10,578 research outputs found
Critical fields of liquids of liquid superconducting metallic hydrogen
Liquid metallic hydrogen, in a fully dissociated state, is predicted at certain densities to pass from dirty to clean and from type II to type I superconducting behavior as temperature is lowered
Cosmological constraints from the X-ray gas mass fraction in relaxed lensing clusters observed with Chandra
We present precise measurements of the X-ray gas mass fraction for a sample
of luminous, relatively relaxed clusters of galaxies observed with the Chandra
Observatory, for which independent confirmation of the mass results is
available from gravitational lensing studies. Parameterizing the total
(luminous plus dark matter) mass profiles using the model of Navarro, Frenk &
White (1997), we show that the X-ray gas mass fractions in the clusters
asymptote towards an approximately constant value at a radius r_2500, where the
mean interior density is 2500 times the critical density of the Universe at the
redshifts of the clusters. Combining the Chandra results on the X-ray gas mass
fraction and its apparent redshift dependence with recent measurements of the
mean baryonic matter density in the Universe and the Hubble Constant determined
from the Hubble Key Project, we obtain a tight constraint on the mean total
matter density of the Universe, Omega_m = 0.30^{+0.04}_{-0.03}, and measure a
positive cosmological constant, Omega_Lambda = 0.95^{+0.48}_{-0.72}. Our
results are in good agreement with recent, independent findings based on
analyses of anisotropies in the cosmic microwave background radiation, the
properties of distant supernovae, and the large-scale distribution of galaxies.Comment: Accepted for publication in MNRAS Letters (6 pages, 3 figures
Resolving the nucleus of Centaurus A at mid-IR wavelengths
We have observed Centaurus A with the MID-infrared Interferometric instrument
(MIDI) at the Very Large Telescope Interferometer (VLTI) at resolutions of 7 -
15 mas (at 12.5 micron) and filled gaps in the (u,v) coverage in comparison to
earlier measurements. We are now able to describe the nuclear emission in terms
of geometric components and derive their parameters by fitting models to the
interferometric data. With simple geometrical models, the best fit is achieved
for an elongated disk with flat intensity profile with diameter 76 +/- 9 mas x
35 +/- 2 mas (1.41 +/- 0.17 pc x 0.65 +/- 0.03 pc) whose major axis is oriented
at a position angle (PA) of 10.1 +/- 2.2 degrees east of north. A point source
contributes 47 +/- 11 % of the nuclear emission at 12.5 micron. There is also
evidence that neither such a uniform nor a Gaussian disk are good fits to the
data. This indicates that we are resolving more complicated small-scale
structure in AGNs with MIDI, as has been seen in Seyfert galaxies previously
observed with MIDI. The PA and inferred inclination i = 62.6 +2.1/-2.6 degrees
of the dust emission are compared with observations of gas and dust at larger
scales.Comment: Accepted for the PASA special issue on Centaurus
Dynamics of the Narrow-Line Region in the Seyfert 2 Galaxy NGC 1068
We present dynamical models based on a study of high-resolution long-slit
spectra of the narrow-line region (NLR) in NGC 1068 obtained with the Space
Telescope Imaging Spectrograph (STIS) aboard The Hubble Space Telescope (HST).
The dynamical models consider the radiative force due to the active galactic
nucleus (AGN), gravitational forces from the supermassive black hole (SMBH),
nuclear stellar cluster, and galactic bulge, and a drag force due to the NLR
clouds interacting with a hot ambient medium. The derived velocity profile of
the NLR gas is compared to that obtained from our previous kinematic models of
the NLR using a simple biconical geometry for the outflowing NLR clouds. The
results show that the acceleration profile due to radiative line driving is too
steep to fit the data and that gravitational forces along cannot slow the
clouds down, but with drag forces included, the clouds can slow down to the
systemic velocity over the range 100--400 pc, as observed. However, we are not
able to match the gradual acceleration of the NLR clouds from ~0 to ~100 pc,
indicating the need for additional dynamical studies.Comment: Paper prepared by emulateapj version 10/09/06 and accepted for print
in Ap
Single-spin Azimuthal Asymmetries in the ``Reduced Twist-3 Approximation''
We consider the single-spin azimuthal asymmetries recently measured at the
HERMES experiment for charged pions produced in semi-inclusive deep inelastic
scattering of leptons off longitudinally polarized protons. Guided by the
experimental results and assuming a vanishing twist-2 transverse quark spin
distribution in the longitudinally polarized nucleon, denoted as ``reduced
twist-3 approximation'', a self-consistent description of the observed
single-spin asymmetries is obtained. In addition, predictions are given for the
z dependence of the single target-spin asymmetry.Comment: 8 pages, 2 figures, typos corrected, very small changes to text,
reference adde
Revealing the large nuclear dust structures in NGC 1068 with MIDI/VLTI
To understand the relation between the small "obscuring torus" and dusty
structures at larger scales (5-10 pc) in NGC 1068, we use ESO's Mid-Infrared
Interferometer (MIDI) with the 1.8 m Auxiliary Telescopes to achieve the
necessary spatial resolution (~ 20-100 millarcsec). We use the chromatic phases
in the data to improve the spatial fidelity of the analysis. We present
interferometric data for NGC 1068 obtained in 2007 and 2012. We find no
evidence of source variability. Many (u,v) points show non-zero chromatic
phases indicating significant asymmetries. Gaussian model fitting of the
correlated fluxes and chromatic phases provides a 3-component best fit with
estimates of sizes, temperatures and positions of the components. A large,
warm, off-center component is required at a distance approximately 90 mas to
the north-west at a PA ~ -18 deg. The dust at 5-10 pc in the polar region
contributes 4 times more to the mid-infrared flux at 12 um than the dust
located at the center. This dust may represent the inner wall of a dusty cone.
If similar regions are heated by the direct radiation from the nucleus, then
they will contribute substantially to the classification of many Seyfert
galaxies as Type 2. Such a region is also consistent in other Seyfert galaxies
(the Circinus galaxy, NGC 3783 and NGC 424).Comment: 21 pages, 10 figures; Accepted for publication on A&
Ex-ante measure of patent quality reveals intrinsic fitness for citation-network growth
We have constructed a fitness parameter, characterizing the intrinsic
attractiveness for patents to be cited, from attributes of the associated
inventions known at the time a patent is granted. This exogenously obtained
fitness is shown to determine the temporal growth of the citation network in
conjunction with mechanisms of preferential attachment and obsolescence-induced
ageing that operate without reference to characteristics of individual patents.
Our study opens a window on understanding quantitatively the interplay of the
rich-gets-richer and fit-gets-richer paradigms that have been suggested to
govern the growth dynamics of real-world complex networks.Comment: 6 pages, 3 figures, RevTex4.1, v2: minor changes, version to appear
as a Rapid Communication in Phys. Rev.
The Insignificance of Global Reheating in the Abell 1068 Cluster: X-Ray Analysis
We report on a Chandra observation of the massive, medium redshift (z=0.1386)
cooling flow cluster Abell 1068. We detect a clear temperature gradient in the
X-ray emitting gas from kT ~ 5 keV in the outer part of the cluster down to
roughly 2 keV in the core, and a striking increase in the metallicity of the
gas toward the cluster center. The total spectrum from the cluster can be fit
by a cooling flow model with a total mass deposition rate of 150 solar
masses/yr. Within the core (r < 30 kpc), the mass depositon rate of 40 solar
masses/yr is comparable to estimates for the star formation rate from optical
data. We find an apparent correlation between the cD galaxy's optical isophotes
and enhanced metallicity isocontours in the central ~100 kpc of the cluster. We
show that the approximate doubling of the metallicity associated with the cD
can be plausibly explained by supernova explosions associated with the cD's
ambient stellar population and the recent starburst. Finally, we calculate the
amount of heating due to thermal conduction and show that this process is
unlikely to offset cooling in Abell 1068.Comment: Accepted for publication in ApJ, 26 pages, 12 b+w figures, 3 color
figure
Testing modified gravity with globular cluster velocity dispersions
Globular clusters (GCs) in the Milky Way have characteristic velocity
dispersions that are consistent with the predictions of Newtonian gravity, and
may be at odds with Modified Newtonian Dynamics (MOND). We discuss a modified
gravity (MOG) theory that successfully predicts galaxy rotation curves, galaxy
cluster masses and velocity dispersions, lensing, and cosmological
observations, yet produces predictions consistent with Newtonian theory for
smaller systems, such as GCs. MOG produces velocity dispersion predictions for
GCs that are independent of the distance from the galactic center, which may
not be the case for MOND. New observations of distant GCs may produce strong
criteria that can be used to distinguish between competing gravitational
theories.Comment: 4 pages, 2 figures; accepted for publication in Ap
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