1,443 research outputs found
Evolution of the AGN UV luminosity function from redshift 7.5
Determinations of the UV luminosity function of AGN at high redshifts are
important for constraining the AGN contribution to reionization and
understanding the growth of supermassive black holes. Recent inferences of the
luminosity function suffer from inconsistencies arising from inhomogeneous
selection and analysis of AGN data. We address this problem by constructing a
sample of more than 80,000 colour-selected AGN from redshift z=0 to 7.5. While
this sample is composed of multiple data sets with spectroscopic redshifts and
completeness estimates, we homogenise these data sets to identical cosmologies,
intrinsic AGN spectra, and magnitude systems. Using this sample, we derive the
AGN UV luminosity function from redshift z=0 to 7.5. The luminosity function
has a double power law form at all redshifts. The break magnitude of the
AGN luminosity function shows a steep brightening from at z=0.7
to at z=6. The faint-end slope significantly steepens
from at to at . In spite of this steepening,
the contribution of AGN to the hydrogen photoionization rate at is
subdominant (< 3%), although it can be non-negligible (~10%) if these
luminosity functions hold down to . Under reasonable assumptions,
AGN can reionize HeII by redshift z=2.9. At low redshifts (z<0.5), AGN can
produce about half of the hydrogen photoionization rate inferred from the
statistics of HI absorption lines in the IGM. Our global analysis of the
luminosity function also reveals important systematic errors in the data,
particularly at z=2.2--3.5, which need to be addressed and incorporated in the
AGN selection function in future in order to improve our results. We make
various fitting functions, luminosity function analysis codes, and homogenised
AGN data publicly available.Comment: 30 pages, 15 figures; accepted in MNRAS; code, data, and various fits
at https://github.com/gkulkarni/QL
Distortions of C-60(4-) studied by infrared spectroscopy
The Jahn-Teller effect plays a crucial role in the explanation of the insulating character of
A(4)C(60) (A = K, Rb, Cs). To detect possible phase transitions arising from the interplay between the
molecular Jahn-Teller distortion and the distorting potential field of the counterions, we measured
the mid-IR spectra of A(4)C(60) compounds in the temperature range 90 - 300 K and found significant
spectral changes with temperature in all three compounds. We also compare these spectra to that
of Na(4)C(60) in its room-temperature polymeric phase, where the distortion is more pronounced and
evident from the structure
Testing goodness-of-fit of random graph models
Random graphs are matrices with independent 0, 1 elements with probabilities
determined by a small number of parameters. One of the oldest model is the
Rasch model where the odds are ratios of positive numbers scaling the rows and
columns. Later Persi Diaconis with his coworkers rediscovered the model for
symmetric matrices and called the model beta. Here we give goodnes-of-fit tests
for the model and extend the model to a version of the block model introduced
by Holland, Laskey, and Leinhard
Critical current of a Josephson junction containing a conical magnet
We calculate the critical current of a
superconductor/ferromagnetic/superconductor (S/FM/S) Josephson junction in
which the FM layer has a conical magnetic structure composed of an in-plane
rotating antiferromagnetic phase and an out-of-plane ferromagnetic component.
In view of the realistic electronic properties and magnetic structures that can
be formed when conical magnets such as Ho are grown with a polycrystalline
structure in thin-film form by methods such as direct current sputtering and
evaporation, we have modeled this situation in the dirty limit with a large
magnetic coherence length (). This means that the electron mean free
path is much smaller than the normalized spiral length which in
turn is much smaller than (with as the length a complete
spiral makes along the growth direction of the FM). In this physically
reasonable limit we have employed the linearized Usadel equations: we find that
the triplet correlations are short ranged and manifested in the critical
current as a rapid oscillation on the scale of . These rapid
oscillations in the critical current are superimposed on a slower oscillation
which is related to the singlet correlations. Both oscillations decay on the
scale of . We derive an analytical solution and also describe a
computational method for obtaining the critical current as a function of the
conical magnetic layer thickness.Comment: Extended version of the published paper. Additional information about
the computational method is included in the appendi
The Comparison of Effects of Liquid Carbon Dioxide and Conventional Flood Cooling on the Machining Conditions During Milling of Nickel-based Superalloys
In this scientific study, the authors have dealt with the slot milling of nickel-based superalloys. These alloys are among the most difficult materials to machine and are widely used in aerospace and energy industries. Due to the properties of the material, slot milling is a particular problem because tool wear happens quickly, and tool breakages are common. When these superalloys are machined, very high temperatures occur in the cutting zone, which cannot leave due to the extremely poor thermal conductivity of the material and will therefore transfer to the edges of the cutting tool, causing it to anneal, break off and fail. So, the researchers initiated a new field of research: cryogenically-assisted machining. In this paper, the authors used two cooling methods, the conventional flood cooling and cryogenic cooling with liquid carbon-dioxide (LCO2). The effects of these cooling methods were tested focusing on the cutting forces, tool wear, chip morphology and surface roughness of the bottom of the slots. The aim was to determine the best cooling methods for these materials. Based on the results, it can be concluded that, LCO2 has a negative effect on cutting forces, tool life and surface roughness. It only has a positive effect on chip formation. It can be see that, the lubricating effect has a greater impact on tool life, tool load and surface roughness of the milled slots than cooling
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