510,158 research outputs found
On the Accuracy of Hyper-local Geotagging of Social Media Content
Social media users share billions of items per year, only a small fraction of
which is geotagged. We present a data- driven approach for identifying
non-geotagged content items that can be associated with a hyper-local
geographic area by modeling the location distributions of hyper-local n-grams
that appear in the text. We explore the trade-off between accuracy, precision
and coverage of this method. Further, we explore differences across content
received from multiple platforms and devices, and show, for example, that
content shared via different sources and applications produces significantly
different geographic distributions, and that it is best to model and predict
location for items according to their source. Our findings show the potential
and the bounds of a data-driven approach to geotag short social media texts,
and offer implications for all applications that use data-driven approaches to
locate content.Comment: 10 page
The diverse hot gas content and dynamics of optically similar low-mass elliptical galaxies
The presence of hot X-ray emitting gas is ubiquitous in massive early-type
galaxies. However, much less is known about the content and physical status of
the hot X-ray gas in low-mass ellipticals. In the present paper we study the
X-ray gas content of four low-mass elliptical galaxies using archival Chandra
X-ray observations. The sample galaxies, NGC821, NGC3379, NGC4278, and NGC4697,
have approximately identical K-band luminosities, and hence stellar masses, yet
their X-ray appearance is strikingly different. We conclude that the unresolved
emission in NGC821 and NGC3379 is built up from a multitude of faint compact
objects, such as coronally active binaries and cataclysmic variables. Despite
the non-detection of X-ray gas, these galaxies may host low density, and hence
low luminosity, X-ray gas components, which undergo a Type Ia supernova (SN Ia)
driven outflow. We detect hot X-ray gas with a temperature of kT ~ 0.35 keV in
NGC4278, the component of which has a steeper surface brightness distribution
than the stellar light. Within the central 50 arcsec (~3.9 kpc) the estimated
gas mass is ~3 x 10^7 M_sun, implying a gas mass fraction of ~0.06%. We
demonstrate that the X-ray gas exhibits a bipolar morphology in the
northeast-southwest direction, indicating that it may be outflowing from the
galaxy. The mass and energy budget of the outflow can be maintained by evolved
stars and SNe Ia, respectively. The X-ray gas in NGC4697 has an average
temperature of kT ~ 0.3 keV, and a significantly broader distribution than the
stellar light. The total gas mass within 90 arcsec (~5.1 kpc) is ~2.1 x 10^8
M_sun, hence the gas mass fraction is ~0.4%. Based on the distribution and
physical parameters of the X-ray gas, we conclude that it is most likely in
hydrostatic equilibrium, although a subsonic outflow may be present.Comment: 14 pages, 8 figures, 3 tables, accepted for publication in Ap
Galactic outflows and the kinematics of damped Lyman alpha absorbers
The kinematics of damped Lyman alpha absorbers (DLAs) are difficult to
reproduce in hierarchical galaxy formation models, particularly the
preponderance of wide systems. We investigate DLA kinematics at z=3 using
high-resolution cosmological hydrodynamical simulations that include a
heuristic model for galactic outflows. Without outflows, our simulations fail
to yield enough wide DLAs, as in previous studies. With outflows, predicted DLA
kinematics are in much better agreement with observations. Comparing two
outflow models, we find that a model based on momentum-driven wind scalings
provides the best match to the observed DLA kinematic statistics of Prochaska &
Wolfe. In this model, DLAs typically arise a few kpc away from galaxies that
would be identified in emission. Narrow DLAs can arise from any halo and galaxy
mass, but wide ones only arise in halos with mass >10^11 Mo, from either large
central or small satellite galaxies. This implies that the success of this
outflow model originates from being most efficient at pushing gas out from
small satellite galaxies living in larger halos. This increases the
cross-section for large halos relative to smaller ones, thereby yielding wider
kinematics. Our simulations do not include radiative transfer effects or
detailed metal tracking, and outflows are modeled heuristically, but they
strongly suggest that galactic outflows are central to understanding DLA
kinematics. An interesting consequence is that DLA kinematics may place
constraints on the nature and efficiency of gas ejection from high-z galaxies.Comment: submitted to MNRA
Machine Learning meets Data-Driven Journalism: Boosting International Understanding and Transparency in News Coverage
Migration crisis, climate change or tax havens: Global challenges need global
solutions. But agreeing on a joint approach is difficult without a common
ground for discussion. Public spheres are highly segmented because news are
mainly produced and received on a national level. Gain- ing a global view on
international debates about important issues is hindered by the enormous
quantity of news and by language barriers. Media analysis usually focuses only
on qualitative re- search. In this position statement, we argue that it is
imperative to pool methods from machine learning, journalism studies and
statistics to help bridging the segmented data of the international public
sphere, using the Transatlantic Trade and Investment Partnership (TTIP) as a
case study.Comment: presented at 2016 ICML Workshop on #Data4Good: Machine Learning in
Social Good Applications, New York, N
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