9,448 research outputs found
Inflowing gas onto a compact obscured nucleus in Arp 299A: Herschel spectroscopic studies of H2O and OH
Aims. We probe the physical conditions in the core of Arp 299A and try to put
constraints to the nature of its nuclear power source. Methods. We used
Herschel Space Observatory far-infrared and submillimeter observations of H2O
and OH rotational lines in Arp 299A to create a multi-component model of the
galaxy. In doing this, we employed a spherically symmetric radiative transfer
code. Results. Nine H2O lines in absorption and eight in emission as well as
four OH doublets in absorption and one in emission, are detected in Arp 299A.
No lines of the 18O isotopologues, which have been seen in compact obscured
nuclei of other galaxies, are detected. The absorption in the ground state OH
doublet at 119 {\mu}m is found redshifted by ~175 km/s compared to other OH and
H2O lines, suggesting a low excitation inflow. We find that at least two
components are required in order to account for the excited molecular line
spectrum. The inner component has a radius of 20-25 pc, a very high infrared
surface brightness (> 3e13 Lsun/kpc^2), warm dust (Td > 90 K), and a large H2
column density (NH2 > 1e24 cm^-2). The outer component is larger (50-100 pc)
with slightly cooler dust (70-90 K). In addition, a much more extended
inflowing component is required to also account for the OH doublet at 119
{\mu}m. Conclusions. The Compton-thick nature of the core makes it difficult to
determine the nature of the buried power source, but the high surface
brightness indicates that it is either an active galactic nucleus and/or a
dense nuclear starburst. The high OH/H2O ratio in the nucleus indicates that
ion-neutral chemistry induced by X-rays or cosmic-rays is important. Finally we
find a lower limit to the 16O/18O ratio of 400 in the nuclear region, possibly
indicating that the nuclear starburst is in an early evolutionary stage, or
that it is fed through a molecular inflow of, at most, solar metallicity.Comment: 14 pages, 13 figures, Accepted for publication in Astronomy and
Astrophysic
Debt management in Brazil : evaluation of the Real Plan and challenges ahead
Brazil's domestic debt has posed two challenges to policymakers: it has grown very fast and, despite progress, remains extremely short in maturity. The authors analyze Brazil's experience with domestic public debt management, searching for policy prescriptions for the next few years. After briefly reviewing the recent history of the country's domestic debt, they decompose the large rise in federal bonded debt in 1995-98, searching for its macroeconomic causes. The main explanations: extremely high interest payments (caused by Brazil's weak fiscal stance and quasi-fixed exchange rate regime) and the accumulation of assets (especially obligations of Brazil's states). Simulations of the net debt path for the near future underscore the importance of a tighter fiscal stance to prevent the debt-to-GDP ratio from growing further. The authors'main policy advice is to foster and rely more on inflation-linked bonds--the least harmful way to lengthen debt maturity.Economic Theory&Research,Banks&Banking Reform,Public Sector Economics&Finance,Payment Systems&Infrastructure,Strategic Debt Management,Economic Theory&Research,Banks&Banking Reform,Strategic Debt Management,Public Sector Economics&Finance,Municipal Financial Management
Unity of pomerons from gauge/string duality
We develop a formalism where the hard and soft pomeron contributions to high
energy scattering arise as leading Regge poles of a single kernel in
holographic QCD. The kernel is obtained using effective field theory inspired
by Regge theory of a 5-d string theory. It describes the exchange of higher
spin fields in the graviton Regge trajectory that are dual to glueball states
of twist two. For a specific holographic QCD model we describe Deep Inelastic
Scattering in the Regge limit of low Bjorken x, finding good agreement with
experimental data from HERA. The observed rise of the effective pomeron
intercept, as the size of the probe decreases, is reproduced by considering the
first four pomeron trajectories. In the case of soft probes, relevant to total
cross sections, the leading hard pomeron trajectory is suppressed, such that in
this kinematical region we reproduce an intercept of 1.09 compatible with the
QCD soft pomeron data. In the spectral region of positive Maldelstam variable t
the first two pomeron trajectories are consistent with current expectations for
the glueball spectrum from lattice simulations
Modeling the H2O submillimeter emission in extragalactic sources
Recent observational studies have shown that H2O emission at (rest)
submillimeter wavelengths is ubiquitous in infrared galaxies, both in the local
and in the early Universe, suggestive of far-infrared pumping of H2O by dust in
warm regions. In this work, models are presented that show that (i) the
highest-lying H2O lines (E_{upper}>400 K) are formed in very warm (T_{dust}>~90
K) regions and require high H2O columns (N_{H2O}>~3x10^{17} cm^{-2}), while
lower lying lines can be efficiently excited with T_{dust}~45-75 K and
N_{H2O}~(0.5-2)x10^{17} cm^{-2}; (ii) significant collisional excitation of the
lowest lying (E_{upper}<200 K) levels, which enhances the overall
L_{H2O}-L_{IR} ratios, is identified in sources where the ground-state para-H2O
1_{11}-0_{00} line is detected in emission; (iii) the H2O-to-infrared (8-1000
um) luminosity ratio is expected to decrease with increasing T_{dust} for all
lines with E_{upper}<~300 K, as has recently been reported in a sample of
LIRGs, but increases with T_{dust} for the highest lying H2O lines
(E_{upper}>400 K); (iv) we find theoretical upper limits for L_{H2O}/L_{IR} in
warm environments, owing to H2O line saturation; (v) individual models are
presented for two very different prototypical galaxies, the Seyfert 2 galaxy
NGC 1068 and the nearest ultraluminous infrared galaxy Arp 220, showing that
the excited submillimeter H2O emission is dominated by far-infrared pumping in
both cases; (vi) the L_{H2O}-L_{IR} correlation previously reported in
observational studies indicates depletion or exhaustion time scales,
t_{dep}=Sigma_{gas}/Sigma_{SFR}, of <~12 Myr for star-forming sources where
lines up to E_{upper}=300 K are detected, in agreement with the values
previously found for (U)LIRGs from HCN millimeter emission...Comment: 13 pages, 13 figure
Impact of Power Allocation and Antenna Directivity in the Capacity of a Multiuser Cognitive Ad Hoc Network
This paper studies the benefits that power control and antenna directivity can bring to the capacity of a multiuser cognitive radio network. The main objective is to optimize the secondary network sum rate under the capacity constraint of the primary network. Exploiting location awareness, antenna directivity, and the power control capability, the cognitive radio ad hoc network can broaden its coverage and improve capacity. Computer simulations show that by employing the proposed method the system performance is significantly enhanced compared to conventional fixed power allocation
Cancer therapeutic potential of combinatorial immuno- and vaso-modulatory interventions
Currently, most of the basic mechanisms governing tumor-immune system
interactions, in combination with modulations of tumor-associated vasculature,
are far from being completely understood. Here, we propose a mathematical model
of vascularized tumor growth, where the main novelty is the modeling of the
interplay between functional tumor vasculature and effector cell recruitment
dynamics. Parameters are calibrated on the basis of different in vivo
immunocompromised Rag1-/- and wild-type (WT) BALB/c murine tumor growth
experiments. The model analysis supports that tumor vasculature normalization
can be a plausible and effective strategy to treat cancer when combined with
appropriate immuno-stimulations. We find that improved levels of functional
tumor vasculature, potentially mediated by normalization or stress alleviation
strategies, can provide beneficial outcomes in terms of tumor burden reduction
and growth control. Normalization of tumor blood vessels opens a therapeutic
window of opportunity to augment the antitumor immune responses, as well as to
reduce the intratumoral immunosuppression and induced-hypoxia due to vascular
abnormalities. The potential success of normalizing tumor-associated
vasculature closely depends on the effector cell recruitment dynamics and tumor
sizes. Furthermore, an arbitrary increase of initial effector cell
concentration does not necessarily imply a better tumor control. We evidence
the existence of an optimal concentration range of effector cells for tumor
shrinkage. Based on these findings, we suggest a theory-driven therapeutic
proposal that optimally combines immuno- and vaso-modulatory interventions
Soft Pomeron in Holographic QCD
We study the graviton Regge trajectory in Holographic QCD as a model for high
energy scattering processes dominated by soft pomeron exchange. This is done by
considering spin J fields from the closed string sector that are dual to
glueball states of even spin and parity. In particular, we construct a model
that governs the analytic continuation of the spin J field equation to the
region of real J < 2, which includes the scattering domain of negative
Maldelstam variable t. The model leads to approximately linear Regge
trajectories and is compatible with the measured values of 1.08 for the
intercept and 0.25 GeV for the slope of the soft pomeron. The intercept
of the secondary pomeron trajectory is in the same region of the subleading
trajectories, made of mesons, proposed by Donnachie and Landshoff, and should
therefore be taken into account.Comment: 7 pages, 5 figures. V2 : The paper has been expanded to provide more
details of the model and results. Added two new figures and two new
references; corrected typo
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