11,756 research outputs found
Forming Galaxies with MOND
Beginning with a simple model for the growth of structure, I consider the
dissipationless evolution of a MOND-dominated region in an expanding Universe
by means of a spherically symmetric N-body code. I demonstrate that the final
virialized objects resemble elliptical galaxies with well-defined relationships
between the mass, radius, and velocity dispersion. These calculations suggest
that, in the context of MOND, massive elliptical galaxies may be formed early
(z > 10) as a result of monolithic dissipationless collapse. Then I reconsider
the classic argument that a galaxy of stars results from cooling and
fragmentation of a gas cloud on a time scale shorter than that of dynamical
collapse. Qualitatively, the results are similar to that of the traditional
picture; moreover, the existence, in MOND, of a density-temperature relation
for virialized, near isothermal objects as well as a mass-temperature relation
implies that there is a definite limit to the mass of a gas cloud where this
condition can be met-- an upper limit corresponding to that of presently
observed massive galaxies.Comment: 9 pages, 9 figures, revised in response to comments of referee. Table
added, extended discussion, accepted MNRA
Superconductivity and Cobalt Oxidation State in Metastable Na(x)CoO(2-delta)*yH2O (x ~ 1/3; y ~ 4x)
We report the synthesis and superconducting properties of a metastable form
of the known superconductor NaxCoO2*yH2O (x ~ 1/3, y ~ 4x). Instead of using
the conventional bromine-acetonitrile mixture for sodium deintercalation, we
use an aqueous bromine solution. Using this method, we oxidize the sample to a
point that the sodium cobaltate becomes unstable, leading to formation of other
products if not controlled. This compound has the same structure as the
reported superconductor, yet it exhibits a systematic variation of the
superconducting transition temperature (Tc) as a function of time. Immediately
after synthesis, this compound is not a superconductor, even though it contains
appropriate amounts of sodium and water. The samples become superconducting
with low Tc values after ~ 90 h. Tc continually increases until it reaches a
maximum value (4.5 K) after about 260 h. Then Tc drops drastically, becoming
non-superconducting approximately 100 h later. Corresponding time-dependent
neutron powder diffraction data shows that the changes in superconductivity
exhibited by the metastable cobaltate correspond to slow formation of oxygen
vacancies in the CoO2 layers. In effect, the formation of these defects
continually reduces the cobalt oxidation state causing the sample to evolve
through its superconducting life cycle. Thus, the dome-shaped superconducting
phase diagram is mapped as a function of cobalt oxidation state using a single
sample. The width of this dome based on the formal oxidation state of cobalt is
very narrow - approximately 0.1 valence units wide. Interestingly, the maximum
Tc in NaxCoO2*yH2O occurs when the cobalt oxidation state is near 3.5. Thus, we
speculate that the maximum Tc occurs near the charge ordered insulating state
that correlates with the average cobalt oxidation state of 3.5.Comment: 22 pages, 9 figures, 1 tabl
Infrared Surface Brightness Fluctuations of the Coma Elliptical NGC 4874 and the Value of the Hubble Constant
We have used the Keck I Telescope to measure K-band surface brightness
fluctuations (SBFs) of NGC 4874, the dominant elliptical galaxy in the Coma
cluster. We use deep HST WFPC2 optical imaging to account for the contamination
due to faint globular clusters and improved analysis techniques to derive
measurements of the SBF apparent magnitude. Using a new SBF calibration which
accounts for the dependence of K-band SBFs on the integrated color of the
stellar population, we measure a distance modulus of 34.99+/-0.21 mag (100+/-10
Mpc) for the Coma cluster. The resulting value of the Hubble constant is 71+/-8
km/s/Mpc, not including any systematic error in the HST Cepheid distance scale.Comment: ApJ Letters, in press. Uses emulateapj5.st
Tuning of magnetic and electronic states by control of oxygen content in lanthanum strontium cobaltites
We report on the magnetic, resistive, and structural studies of perovskite
LaSrCoO. By using the relation of synthesis
temperature and oxygen partial pressure to oxygen stoichiometry obtained from
thermogravimetric analysis, we have synthesized a series of samples with
precisely controlled . These samples show three structural
phases at , , , and two-phase
behavior for other oxygen contents. The stoichiometric material with
is a cubic ferromagnetic metal with the Curie temperature K. The increase of to 0.15 is followed by a linear decrease of
to 160 K and a metal-insulator transition near the
boundary of the cubic structure range. Further increase of results in
formation of a tetragonal phase for
and a brownmillerite phase for . At low
temperatures, these are weak ferromagnetic insulators (canted antiferromagnets)
with magnetic transitions at and 120 K, respectively. At
higher temperatures, the phase is -type
antiferromagnetic between 230 K and 360 K. Low temperature magnetic
properties of this system for can be described in terms of a
mixture of Co ions in the low-spin state and Co ions in the
intermediate-spin state and a possible spin transition of Co to the
intermediate-spin state above . For , there appears to
be a combination of Co and Co ions, both in the high-spin state
with dominating antiferromagnetic interactions.Comment: RevTeX, 9 pages, 7 figures, to be published in Physical Review
Asymptotic Behavior of Ext functors for modules of finite complete intersection dimension
Let be a local ring, and let and be finitely generated
-modules such that has finite complete intersection dimension. In this
paper we define and study, under certain conditions, a pairing using the
modules \Ext_R^i(M,N) which generalizes Buchweitz's notion of the Herbrand
diference. We exploit this pairing to examine the number of consecutive
vanishing of \Ext_R^i(M,N) needed to ensure that \Ext_R^i(M,N)=0 for all
. Our results recover and improve on most of the known bounds in the
literature, especially when has dimension at most two
High transport currents in mechanically reinforced MgB2 wires
We prepared and characterized monofilamentary MgB2 wires with a mechanically
reinforced composite sheath of Ta(Nb)/Cu/steel, which leads to dense filaments
and correspondingly high transport currents up to Jc = 10^5 A/cm^2 at 4.2 K,
self field. The reproducibility of the measured transport currents was
excellent and not depending on the wire diameter. Using different precursors,
commercial reacted powder or an unreacted Mg/B powder mixture, a strong
influence on the pinning behaviour and the irreversibility field was observed.
The critical transport current density showed a nearly linear temperature
dependency for all wires being still 52 kA/cm^2 at 20 K and 23 kA/cm^2 at 30 K.
Detailed data for Jc(B,T) and Tc(B) were measured.Comment: 21 pages, 13 figures, revised version, to be published in Supercond.
Sci. Techno
Negative thermal expansion of MgB in the superconducting state and anomalous behavior of the bulk Gr\"uneisen function
The thermal expansion coefficient of MgB is revealed to change
from positive to negative on cooling through the superconducting transition
temperature . The Gr\"uneisen function also becomes negative at
followed by a dramatic increase to large positive values at low temperature.
The results suggest anomalous coupling between superconducting electrons and
low-energy phonons.Comment: 5 figures. submitted to Phys. Rev. Let
Harmonic analysis of iterated function systems with overlap
In this paper we extend previous work on IFSs without overlap. Our method
involves systems of operators generalizing the more familiar Cuntz relations
from operator algebra theory, and from subband filter operators in signal
processing.Comment: 37 page
The existence problem for dynamics of dissipative systems in quantum probability
Motivated by existence problems for dissipative systems arising naturally in
lattice models from quantum statistical mechanics, we consider the following
-algebraic setting: A given hermitian dissipative mapping is
densely defined in a unital -algebra . The identity
element in is also in the domain of . Completely
dissipative maps are defined by the requirement that the induced maps,
, are dissipative on the by complex
matrices over for all . We establish the existence of different
types of maximal extensions of completely dissipative maps. If the enveloping
von Neumann algebra of is injective, we show the existence of an
extension of which is the infinitesimal generator of a quantum
dynamical semigroup of completely positive maps in the von Neumann algebra. If
is a given well-behaved *-derivation, then we show that each of the
maps and is completely dissipative.Comment: 24 pages, LaTeX/REVTeX v. 4.0, submitted to J. Math. Phys.; PACS 02.,
02.10.Hh, 02.30.Tb, 03.65.-w, 05.30.-
The history of mass assembly of faint red galaxies in 28 galaxy clusters since z=1.3
We measure the relative evolution of the number of bright and faint (as faint
as 0.05 L*) red galaxies in a sample of 28 clusters, of which 16 are at 0.50<=
z<=1.27, all observed through a pair of filters bracketing the 4000 Angstrom
break rest-frame. The abundance of red galaxies, relative to bright ones, is
constant over all the studied redshift range, 0<z<1.3, and rules out a
differential evolution between bright and faint red galaxies as large as
claimed in some past works. Faint red galaxies are largely assembled and in
place at z=1.3 and their deficit does not depend on cluster mass, parametrized
by velocity dispersion or X-ray luminosity. Our analysis, with respect to
previous one, samples a wider redshift range, minimizes systematics and put a
more attention to statistical issues, keeping at the same time a large number
of clusters.Comment: MNRAS, 386, 1045. Half a single sentence (in sec 4.4) change
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