12,255 research outputs found
Evolution of the UV Excess in Early-Type Galaxies
We examine the UV emission from luminous early-type galaxies as a function of
redshift. We perform a stacking analysis using Galaxy Evolution Explorer
(GALEX) images of galaxies in the NOAO Deep Wide Field Survey (NDWFS) Bo\"otes
field and examine the evolution in the UV colors of the average galaxy. Our
sample, selected to have minimal ongoing star formation based on the optical to
mid-IR SEDs of the galaxies, includes 1843 galaxies spanning the redshift range
. We find evidence that the strength of the UV excess
decreases, on average, with redshift, and our measurements also show moderate
disagreement with previous models of the UV excess. Our results show little
evolution in the shape of the UV continuum with redshift, consistent either
with the binary model for the formation of Extreme Horizontal Branch (EHB)
stars or with no evolution in EHB morphology with look-back time. However, the
binary formation model predicts that the strength of the UV excess should also
be relatively constant, in contradiction with our measured results. Finally, we
see no significant influence of a galaxy's environment on the strength of its
UV excess.Comment: 30 pages, 10 figures; accepted by ApJ. Modified from original version
to reflect referee's comment
The size, density, and formation of the Orcus-Vanth system in the Kuiper belt
The Kuiper belt object Orcus and its satellite Vanth form an unusual system
in the Kuiper belt. Orcus is amongst the largest objects known in the Kuiper
belt, but the relative size of Vanth is much larger than that of the tiny
satellites of the other large objects. From Hubble Space Telescope observations
we find that Orcus and Vanth have different visible colors and that Vanth does
not share the water ice absorption feature seen in the infrared spectrum of
Orcus. We also find that Vanth has a nearly face-on circular orbit with a
period of 9.5393 +-0.0001 days and semimajor axis of 8980+-20 km, implying a
system mass of 6.32+- 0.01 X 10^20 kg or 3.8% the mass of dwarf planet Eris.
From Spitzer Space Telescope observations we find that the thermal emission
is consistent with a single body with diameter 940+-70 km and a geometric
albedo of 0.28+-0.04. Assuming equal densities and albedos, this measurements
implies sizes of Orcus and Vanth of 900 and 280 km, respectively, and a mass
ratio of 33. Assuming a factor of 2 lower albedo for the non-icy Vanth,
however, implies sizes of 820 and 640 km and a mass ratio of 2. The measured
density depends on the assumed albedo ratio of the two objects but is
approximately 1.5+-0.3 g cm^-3$, midway between typical densities measured for
larger and for smaller objects. The orbit and mass ratio is consistent with
formation from a giant impact and subsequent outward tidal evolution and even
consistent with the system having now achieved a double synchronous state. The
system can equally well be explained, however, by initial eccentric capture,
Kozai cycling which increases the eccentricity and decreases the pericenter of
the orbit of Vanth, and subsequent tidal evolution inward.Comment: Submitted to A
The evolution of interstellar clouds in a streaming hot plasma including heat conduction
To examine the evolution of giant molecular clouds in the stream of a hot
plasma we performed two-dimensional hydrodynamical simulations that take full
account of self-gravity, heating and cooling effects and heat conduction by
electrons. We use the thermal conductivity of a fully ionized hydrogen plasma
proposed by Spitzer and a saturated heat flux according to Cowie & McKee in
regions where the mean free path of the electrons is large compared to the
temperature scaleheight. Significant structural and evolutionary differences
occur between simulations with and without heat conduction. Dense clouds in
pure dynamical models experience dynamical destruction by Kelvin-Helmholtz (KH)
instability. In static models heat conduction leads to evaporation of such
clouds. Heat conduction acting on clouds in a gas stream smooths out steep
temperature and density gradients at the edge of the cloud because the
conduction timescale is shorter than the cooling timescale. This diminishes the
velocity gradient between the streaming plasma and the cloud, so that the
timescale for the onset of KH instabilities increases, and the surface of the
cloud becomes less susceptible to KH instabilities. The stabilisation effect of
heat conduction against KH instability is more pronounced for smaller and less
massive clouds. As in the static case more realistic cloud conditions allow
heat conduction to transfer hot material onto the cloud's surface and to mix
the accreted gas deeper into the cloud.Comment: 19 pages, 12 figures, accepted in Astronomy and Astrophysic
The Mid-IR and X-ray Selected QSO Luminosity Function
We present the J-band luminosity function of 1838 mid-infrared and X-ray
selected AGNs in the redshift range 0<z<5.85. These luminosity functions are
constructed by combining the deep multi-wavelength broad-band observations from
the UV to the mid-IR of the NDWFS Bootes field with the X-ray observations of
the XBootes survey and the spectroscopic observations of the same field by
AGES. Our sample is primarily composed of IRAC-selected AGNs, targeted using
modifications of the Stern et al.(2005) criteria, complemented by MIPS 24
microns and X-ray selected AGNs to alleviate the biases of IRAC mid-IR
selection against z~4.5 quasars and AGNs faint with respect to their hosts.
This sample provides an accurate link between low and high redshift AGN
luminosity functions and does not suffer from the usual incompleteness of
optical samples at z~3. We find that the space density of the brightest quasars
strongly decreases from z=3 to z=0, while the space density of faint quasars is
at least flat, and possibly increasing, over the same redshift range. At z>3 we
observe a decrease in the space density of quasars of all brightnesses. We
model the luminosity function by a double power-law and find that its evolution
cannot be described by either pure luminosity or pure density evolution, but
must be a combination of both. Our best-fit model has bright and faint
power-law indices consistent with the low redshift measurements based on the
2QZ and 2SLAQ surveys and it generally agrees with the number of bright quasars
predicted by other LFs at all redshifts. If we construct the QSO luminosity
function using only the IRAC-selected AGNs, we find that the biases inherent to
this selection method significantly modify the behavior of phi*(z) only for z<1
and have no significant impact upon the characteristic magnitude M*_J(z).Comment: Corrected minor typo in equations (4) and (6). Accepted for
publication in The Astrophysical Journal. 56 pages + 6 tables + 16 figure
Enhancement of low-mass dileptons in heavy-ion collisions
Using a relativistic transport model for the expansion stage of S+Au
collisions at 200 GeV/nucleon, we show that the recently observed enhancement
of low-mass dileptons by the CERES collaboration can be explained by the
decrease of vector meson masses in hot and dense hadronic matter.Comment: 12 pages, RevTeX, 3 figures available from [email protected]
The Mid-Infrared Properties of X-ray Sources
We combine the results of the Spitzer IRAC Shallow Survey and the Chandra
XBootes Survey of the 8.5 square degrees Bootes field of the NOAO Deep Wide-
Field Survey to produce the largest comparison of mid-IR and X-ray sources to
date. The comparison is limited to sources with X-ray fluxes >8x10-15 erg
cm-2s-1 in the 0.5-7.0 keV range and mid-IR sources with 3.6 um fluxes brighter
than 18.4 mag (12.3 uJy). In this most sensitive IRAC band, 85% of the 3086
X-ray sources have mid-IR counterparts at an 80% confidence level based on a
Bayesian matching technique. Only 2.5% of the sample have no IRAC counterpart
at all based on visual inspection. Even for a smaller but a significantly
deeper Chandra survey in the same field, the IRAC Shallow Survey recovers most
of the X-ray sources. A majority (65%) of the Chandra sources detected in all
four IRAC bands occupy a well-defined region of IRAC [3.6] - [4.5] vs [5.8] -
[8.0] color-color space. These X-ray sources are likely infrared luminous,
unobscured type I AGN with little mid-infrared flux contributed by the AGN host
galaxy. Of the remaining Chandra sources, most are lower luminosity type I and
type II AGN whose mid-IR emission is dominated by the host galaxy, while
approximately 5% are either Galactic stars or very local galaxies.Comment: Accepted for publication in Ap
Why growth equals power - and why it shouldn't : constructing visions of China
When discussing the success of China's transition from socialism, there is a tendency to focus on growth figures as an indication of performance. Whilst these figures are
indeed impressive, we should not confuse growth with development and assume that the former necessarily automatically generates the latter. Much has been done to
reduce poverty in China, but the task is not as complete as some observers would suggest; particularly in terms of access to health, education and welfare, and also in
dealing with relative (rather than absolute) depravation and poverty. Visions of China have been constructed that exaggerate Chinese development and power in the global
system partly to serve political interests, but partly due to the failure to consider the relationship between growth and development, partly due to the failure to disaggregate
who gets what in China, and partly due to the persistence of inter-national conceptions of globalised production, trade, and financial flows
Homogenization in magnetic-shape-memory polymer composites
Magnetic-shape-memory materials (e.g. specific NiMnGa alloys) react with a
large change of shape to the presence of an external magnetic field. As an
alternative for the difficult to manifacture single crystal of these alloys we
study composite materials in which small magnetic-shape-memory particles are
embedded in a polymer matrix. The macroscopic properties of the composite
depend strongly on the geometry of the microstructure and on the
characteristics of the particles and the polymer.
We present a variational model based on micromagnetism and elasticity, and
derive via homogenization an effective macroscopic model under the assumption
that the microstructure is periodic. We then study numerically the resulting
cell problem, and discuss the effect of the microstructure on the macroscopic
material behavior. Our results may be used to optimize the shape of the
particles and the microstructure.Comment: 17 pages, 4 figure
Electrical discharge coating of nanostructured TiC-Fe cermets on 304 stainless steel
The electrical discharge coating (EDC) process, as used for the development of TiC-Fe cermet coatings on 304 stainless steel, has been investigated as a function of increasing current (2–19 A) and pulse-on time (2–64 μs). Coating morphologies, comprising of a mixture of TiC, γ-Fe, ά-Fe and amorphous carbon, were characterised using the combined techniques of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD) and cross-sectional transmission electron microscopy (TEM). The developed coatings exhibited variable hardness values, up to an order of magnitude higher than that of the substrate, depending on the content and dispersion of nanostructured TiC particles within the Fe matrix. Coating hardness was found to increase with increasing current, but decrease under conditions of high pulse-on times, reflecting differences in the amount of TiC incorporated into the coatings. Optimised coatings were achieved using conditions of low processing energy which minimised the development of pores and cracks
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