27,980 research outputs found
The excitation of near-infrared H2 emission in NGC 253
Because of its large angular size and proximity to the Milky Way, NGC 253, an
archetypal starburst galaxy, provides an excellent laboratory to study the
intricacies of this intense episode of star formation. We aim to characterize
the excitation mechanisms driving the emission in NGC 253. Specifically we aim
to distinguish between shock excitation and UV excitation as the dominant
driving mechanism, using Br\gamma, H_2 and [FeII] as diagnostic emission line
tracers. Using SINFONI observations, we create linemaps of Br\gamma,
[FeII]_{1.64}, and all detected H_2 transitions. By using symmetry arguments of
the gas and stellar gas velocity field, we find a kinematic center in agreement
with previous determinations. The ratio of the 2-1 S(1) to 1-0 S(1) H_2
transitions can be used as a diagnostic to discriminate between shock and
fluorescent excitation. Using the 1-0 S(1)/2-1 S(1) line ratio as well as
several other H_2 line ratios and the morphological comparison between H_2 and
Br\gamma and [FeII], we find that excitation from UV photons is the dominant
excitation mechanisms throughout NGC 253. We employ a diagnostic energy level
diagram to quantitatively differentiate between mechanisms. We compare the
observed energy level diagrams to PDR and shock models and find that in most
regions and over the galaxy as a whole, fluorescent excitation is the dominant
mechanism exciting the H_2 gas. We also place an upper limit of the percentage
of shock excited H_2 at 29%. We find that UV radiation is the dominant
excitation mechanism for the H_2 emission. The H_2 emission does not correlate
well with Br\gamma but closely traces the PAH emission, showing that not only
is H_2 fluorescently excited, but it is predominately excited by slightly lower
mass stars than O stars which excite Br\gamma, such as B stars
Neural spike train synchronization indices: Definitions, interpretations, and applications
A comparison of previously defined spike train synchronization indices is undertaken within a stochastic point process framework. The second-order cumulant density (covariance density) is shown to be common to all the indices. Simulation studies were used to investigate the sampling variability of a single index based on the second-order cumulant. The simulations used a paired motoneurone model and a paired regular spiking cortical neurone model. The sampling variability of spike trains generated under identical conditions from the paired motoneurone model varied from 50% to 160% of the estimated value. On theoretical grounds, and on the basis of simulated data a rate dependence is present in all synchronization indices. The application of coherence and pooled coherence estimates to the issue of synchronization indices is considered. This alternative frequency domain approach allows an arbitrary number of spike train pairs to be evaluated for statistically significant differences, and combined into a single population measure. The pooled coherence framework allows pooled time domain measures to be derived, application of this to the simulated data is illustrated. Data from the cortical neurone model is generated over a wide range of firing rates (1-250 spikes/s). The pooled coherence framework correctly characterizes the sampling variability as not significant over this wide operating range. The broader applicability of this approach to multielectrode array data is briefly discussed
Social and Economic Decline as Factors in Conflict in the Caucasus
We argue that the conflicts in the Caucasus are the result of the abrogation by the elite of the earlier, Soviet era, social contract. This process was accompanied by the collapse of the formal economy; evidenced by huge national income compression, falling public goods provision, and growing inequality and poverty. In the absence of state provision of basic amenities and governance, ordinary people are compelled to fall back on kinship ties. Declining standards of governance facilitate state-sponsored corruption and criminality in a setting where the shadow economic activity is increasingly important to individual survival strategies. Oil pipelines and the right to control the transit of goods both legal and illegal also underlie conflict in the region. Criminality has replaced ethnicity as the major motivation for conflict and conflict per se has become a lucrative source of income.Caucasus, conflict, natural resources
Neutral carbon and CO in 76 (U)LIRGs and starburst galaxy centers A method to determine molecular gas properties in luminous galaxies
We present fluxes in both neutral carbon [CI] lines at the centers of 76
galaxies with FIR luminosities between 10^{9} and 10^{12} L(o) obtained with
Herschel-SPIRE and with ground-based facilities, along with the J=7-6, J=4-3,
J=2-1 12CO and J=2-1 13CO line fluxes. We investigate whether these lines can
be used to characterize the molecular ISM of the parent galaxies in simple ways
and how the molecular gas properties define the model results. In most
starburst galaxies, the [CI]/13CO flux ratio is much higher than in Galactic
star-forming regions, and it is correlated to the total FIR luminosity. The
[CI](1-0)/CO(4-3), the [CI](2-1) (2-1)/CO(7-6), and the [CI] (2-1)/(1-0) flux
ratios are also correlated, and trace the excitation of the molecular gas. In
the most luminous infrared galaxies (LIRGs), the ISM is fully dominated by
dense and moderately warm gas clouds that appear to have low [C]/[CO] and
[13CO]/[12CO] abundances. In less luminous galaxies, emission from gas clouds
at lower densities becomes progressively more important, and a multiple-phase
analysis is required to determine consistent physical characteristics. Neither
the CO nor the [CI] velocity-integrated line fluxes are good predictors of H2
column densities in individual galaxies, and X(CI) conversion factors are not
superior to X(CO) factors. The methods and diagnostic diagrams outlined in this
paper also provide a new and relatively straightforward means of deriving the
physical characteristics of molecular gas in high-redshift galaxies up to z=5,
which are otherwise hard to determine
Helicity dynamics in stratified turbulence in the absence of forcing
A numerical study of decaying stably-stratified flows is performed.
Relatively high stratification and moderate Reynolds numbers are considered,
and a particular emphasis is placed on the role of helicity (velocity-vorticity
correlations). The problem is tackled by integrating the Boussinesq equations
in a periodic cubical domain using different initial conditions: a non-helical
Taylor-Green (TG) flow, a fully helical Beltrami (ABC) flow, and random flows
with a tunable helicity. We show that for stratified ABC flows helicity
undergoes a substantially slower decay than for unstratified ABC flows. This
fact is likely associated to the combined effect of stratification and large
scale coherent structures. Indeed, when the latter are missing, as in random
flows, helicity is rapidly destroyed by the onset of gravitational waves. A
type of large-scale dissipative "cyclostrophic" balance can be invoked to
explain this behavior. When helicity survives in the system it strongly affects
the temporal energy decay and the energy distribution among Fourier modes. We
discover in fact that the decay rate of energy for stratified helical flows is
much slower than for stratified non-helical flows and can be considered with a
phenomenological model in a way similar to what is done for unstratified
rotating flows. We also show that helicity, when strong, has a measurable
effect on the Fourier spectra, in particular at scales larger than the buoyancy
scale for which it displays a rather flat scaling associated with vertical
shear
Large-scale anisotropy in stably stratified rotating flows
We present results from direct numerical simulations of the Boussinesq
equations in the presence of rotation and/or stratification, both in the
vertical direction. The runs are forced isotropically and randomly at small
scales and have spatial resolutions of up to grid points and Reynolds
numbers of . We first show that solutions with negative energy
flux and inverse cascades develop in rotating turbulence, whether or not
stratification is present. However, the purely stratified case is characterized
instead by an early-time, highly anisotropic transfer to large scales with
almost zero net isotropic energy flux. This is consistent with previous studies
that observed the development of vertically sheared horizontal winds, although
only at substantially later times. However, and unlike previous works, when
sufficient scale separation is allowed between the forcing scale and the domain
size, the total energy displays a perpendicular (horizontal) spectrum with
power law behavior compatible with , including in the
absence of rotation. In this latter purely stratified case, such a spectrum is
the result of a direct cascade of the energy contained in the large-scale
horizontal wind, as is evidenced by a strong positive flux of energy in the
parallel direction at all scales including the largest resolved scales
Isotropisation at small scales of rotating helically-driven turbulence
We present numerical evidence of how three-dimensionalization occurs at small
scale in rotating turbulence with Beltrami (ABC) forcing, creating helical
flow. The Zeman scale at which the inertial and eddy turn-over
times are equal is more than one order of magnitude larger than the dissipation
scale, with the relevant domains (large-scale inverse cascade of energy, dual
regime in the direct cascade of energy and helicity , and dissipation)
each moderately resolved. These results stem from the analysis of a large
direct numerical simulation on a grid of points, with Rossby and
Reynolds numbers respectively equal to 0.07 and . At scales
smaller than the forcing, a helical wave-modulated inertial law for the energy
and helicity spectra is followed beyond by Kolmogorov spectra
for and . Looking at the two-dimensional slow manifold, we also show
that the helicity spectrum breaks down at , a clear sign of
recovery of three-dimensionality in the small scales.Comment: 13 pages, 6 figure
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