9,826 research outputs found
Time-resolved infrared emission from radiation-driven central obscuring structures in Active Galactic Nuclei
The central engines of Seyfert galaxies are thought to be enshrouded by
geometrically thick gas and dust structures. In this article, we derive
observable properties for a self-consistent model of such toroidal gas and dust
distributions, where the geometrical thickness is achieved and maintained with
the help of X-ray heating and radiation pressure due to the central engine.
Spectral energy distributions (SEDs) and images are obtained with the help of
dust continuum radiative transfer calculations with RADMC-3D. For the first
time, we are able to present time-resolved SEDs and images for a physical model
of the central obscurer. Temporal changes are mostly visible at shorter
wavelengths, close to the combined peak of the dust opacity as well as the
central source spectrum and are caused by variations in the column densities of
the generated outflow. Due to the three-component morphology of the
hydrodynamical models -- a thin disc with high density filaments, a surrounding
fluffy component (the obscurer) and a low density outflow along the rotation
axis -- we find dramatic differences depending on wavelength: whereas the
mid-infrared images are dominated by the elongated appearance of the outflow
cone, the long wavelength emission is mainly given by the cold and dense disc
component. Overall, we find good agreement with observed characteristics,
especially for those models, which show clear outflow cones in combination with
a geometrically thick distribution of gas and dust, as well as a geometrically
thin, but high column density disc in the equatorial plane.Comment: 16 pages, 12 figures, accepted for publication in MNRA
Line formation in solar granulation VI. [C I], C I, CH and C2 lines and the photospheric C abundance
The solar photospheric carbon abundance has been determined from [C I], C I,
CH vibration-rotation, CH A-X electronic and C2 Swan electronic lines by means
of a time-dependent, 3D, hydrodynamical model of the solar atmosphere.
Departures from LTE have been considered for the C I lines. These turned out to
be of increasing importance for stronger lines and are crucial to remove a
trend in LTE abundances with the strengths of the lines. Very gratifying
agreement is found among all the atomic and molecular abundance diagnostics in
spite of their widely different line formation sensitivities. The mean of the
solar carbon abundance based on the four primary abundance indicators ([C I], C
I, CH vibration-rotation, C_2 Swan) is log C = 8.39 +/- 0.05, including our
best estimate of possible systematic errors. Consistent results also come from
the CH electronic lines, which we have relegated to a supporting role due to
their sensitivity to the line broadening. The new 3D based solar C abundance is
significantly lower than previously estimated in studies using 1D model
atmospheres.Comment: Accepted for A&A, 13 page
Compressed k2-Triples for Full-In-Memory RDF Engines
Current "data deluge" has flooded the Web of Data with very large RDF
datasets. They are hosted and queried through SPARQL endpoints which act as
nodes of a semantic net built on the principles of the Linked Data project.
Although this is a realistic philosophy for global data publishing, its query
performance is diminished when the RDF engines (behind the endpoints) manage
these huge datasets. Their indexes cannot be fully loaded in main memory, hence
these systems need to perform slow disk accesses to solve SPARQL queries. This
paper addresses this problem by a compact indexed RDF structure (called
k2-triples) applying compact k2-tree structures to the well-known
vertical-partitioning technique. It obtains an ultra-compressed representation
of large RDF graphs and allows SPARQL queries to be full-in-memory performed
without decompression. We show that k2-triples clearly outperforms
state-of-the-art compressibility and traditional vertical-partitioning query
resolution, remaining very competitive with multi-index solutions.Comment: In Proc. of AMCIS'201
Relativistic and slowing down: the flow in the hotspots of powerful radio galaxies and quasars
Pairs of radio emitting jets with lengths up to several hundred kiloparsecs
emanate from the central region (the `core') of radio loud active galaxies.
In the most powerful of them, these jets terminate in the `hotspots', compact
high brightness regions, where the jet flow collides with the intergalactic
medium (IGM). Although it has long been established that in their inner
(parsec) regions these jet flows are relativistic, it is still not clear
if they remain so at their largest (hundreds of kiloparsec) scales. We argue
that the X-ray, optical and radio data of the hotspots, despite their
at-first-sight disparate properties, can be unified in a scheme involving a
relativistic flow upstream of the hotspot that decelerates to the
sub-relativistic speed of its inferred advance through the IGM and viewed at
different angles to its direction of motion. This scheme, besides providing an
account of the hotspot spectral properties with jet orientation, it also
suggests that the large-scale jets remain relativistic all the way to the
hotspots.Comment: to appear in ApJ
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