3,556 research outputs found
Searching for high-redshift centimeter-wave continuum, line and maser emission using the Square Kilometer Array
We discuss the detection of redshifted line and continuum emission at radio
wavelengths using a Square Kilometer Array (SKA), specifically from
low-excitation rotational molecular line transitions of CO and HCN (molecular
lines), the recombination radiation from atomic transitions in almost-ionized
hydrogen (radio recombination lines; RRLs), OH and water maser lines, as well
as from synchrotron and free-free continuum radiation and HI 21-cm line
radiation. The detection of radio lines with the SKA offers the prospect to
determine the redshifts and thus exact luminosities for some of the most
distant and optically faint star-forming galaxies and active galactic nuclei
(AGN), even those galaxies that are either deeply enshrouded in interstellar
dust or shining prior to the end of reionization. Moreover, it provides an
opportunity to study the astrophysical conditions and resolved morphologies of
the most active regions in galaxies during the most active phase of star
formation at redshift z~2. A sufficiently powerful and adaptable SKA correlator
will enable wide-field three-dimensional redshift surveys at chosen specific
high redshifts, and will allow new probes of the evolution of large-scale
structure (LSS) in the distribution of galaxies. The detection of molecular
line radiation favours pushing the operating frequencies of SKA up to at least
26 GHz, and ideally to 40 GHz, while very high redshift maser emissions
requires access to about 100 MHz. To search for LSS the widest possible
instantaneous field of view would be advantageous.Comment: 12 pages, 2 figures. To appear in "Science with the Square Kilometer
Array," eds. C. Carilli and S. Rawlings, New Astronomy Reviews (Elsevier:
Amsterdam
A Dense Gas Trigger for OH Megamasers
HCN and CO line diagnostics provide new insight into the OH megamaser (OHM)
phenomenon, suggesting a dense gas trigger for OHMs. We identify three physical
properties that differentiate OHM hosts from other starburst galaxies: (1) OHMs
have the highest mean molecular gas densities among starburst galaxies; nearly
all OHM hosts have = 10^3-10^4 cm^-3 (OH line-emitting clouds likely
have n(H2) > 10^4 cm^-3). (2) OHM hosts are a distinct population in the
nonlinear part of the IR-CO relation. (3) OHM hosts have exceptionally high
dense molecular gas fractions, L(HCN)/L(CO)>0.07, and comprise roughly half of
this unusual population. OH absorbers and kilomasers generally follow the
linear IR-CO relation and are uniformly distributed in dense gas fraction and
L(HCN), demonstrating that OHMs are independent of OH abundance. The fraction
of non-OHMs with high mean densities and high dense gas fractions constrains
beaming to be a minor effect: OHM emission solid angle must exceed 2 pi
steradians. Contrary to conventional wisdom, IR luminosity does not dictate OHM
formation; both star formation and OHM activity are consequences of tidal
density enhancements accompanying galaxy interactions. The OHM fraction in
starbursts is likely due to the fraction of mergers experiencing a temporal
spike in tidally driven density enhancement. OHMs are thus signposts marking
the most intense, compact, and unusual modes of star formation in the local
universe. Future high redshift OHM surveys can now be interpreted in a star
formation and galaxy evolution context, indicating both the merging rate of
galaxies and the burst contribution to star formation.Comment: 5 pages, 3 figures, 1 table, accepted by ApJ Letter
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Precision Extruding Deposition and Characterization of Cellular Poly-e-Caprolactone Tissue Scaffolds
Successes in scaffold guided tissue engineering require scaffolds to have specific macroscopic
geometries and internal architectures in order to provide the needed biological and biophysical
functions. Freeform fabrication provides an effective process tool to manufacture many advanced
scaffolds with designed properties. This paper reports our recent study on using a novel Precision
Extruding Deposition (PED) process technique to directly fabricate cellular Poly-ε-Caprolactone
(PCL) scaffolds. Scaffolds with a controlled pore size of 250 µm and designed structural
orientations were fabricated. The scaffold morphology, internal micro-architecture and
mechanical properties were evaluated using SEM, Micro-Computed Tomography (µ-CT) and the
mechanical testing. Preliminary biological study was also conducted to investigate the cell
responses to the as-fabricated tissue scaffolds. The results and the characterizations demonstrate
the viability of the PED process to the scaffold fabrication as well as a good mechanical
property, structural integrity, controlled pore size, pore interconnectivity, and the anticipated
biological compatibility of the as-fabricated PCL scaffolds.The authors acknowledge the NSF-0219176 project funding support to graduate students
Andrew Darling and Saif Khalil, and the ONR research funding support to graduate student
Lauren Shor.Mechanical Engineerin
Ab initio Quantum and ab initio Molecular Dynamics of the Dissociative Adsorption of Hydrogen on Pd(100)
The dissociative adsorption of hydrogen on Pd(100) has been studied by ab
initio quantum dynamics and ab initio molecular dynamics calculations. Treating
all hydrogen degrees of freedom as dynamical coordinates implies a high
dimensionality and requires statistical averages over thousands of
trajectories. An efficient and accurate treatment of such extensive statistics
is achieved in two steps: In a first step we evaluate the ab initio potential
energy surface (PES) and determine an analytical representation. Then, in an
independent second step dynamical calculations are performed on the analytical
representation of the PES. Thus the dissociation dynamics is investigated
without any crucial assumption except for the Born-Oppenheimer approximation
which is anyhow employed when density-functional theory calculations are
performed. The ab initio molecular dynamics is compared to detailed quantum
dynamical calculations on exactly the same ab initio PES. The occurence of
quantum oscillations in the sticking probability as a function of kinetic
energy is addressed. They turn out to be very sensitive to the symmetry of the
initial conditions. At low kinetic energies sticking is dominated by the
steering effect which is illustrated using classical trajectories. The steering
effects depends on the kinetic energy, but not on the mass of the molecules.
Zero-point effects lead to strong differences between quantum and classical
calculations of the sticking probability. The dependence of the sticking
probability on the angle of incidence is analysed; it is found to be in good
agreement with experimental data. The results show that the determination of
the potential energy surface combined with high-dimensional dynamical
calculations, in which all relevant degrees of freedon are taken into account,
leads to a detailed understanding of the dissociation dynamics of hydrogen at a
transition metal surface.Comment: 15 pages, 9 figures, subm. to Phys. Rev.
Aharonov-Bohm-like effect for light propagating in nematics with disclinations
Using a geometric approach for the propagation of light in anisotropic media,
we investigate what effect the director field of disclinations may have on the
polarization state of light. Parallel transport around the defect, of the
spinor describing the polarization, indicates the acquisition of a topological
phase, in analogy with the Aharonov-Bohm effect.Comment: 6 pages, to appear in Europhysics Letter
A Search for OH Megamasers at z > 0.1. III. The Complete Survey
We present the final results from the Arecibo Observatory OH megamaser
survey. We discuss in detail the properties of the remaining 18 OH megamasers
detected in the survey, including 3 redetections. We place upper limits on the
OH emission from 85 nondetections and examine the properties of 25 ambiguous
cases for which the presence or absence of OH emission could not be determined.
The complete survey has discovered 50 new OH megamasers (OHMs) in
(ultra)luminous infrared galaxies ([U]LIRGs) which doubles the sample of known
OHMs and increases the sample at z>0.1 sevenfold. The Arecibo OH megamaser
survey indicates that the OHM fraction in LIRGs is an increasing function of
the far-IR luminosity (L_{FIR}) and far-IR color, reaching a fraction of
roughly one third in the warmest ULIRGs. Significant relationships between OHMs
and their hosts are few, primarily due to a mismatch in size scales of measured
properties and an intrinsic scatter in OHM properties roughly equal to the span
of the dataset. We investigate relationships between OHMs and their hosts with
a variety of statistical tools including survival analysis, partial correlation
coefficients, and a principal component analysis. There is no apparent OH
megamaser ``fundamental plane.'' We compile data on all previously known OHMs
and evaluate the possible mechanisms and relationships responsible for OHM
production in merging systems. The OH-FIR relationship is reexamined using the
doubled OHM sample and found to be significantly flatter than previously
thought: L_{OH} ~ L_{FIR}^{1.2 +/- 0.1}. This near-linear dependence suggests a
mixture of saturated and unsaturated masers, either within individual galaxies
or across the sample.Comment: 28 pages, 14 figures, accepted by AJ. (AASTeX, includes emulateapj5
and onecolfloat5
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