26,971 research outputs found
A Promising Change in the Selection of the Circular Polarization Excitation Used in the Measurement of Eu(III) Circularly Polarized Luminescence
A judicious change in the selected transition used for circular polarization excitation will overcome the low oscillator strength limitation of the currently allowed magnetic-dipole 5D1 ← 7F2 (Eu(III)) transition chosen for circularly polarized luminescence (CPL) measurement. The proposed allowed magnetic-dipole 5D1 ← 7F0 (Eu(III)) transition will facilitate the detection of CPL from the Eu(III) systems of interest. CPL on the acetonitrile solution of the chiral tris complex of Eu(III) with (R,R)-N,N′-bis(1-phenylethyl)-2,6-pyridinedicarboxamide ([Eu((R,R)-1)3]3+), recently suggested as an effective and reliable CPL calibrating agent, confirms the feasibility of the proposed experimental procedure. A comparable CPL activity exhibited by the acetonitrile solution of [Eu((R,R)-1)3]3+ following direct excitation in the spectral range of the 5D1 ← 7F0 transition and upon indirect excitation through the ligand absorption bands (λexc = 308 nm) was observed. This confirms that the recommended magnetic-dipole allowed absorption transition, 5D1 ← 7F0, is the transition to be considered in the measurement of CPL. This work provides critical direction for the continued instrumental improvements that can be done for developing CPL into a biomolecular structural probe
A high-resolution mm and cm study of the obscured LIRG NGC 4418 - A compact obscured nucleus fed by in-falling gas?
The aim of this study is to constrain the dynamics, structure and feeding of
the compact nucleous of NGC4418, and to reveal the nature of the main hidden
power source: starburst or AGN. We obtained high spatial resolution
observations of NGC4418 at 1.4 and 5 GHz with MERLIN, and at 230 and 270 GHz
with the SMA very extended configuration. We use the continuum morphology and
flux density to estimate the size of the emitting region, the star formation
rate and the dust temperature. Emission lines are used to study the kinematics
through position-velocity diagrams. Molecular emission is studied with
population diagrams and by fitting an LTE synthetic spectrum. We detect bright
1mm line emission from CO, HC3N, HNC and C34S, and 1.4 GHz absorption from HI.
The CO 2-1 emission and HI absorption can be fit by two velocity components at
2090 and 2180 km s-1. We detect vibrationally excited HC3N and HNC, with Tvib
300K. Molecular excitation is consistent with a layered temperature structure,
with three main components at 80, 160 and 300 K. For the hot component we
estimate a source size of less than 5 pc. The nuclear molecular gas surface
density of 1e4 Msun pc-2 is extremely high, and similar to that found in the
ultra-luminous infrared galaxy (ULIRG) Arp220. Our observations confirm the the
presence of a molecular and atomic in-flow, previously suggested by Herschel
observations, which is feeding the activity in the center of NGC4418. Molecular
excitation confirms the presence of a very compact, hot dusty core. If a
starburst is responsible for the observed IR flux, this has to be at least as
extreme as the one in Arp220, with an age of 3-10 Myr and a star formation rate
>10 Msun yr-1. If an AGN is present, it must be extremely Compton-thick.Comment: 18 pages, 11 figures, Accepted for publication by A&A on 10/6/201
Out of equilibrium quantum field dynamics of an initial thermal state after a change in the external field
The effects of the initial temperature in the out of equilibrium quantum
field dynamics in the presence of an homogeneous external field are
investigated. We consider an initial thermal state of temperature T for a
constant external field J. A subsequent sign flip of the external field, J to
-J, gives rise to an out of equilibrium nonperturbative quantum field dynamics.
The dynamics is studied here for the symmetry broken lambda(Phi^2)^2 scalar N
component field theory in the large N limit. We find a dynamical effective
potential for the expectation value that helps to understand the dynamics. The
dynamics presents two regimes defined by the presence or absence of a temporal
trapping close to the metastable equilibrium position of the potential. The two
regimes are separated by a critical value of the external field that depends on
the initial temperature. The temporal trapping is shorter for larger initial
temperatures or larger external fields. Parametric resonances and spinodal
instabilities amplify the quantum fluctuations in the field components
transverse to the external field. When there is a temporal trapping this is the
main mechanism that allows the system to escape from the metastable state for
large N. Subsequently backreaction stops the growth of the quantum fluctuations
and the system enters a quasiperiodic regime.Comment: LaTeX, 19 pages, 12 .eps figures, improved version to appear in Phys
Rev
A Low-cost Multi-channel Analogue Signal Generator
A scalable multi-channel analogue signal generator is presented. It uses a commercial low-cost graphics card with multiple outputs in a standard PC as signal source. Each color signal serves as independent channel to generate an analogue signal. A custom-built external PCB was developed to adjust the graphics card output voltage levels for a specific task, which needed differential signals. The system furthermore comprises a software package to program the signal shape. The implementation of the signal generator is presented as well as an application where it was successfully utilized
A precessing molecular jet signaling an obscured, growing supermassive black hole in NGC1377?
With high resolution (0."25 x 0."18) ALMA CO 3-2 observations of the nearby
(D=21 Mpc, 1"=102 pc), extremely radio-quiet galaxy NGC1377, we have discovered
a high-velocity, very collimated nuclear outflow which we interpret as a
molecular jet with a projected length of +-150 pc. Along the jet axis we find
strong velocity reversals where the projected velocity swings from -150 km/s to
+150 km/s. A simple model of a molecular jet precessing around an axis close to
the plane of the sky can reproduce the observations. The velocity of the
outflowing gas is difficult to constrain due to the velocity reversals but we
estimate it to be between 240 and 850 km/s and the jet to precess with a period
P=0.3-1.1 Myr. The CO emission is clumpy along the jet and the total molecular
mass in the high-velocity (+-(60 to 150 km/s)) gas lies between 2e6 Msun (light
jet) and 2e7 Msun (massive jet). There is also CO emission extending along the
minor axis of NGC1377. It holds >40% of the flux in NGC1377 and may be a
slower, wide-angle molecular outflow which is partially entrained by the
molecular jet.
We discuss the driving mechanism of the molecular jet and suggest that it is
either powered by a very faint radio jet or by an accretion disk-wind similar
to those found towards protostars. The nucleus of NGC1377 harbours intense
embedded activity and we detect emission from vibrationally excited HCN J=4-3
v_2=1f which is consistent with hot gas and dust. We find large columns of H2
in the centre of NGC1377 which may be a sign of a high rate of recent gas
infall. The dynamical age of the molecular jet is short (<1 Myr), which could
imply that it is young and consistent with the notion that NGC1377 is caught in
a transient phase of its evolution. However, further studies are required to
determine the age of the molecular jet, its mass and the role it is playing in
the growth of the nucleus of NGC1377.Comment: This is a revised and expanded version of a previous submission which
now has 13 pages, 6 figures (+ 4 in the Appendix) and is accepted for
publication in Astronomy & Astrophysic
A 3-D Multilateration: A Precision Geodetic Measurement System
A system was designed with the capability of determining 1-cm accuracy station positions in three dimensions using pulsed laser earth satellite tracking stations coupled with strictly geometric data reduction. With this high accuracy, several crucial geodetic applications become possible, including earthquake hazards assessment, precision surveying, plate tectonics, and orbital determination
Luminous, pc-scale CO 6-5 emission in the obscured nucleus of NGC1377
High resolution submm observations are important in probing the morphology,
column density and dynamics of obscured active galactic nuclei (AGNs). With
high resolution (0.06 x 0.05) ALMA 690 GHz observations we have found bright
(TB >80 K) and compact (FWHM 10x7 pc) CO 6-5 line emission in the nucleus of
the extremely radio-quiet galaxy NGC1377. The integrated CO 6-5 intensity is
aligned with the previously discovered jet/outflow of NGC1377 and is tracing
the dense (n>1e4 cm-3), hot gas at the base of the outflow. The velocity
structure is complex and shifts across the jet/outflow are discussed in terms
of jet-rotation or separate, overlapping kinematical components. High velocity
gas (deltaV +-145 km/s) is detected inside r<2-3 pc and we suggest that it is
emerging from an inclined rotating disk or torus of position angle PA=140+-20
deg with a dynamical mass of approx 3e6 Msun. This mass is consistent with that
of a supermassive black hole (SMBH), as inferred from the M-sigma relation. The
gas mass of the proposed disk/torus constitutes <3% of the nuclear dynamical
mass. In contrast to the intense CO 6-5 line emission, we do not detect dust
continuum with an upper limit of S(690GHz)<2mJy. The corresponding, 5 pc, H2
column density is estimated to N(H2)<3e23 cm-2, which is inconsistent with a
Compton Thick (CT) source. We discuss the possibility that CT obscuration may
be occuring on small (subparsec) or larger scales. From SED fitting we suggest
that half of the IR emission of NGC1377 is nuclear and the rest (mostly the
far-infrared (FIR)) is more extended. The extreme radio quietness, and the lack
of emission from other star formation tracers, raise questions on the origin of
the FIR emission. We discuss the possibility that it is arising from the
dissipation of shocks in the molecular jet/outflow or from irradiation by the
nuclear source along the poles.Comment: 7 pages, 5 figures, submitted to Astronomy and Astrophysic
Coherent versus Incoherent Light Scattering from a Quantum Dot
We analyze the light scattered by a single InAs quantum dot interacting with
a resonant continuous-wave laser. High resolution spectra reveal clear
distinctions between coherent and incoherent scattering, with the laser
intensity spanning over four orders of magnitude. We find that the fraction of
coherently scattered photons can approach unity under sufficiently weak or
detuned excitation, ruling out pure dephasing as a relevant decoherence
mechanism. We show how spectral diffusion shapes spectra, correlation
functions, and phase-coherence, concealing the ideal radiatively-broadened
two-level system described by Mollow.Comment: to appear in PRB 85, 23531
Instabilities and Clumping in Type Ia Supernova Remnants
We present two-dimensional high-resolution hydrodynamical simulations in
spherical polar coordinates of a Type Ia supernova interacting with a constant
density interstellar medium. The ejecta are assumed to be freely expanding with
an exponential density profile. The interaction gives rise to a double-shocked
structure susceptible to hydrodynamic instabilities. The Rayleigh-Taylor
instability initially grows, but the Kelvin-Helmholtz instability takes over,
producing vortex rings. The nonlinear instability initially evolves toward
longer wavelengths and eventually fades away when the reverse shock front is in
the flatter part of the supernova density distribution. Based on observations
of X-ray knots and the protrusion in the southeast outlin of Tycho's supernova
remnant, we include clumping in the ejecta. The clump interaction with the
reverse shock induces Rayleigh-Taylor and Kelvin-Helmholtz instabilities on the
clump surface that facilitate fragmentation. In order to survive crushing and
to have a bulging effect on the forward shock, the clump's initial density
ratio to the surrounding ejecta must be at least 100 for the conditions in
Tycho's remnant. The 56Ni bubble effect may be important for the development of
clumpiness in the ejecta. The observed presence of an Fe clump would then
require a non-radioactive origin for this Fe, possibly 54Fe. The large radial
distance of the X-ray emitting Si and S ejecta from the remnant center
indicates that they were initially in clumps.Comment: 27 pages, 4 postscript figures, 5 GIF figures submitted to
Astrophysical Journa
Exploring the molecular chemistry and excitation in obscured luminous infrared galaxies: An ALMA mm-wave spectral scan of NGC 4418
We obtained an ALMA Cycle 0 spectral scan of the dusty LIRG NGC 4418,
spanning a total of 70.7 GHz in bands 3, 6, and 7. We use a combined local
thermal equilibrium (LTE) and non-LTE (NLTE) fit of the spectrum in order to
identify the molecular species and derive column densities and excitation
temperatures. We derive molecular abundances and compare them with other
Galactic and extragalactic sources by means of a principal component analysis.
We detect 317 emission lines from a total of 45 molecular species, including 15
isotopic substitutions and six vibrationally excited variants. Our LTE/NLTE fit
find kinetic temperatures from 20 to 350 K, and densities between 10 and
10 cm. The spectrum is dominated by vibrationally excited HCN,
HCN, and HNC, with vibrational temperatures from 300 to 450 K. We find high
abundances of HCN, SiO, HS, and c-HCCCH and a low CHOH abundance. A
principal component analysis shows that NGC 4418 and Arp 220 share very similar
molecular abundances and excitation, which clearly set them apart from other
Galactic and extragalactic environments. The similar molecular abundances
observed towards NCG 4418 and Arp 220 are consistent with a hot gas-phase
chemistry, with the relative abundances of SiO and CHOH being regulated by
shocks and X-ray driven dissociation. The bright emission from vibrationally
excited species confirms the presence of a compact IR source, with an effective
diameter 350 K. The molecular abundances
and the vibrationally excited spectrum are consistent with a young
AGN/starburst system. We suggest that NGC 4418 may be a template for a new kind
of chemistry and excitation, typical of compact obscured nuclei (CON). Because
of the narrow line widths and bright molecular emission, NGC 4418 is the ideal
target for further studies of the chemistry in CONs.Comment: accepted by A&A on 29/06/201
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