403 research outputs found
The 21-cm signature of the first stars during the Lyman–Werner feedback era
The formation of the first stars is an exciting frontier area in astronomy. Early redshifts (z ∼ 20) have become observationally promising as a result of a recently recognized effect of a supersonic relative velocity between the dark matter and gas. This effect produces prominent structure on 100 comoving Mpc scales, which makes it much more feasible to detect 21-cm fluctuations from the epoch of first heating. We use semi-numerical hybrid methods to follow for the first time the joint evolution of the X-ray and Lyman–Werner radiative backgrounds, including the effect of the supersonic streaming velocity on the cosmic distribution of stars. We incorporate self-consistently the negative feedback on star formation induced by the Lyman–Werner radiation, which dissociates molecular hydrogen and thus suppresses gas cooling. We find that the feedback delays the X-ray heating transition by Δz ∼ 2, but leaves a promisingly large fluctuation signal over a broad redshift range. The large-scale power spectrum is predicted to reach a maximal signal-to-noise ratio of S/N ∼ 3–4 at z ∼ 18 (for a projected first-generation instrument), with S/N >1 out to z ∼ 22–23. We hope to stimulate additional numerical simulations as well as observational efforts focused on the epoch prior to cosmic reionization
Demonstrating the Feasibility of Line Intensity Mapping Using Mock Data of Galaxy Clustering from Simulations
Visbal & Loeb (2010) have shown that it is possible to measure the clustering
of galaxies by cross correlating the cumulative emission from two different
spectral lines which originate at the same redshift. Through this cross
correlation, one can study galaxies which are too faint to be individually
resolved. This technique, known as intensity mapping, is a promising probe of
the global properties of high redshift galaxies. Here, we test the feasibility
of such measurements with synthetic data generated from cosmological dark
matter simulations. We use a simple prescription for associating galaxies with
dark matter halos and create a realization of emitted radiation as a function
of angular position and wavelength over a patch of the sky. This is then used
to create synthetic data for two different hypothetical instruments, one aboard
the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) and another
consisting of a pair of ground based radio telescopes designed to measure the
CO(1-0) and CO(2-1) emission lines. We find that the line cross power spectrum
can be measured accurately from the synthetic data with errors consistent with
the analytical prediction of Visbal & Loeb (2010). Removal of astronomical
backgrounds and masking bright line emission from foreground contaminating
galaxies do not prevent accurate cross power spectrum measurements.Comment: 12 pages, 6 figures, Submitted to JCA
Steric and Electronic Effects in N-Heterocyclic Carbene Gold(III) Complexes: An Experimental and Computational Study
A series of neutral acridine-based gold(III)-NHC complexes containing the pentafluorophenyl (–C6F5) group were synthesized. All of the complexes were fully characterized by analytical techniques. The square planar geometry around the gold center was confirmed by X-ray diffraction analysis for complexes 1 (Trichloro [1-methyl-3-(9-acridine)imidazol-2-ylidene]gold(III)) and 2 (Chloro-bis(pentafluorophenyl)[1-methyl-3-(9-acridine)imidazol-2-ylidene]gold(III)). In both cases, the acridine rings play a key role in the crystal packing of the solid structures by mean of π–π stacking interactions, with centroid–centroid and interplanar distances being similar to those found in other previously reported acridine-based Au(I)-NHC complexes. A different reactivity when using a bulkier N-heterocyclic carbene ligand such as 1,3-bis-(2,6-diisopropylphenyl)-2-imidazolidinylidene (SIPr) was observed. While the use of the acridine-based NHC ligand led to the expected organometallic gold(III) species, the steric hindrance of the bulky SIPr ligand led to the formation of the corresponding imidazolinium cation stabilized by the tetrakis(pentafluorophenyl)aurate(III) [Au(C6F5)4]− anion. Computational experiments were carried out in order to figure out the ground state electronic structure and the binding formation energy of the complexes and, therefore, to explain the observed reactivity
Gold Catalyzed Multicomponent Reactions Beyond A3 Coupling
The preparation of complex architectures has inspired the search for new methods and new processes in organic synthesis. Multicomponent reactions have become an interesting approach to achieve such molecular diversity and complexity. This review intends to illustrate important gold-catalyzed examples for the past ten years leading to interesting skeletons involved in biologically active compounds
Volumetric measurements and simulations of the vortex structures generated by low aspect ratio plunging wings
Volumetric three-component velocimetry measurements have been performed on low aspect ratio wings undergoing a small amplitude pure plunging motion. This study focuses on the vortex flows generated by rectangular and elliptical wings set to a fixed geometric angle of attack of α = 20°. An investigation into the effect of Strouhal number illustrates the highly three-dimensional nature of the leading edge vortex as well as its inherent ability to improve lift performance. Computational simulations show good agreement with experimental results, both demonstrating the complex interaction between leading, trailing, and tip vortices generated in each cycle. The leading edge vortex, in particular, may deform significantly throughout the cycle, in some cases developing strong spanwise undulations. These are at least both Strouhal number and planform dependent. One or two arch-type vortical structures may develop, depending on the aspect ratio and Strouhal number. At sufficiently high Strouhal numbers, a tip vortex ring may also develop, propelling itself away from the wing in the spanwise direction due to self-induced velocity
Phase diagrams of period-4 spin chains consisting of three kinds of spins
We study a period-4 antiferromagnetic mixed quantum spin chain consisting of
three kinds of spins. When the ground state is singlet, the spin magnitudes in
a unit cell are arrayed as (s-t, s, s+t, s) with integer or half-odd integer s
and t (0 <= t < s). The spin Hamiltonian is mapped onto a nonlinear sigma model
(NLSM) in a previously developed method. The resultant NLSM includes only two
independent parameters originating from four exchange constants for fixed s and
t. The topological angle in the NLSM determines the gapless phase boundaries
between disordered phases in the parameter space. The phase diagrams for
various s and t shows rich structures. We systematically explain the phases in
the singlet-cluster-solid picture.Comment: 8 pages (16 figures included
ON SPURIOUS BEHAVIOR OF CFD SIMULATIONS
Spurious behavior in underresolved grids and:or semi-implicit temporal discretizations for four computational fluid dynamics (CFD) simulations are studied. The numerical simulations consist of (a) a 1-D chemically relaxed non-equilibrium flow model, (b) the direct numerical simulation (DNS) of 2D incompressible flow over a backward facing step, (c) a loosely coupled approach for a 2D fluid–structure interaction, and (d) a 3D unsteady compressible flow simulation of vortex breakdown on delta wings. These examples were chosen based on their non-apparent spurious behaviors that were difficult to detect without extensive grid and:or temporal refinement studies and without some knowledge from dynamical systems theory. Studies revealed the various possible dangers of misinterpreting numerical simulation of realistic complex flows that are constrained by available computing power. In large scale computations, underresolved grids, semi-implicit procedures, loosely coupled implicit procedures, and insufficiently long-time integration in DNS are most often unavoidable. Consequently, care must be taken in both computation and in interpretation of the numerical data. The results presented confirm the important role that dynamical systems theory can play in the understanding of the non-linear behavior of numerical algorithms and in aiding the identification of the sources of numerical uncertainties in CFD
Measuring the 3D Clustering of Undetected Galaxies Through Cross Correlation of their Cumulative Flux Fluctuations from Multiple Spectral Lines
We discuss a method for detecting the emission from high redshift galaxies by
cross correlating flux fluctuations from multiple spectral lines. If one can
fit and subtract away the continuum emission with a smooth function of
frequency, the remaining signal contains fluctuations of flux with frequency
and angle from line emitting galaxies. Over a particular small range of
observed frequencies, these fluctuations will originate from sources
corresponding to a series of different redshifts, one for each emission line.
It is possible to statistically isolate the fluctuations at a particular
redshift by cross correlating emission originating from the same redshift, but
in different emission lines. This technique will allow detection of clustering
fluctuations from the faintest galaxies which individually cannot be detected,
but which contribute substantially to the total signal due to their large
numbers. We describe these fluctuations quantitatively through the line cross
power spectrum. As an example of a particular application of this technique, we
calculate the signal-to-noise ratio for a measurement of the cross power
spectrum of the OI(63 micron) and OIII(52 micron) fine structure lines with the
proposed Space Infrared Telescope for Cosmology and Astrophysics. We find that
the cross power spectrum can be measured beyond a redshift of z=8. Such
observations could constrain the evolution of the metallicity, bias, and duty
cycle of faint galaxies at high redshifts and may also be sensitive to the
reionization history through its effect on the minimum mass of galaxies. As
another example, we consider the cross power spectrum of CO line emission
measured with a large ground based telescope like CCAT and 21-cm radiation
originating from hydrogen in galaxies after reionization with an interferometer
similar in scale to MWA, but optimized for post-reionization redshifts.Comment: 21 pages, 6 figures; Replaced with version accepted by JCAP; Added an
example of cross correlating CO line emission and 21cm line emission from
galaxies after reionizatio
- …