is regulated by star formation feedback,
and is expected to be nearly independent of their absolute luminosity. The
results of our theoretical models reproduce the observed correlation.Comment: 35 pages, 13 figure
High cooperativity coupling of electron-spin ensembles to superconducting cavities
Electron spins in solids are promising candidates for quantum memories for
superconducting qubits because they can have long coherence times, large
collective couplings, and many quantum bits can be encoded into the spin-waves
of a single ensemble. We demonstrate the coupling of electron spin ensembles to
a superconducting transmission-line resonator at coupling strengths greatly
exceeding the cavity decay rate and comparable to spin linewidth. We also use
the enhanced coupling afforded by the small cross-section of the transmission
line to perform broadband spectroscopy of ruby at millikelvin temperatures at
low powers. In addition, we observe hyperfine structure in diamond P1 centers
and time domain saturation-relaxation of the spins.Comment: 4pgs, 4 figure
Hearing and dementia: from ears to brain
The association between hearing impairment and dementia has emerged as a major public health challenge, with significant opportunities for earlier diagnosis, treatment and prevention. However, the nature of this association has not been defined. We hear with
our brains, particularly within the complex soundscapes of everyday life: neurodegenerative pathologies target the auditory brain,
and are therefore predicted to damage hearing function early and profoundly. Here we present evidence for this proposition, based
on structural and functional features of auditory brain organization that confer vulnerability to neurodegeneration, the extensive,
reciprocal interplay between ‘peripheral’ and ‘central’ hearing dysfunction, and recently characterized auditory signatures of canonical neurodegenerative dementias (Alzheimer’s disease, Lewy body disease and frontotemporal dementia). Moving beyond any simple dichotomy of ear and brain, we argue for a reappraisal of the role of auditory cognitive dysfunction and the critical coupling of
brain to peripheral organs of hearing in the dementias. We call for a clinical assessment of real-world hearing in these diseases that
moves beyond pure tone perception to the development of novel auditory ‘cognitive stress tests’ and proximity markers for the
early diagnosis of dementia and management strategies that harness retained auditory plasticit
Supernova Enrichment of Dwarf Spheroidal Galaxies
(Abridged) Many dwarf galaxies exhibit sub-Solar metallicities, with some
star-to-star variation, despite often containing multiple generations of stars.
The total metal content in these systems is much less than expected from the
heavy element production of massive stars in each episode of star formation.
Such a deficiency implies that a substantial fraction of the enriched material
has been lost from these small galaxies. Mass ejection from dwarf galaxies may
have important consequences for the evolution of the intergalactic medium and
for the evolution of massive galaxies, which themselves may have formed via the
merger of smaller systems. We report here the results of three-dimensional
simulations of the evolution of supernova-enriched gas within dwarf spheroidal
galaxies (dSph's), with the aim of determining the retention efficiency of
supernova ejecta. We consider two galaxy models, selected to represent opposite
ends of the dSph sequence. For each model galaxy we investigate a number of
scenarios, ranging from a single supernova in smooth gas distributions to more
complex multiple supernovae in highly disturbed gas distributions. The results
of these investigations suggest that, for low star-formation efficiencies, it
is difficult to completely expel the enriched material from the galaxy. Most of
the enriched gas is, however, lost from the core of the galaxy following
multiple supernovae, especially if the interstellar medium is already highly
disturbed by processes such as photo-ionization and stellar winds. If
subsequent star formation occurs predominantly within the core where most of
the residual gas is concentrated, then these results could explain the poor
self-enrichment efficiency observed in dwarf galaxies.Comment: 29 pages, 10 figures, to appear in Astrophysical Journa
Continuity properties of measurable group cohomology
A version of group cohomology for locally compact groups and Polish modules
has previously been developed using a bar resolution restricted to measurable
cochains. That theory was shown to enjoy analogs of most of the standard
algebraic properties of group cohomology, but various analytic features of
those cohomology groups were only partially understood.
This paper re-examines some of those issues. At its heart is a simple
dimension-shifting argument which enables one to `regularize' measurable
cocycles, leading to some simplifications in the description of the cohomology
groups. A range of consequences are then derived from this argument.
First, we prove that for target modules that are Fr\'echet spaces, the
cohomology groups agree with those defined using continuous cocycles, and hence
they vanish in positive degrees when the acting group is compact. Using this,
we then show that for Fr\'echet, discrete or toral modules the cohomology
groups are continuous under forming inverse limits of compact base groups, and
also under forming direct limits of discrete target modules.
Lastly, these results together enable us to establish various circumstances
under which the measurable-cochains cohomology groups coincide with others
defined using sheaves on a semi-simplicial space associated to the underlying
group, or sheaves on a classifying space for that group. We also prove in some
cases that the natural quotient topologies on the measurable-cochains
cohomology groups are Hausdorff.Comment: 52 pages. [Nov 22, 2011:] Major re-write with Calvin C. Moore as new
co-author. Results from previous version strengthened and several new results
added. [Nov 25, 2012:] Final version now available at springerlink.co
Renal Foreign Bodies
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/67109/2/10.1177_000992286900800511.pd
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