1,683 research outputs found
Neogeography: The Challenge of Channelling Large and Ill-Behaved Data Streams
Neogeography is the combination of user generated data and experiences with mapping technologies. In this article we present a research project to extract valuable structured information with a geographic component from unstructured user generated text in wikis, forums, or SMSes. The extracted information should be integrated together to form a collective knowledge about certain domain. This structured information can be used further to help users from the same domain who want to get information using simple question answering system. The project intends to help workers communities in developing countries to share their knowledge, providing a simple and cheap way to contribute and get benefit using the available communication technology
The Geometry of Mass Outflows and Fueling Flows in the Seyfert 2 Galaxy Mrk 3
We present a study of the resolved emission-line regions and an inner
dust/gas disk in the Seyfert 2 galaxy Mrk 3, based on Hubble Space Telescope
observations. We show that the extended narrow-line region (ENLR), spanning ~4
kpc, is defined by the intersection of the ionizing bicone of radiation from
the AGN and the inner disk, which is not coplanar with the large-scale stellar
disk. This intersection leads to different position and opening angles of the
ENLR compared to the narrow-line region (NLR). A number of emission-line arcs
in the ENLR appear to be continuations of dust lanes in the disk, supporting
this geometry. The NLR, which consists of outflowing emission-line knots
spanning the central ~650 pc, is in the shape of a backwards S. This shape may
arise from rotation of the gas, or it may trace the original fueling flow close
to the nucleus that was ionized after the AGN turned on.Comment: 22 page, 7 figures, accepted for publication in the Astronomical
Journa
Differential Games of Competition in Online Content Diffusion
Access to online contents represents a large share of the Internet traffic.
Most such contents are multimedia items which are user-generated, i.e., posted
online by the contents' owners. In this paper we focus on how those who provide
contents can leverage online platforms in order to profit from their large base
of potential viewers.
Actually, platforms like Vimeo or YouTube provide tools to accelerate the
dissemination of contents, i.e., recommendation lists and other re-ranking
mechanisms. Hence, the popularity of a content can be increased by paying a
cost for advertisement: doing so, it will appear with some priority in the
recommendation lists and will be accessed more frequently by the platform
users.
Ultimately, such acceleration mechanism engenders a competition among online
contents to gain popularity. In this context, our focus is on the structure of
the acceleration strategies which a content provider should use in order to
optimally promote a content given a certain daily budget. Such a best response
indeed depends on the strategies adopted by competing content providers. Also,
it is a function of the potential popularity of a content and the fee paid for
the platform advertisement service.
We formulate the problem as a differential game and we solve it for the
infinite horizon case by deriving the structure of certain Nash equilibria of
the game
The Kinematic Properties of the Extended Disks of Spiral Galaxies: A Sample of Edge-On Galaxies
We present a kinematic study of the outer regions (R_25<R<2 R_25) of 17
edge-on disk galaxies. Using deep long-slit spectroscopy (flux sensitivity a
few 10^-19 erg s^-1 cm^-2 arcsec^-2), we search for H-alpha emission, which
must be emitted at these flux levels by any accumulation of hydrogen due to the
presence of the extragalactic UV background and any other, local source of UV
flux. We present results from the individual galaxy spectra and a stacked
composite. We detect H-alpha in many cases well beyond R_25 and sometimes as
far as 2 R_25. The combination of sensitivity, spatial resolution, and
kinematic resolution of this technique thus provides a powerful complement to
21-cm observations. Kinematics in the outer disk are generally disk-like (flat
rotation curves, small velocity dispersions) at all radii, and there is no
evidence for a change in the velocity dispersion with radius. We place strong
limits, few percent, on the existence of counter-rotating gas out to 1.5 R_25.
These results suggest that thin disks extend well beyond R_25; however, we also
find a few puzzling anomalies. In ESO 323-G033 we find two emission regions
that have velocities close to the systemic velocity rather than the expected
rotation velocity. These low relative velocities are unlikely to be simply due
to projection effects and so suggest that these regions are not on disk-plane,
circular orbits. In MCG-01-31-002 we find emission from gas with a large
velocity dispersion that is co-rotating with the inner disk.Comment: 18 pages, 14 figures, accepted for publication in Ap
What Powers Ultra-luminous IRAS Galaxies?
We present an ISO SWS and ISOPHOT-S, mid-infrared spectroscopic survey of 15
ultra-luminous IRAS galaxies. We combine the survey results with a detailed
case study, based on near-IR and mm imaging spectroscopy, of one of the sample
galaxies (UGC 5101). We compare the near- and mid-IR characteristics of these
ultra-luminous galaxies to ISO and literature data of thirty starburst and
active galactic nuclei (AGN), template galaxies. We find that
1) 70-80% of the ultra-luminous IRAS galaxies in our sample are predominantly
powered by recently formed massive stars. 20-30% are powered by a central AGN.
These conclusions are based on a new infrared 'diagnostic diagram' involving
the ratio of high to low excitation mid-IR emission lines on the one hand, and
on the strength of the 7.7um PAH feature on the other hand.
2) at least half of the sources probably have simultaneously an active
nucleus and starburst activity in a 1-2 kpc diameter circum-nuclear disk/ring.
3) the mid-infrared emitting regions are highly obscured. After correction
for these extinctions, we estimate that the star forming regions in ULIRGs have
ages between 10^7 and 10^8 years, similar to but somewhat larger than those
found in lower luminosity starburst galaxies.
4) in the sample we have studied there is no obvious trend for the AGN
component to dominate in the most compact, and thus most advanced mergers.
Instead, at any given time during the merger evolution, the time dependent
compression of the circum-nuclear interstellar gas, the accretion rate onto the
central black hole and the associated radiation efficiency may determine
whether star formation or AGN activity dominates the luminosity of the system.Comment: 63 pages postscript (ex. MS Word), 11 postscript and 2 gif figures,
submitted to ApJ. See also
http://www.mpe-garching.mpg.de/ISO/preprint/MPE-IR-97003.htm
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