1,725 research outputs found
Deep rest-frame far-UV spectroscopy of the giant Lyman-alpha emitter 'Himiko'
We present deep 10h VLT/XSHOOTER spectroscopy for an extraordinarily luminous
and extended Lya emitter at z=6.595 referred to as Himiko and first discussed
by Ouchi et al. (2009), with the purpose of constraining the mechanisms
powering its strong emission. Complementary to the spectrum, we discuss NIR
imaging data from the CANDELS survey. We find neither for HeII nor any metal
line a significant excess, with 3 sigma upper limits of 6.8, 3.1, and
5.8x10^{-18} erg/s/cm^2 for CIV 1549, HeII 1640, CIII]
1909, respectively, assuming apertures with 200 km/s widths and offset
by -250 km/s w.r.t to the peak Lya redshift. These limits provide strong
evidence that an AGN is not a major contribution to Himiko's Lya flux. Strong
conclusions about the presence of PopIII star-formation or gravitational
cooling radiation are not possible based on the obtained HeII upper limit. Our
Lya spectrum confirms both spatial extent and flux (8.8+/-0.5x10^{-17}
erg/s/cm^2) of previous measurements. In addition, we can unambiguously exclude
any remaining chance of it being a lower redshift interloper by significantly
detecting a continuum redwards of Lya, while being undetected bluewards
Urban skylines from Schelling model
We propose a metapopulation version of the Schelling model where two kinds of
agents relocate themselves, with unconstrained destination, if their local
fitness is lower than a tolerance threshold. We show that, for small values of
the latter, the population redistributes highly heterogeneously among the
available places. The system thus stabilizes on these heterogeneous skylines
after a long quasi-stationary transient period, during which the population
remains in a well mixed phase. Varying the tolerance passing from large to
small values, we identify three possible global regimes: microscopic clusters
with local coexistence of both kinds of agents, macroscopic clusters with local
coexistence (soft segregation), macroscopic clusters with local segregation but
homogeneous densities (hard segregation). The model is studied numerically and
complemented with an analytical study in the limit of extremely large node
capacity.Comment: 16 pages, 10 figure
The Ionized Gas Kinematics of the LMC-Type Galaxy NGC 1427A in the Fornax Cluster
NGC 1427A is a LMC-like irregular galaxy in the Fornax cluster with an
extended pattern of strong star formation around one of its edges, which is
probably due to some kind of interaction with the cluster environment. We
present H-alpha velocities within NGC 1427A, obtained through long-slit
spectroscopy at seven different positions, chosen to fall on the brightest HII
regions of the galaxy. Due to its location very near the center of the cluster
this object is an excellent candidate to study the effects that the cluster
environment has on gas-rich galaxies embedded in it. The rotation of NGC 1427A
is modeled in two different ways. The global ionized gas kinematics is
reasonably well described by solid-body rotation, although on small scales it
shows a chaotic behaviour. In this simple model, the collision with a smaller
member of the cluster as being responsible for the peculiar morphology of NGC
1427A is very unlikely, since the only candidate intruder falls smoothly into
the general velocity pattern of the main galaxy. In a more elaborate model, for
which we obtain a better solution, this object does not lie in the same plane
of NGC 1427A, in which case we identify it as a satellite bound to the galaxy.
These results are discussed in the context of a normal irregular versus one
interacting with some external agent. Based on several arguments and
quantitative estimates, we argue that the passage through the hot intracluster
gas of the Fornax cluster is a very likely scenario to explain the
morphological properties of NGC 1427A.Comment: 31 pages, LaTeX2e, uses aas2pp4.sty and psfig.sty, including 7
Postscript figures; accepted for publication in ApJ, Vol. 530, February 200
Continuous Nearest Neighbor Queries over Sliding Windows
Abstract—This paper studies continuous monitoring of nearest neighbor (NN) queries over sliding window streams. According to this model, data points continuously stream in the system, and they are considered valid only while they belong to a sliding window that contains 1) the W most recent arrivals (count-based) or 2) the arrivals within a fixed interval W covering the most recent time stamps (time-based). The task of the query processor is to constantly maintain the result of long-running NN queries among the valid data. We present two processing techniques that apply to both count-based and time-based windows. The first one adapts conceptual partitioning, the best existing method for continuous NN monitoring over update streams, to the sliding window model. The second technique reduces the problem to skyline maintenance in the distance-time space and precomputes the future changes in the NN set. We analyze the performance of both algorithms and extend them to variations of NN search. Finally, we compare their efficiency through a comprehensive experimental evaluation. The skyline-based algorithm achieves lower CPU cost, at the expense of slightly larger space overhead. Index Terms—Location-dependent and sensitive, spatial databases, query processing, nearest neighbors, data streams, sliding windows.
A Magellan M2FS Spectroscopic Survey of Galaxies at 5.5<z<6.8: Program Overview and a Sample of the Brightest Lyman-alpha Emitters
We present a spectroscopic survey of high-redshift, luminous galaxies over
four square degrees on the sky, aiming to build a large and homogeneous sample
of Ly emitters (LAEs) at and 6.5, and Lyman-break
galaxies (LBGs) at . The fields that we choose to observe are
well-studied, such as SXDS and COSMOS. They have deep optical imaging data in a
series of broad and narrow bands, allowing efficient selection of galaxy
candidates. Spectroscopic observations are being carried out using the
multi-object spectrograph M2FS on the Magellan Clay telescope. M2FS is
efficient to identify high-redshift galaxies, owing to its 256 optical fibers
deployed over a circular field-of-view 30 arcmin in diameter. We have observed
square degrees. When the program is completed, we expect to identify
more than 400 bright LAEs at and 6.5, and a substantial number of
LBGs at . This unique sample will be used to study a variety of galaxy
properties and to search for large protoclusters. Furthermore, the statistical
properties of these galaxies will be used to probe cosmic reionization. We
describe the motivation, program design, target selection, and M2FS
observations. We also outline our science goals, and present a sample of the
brightest LAEs at and 6.5. This sample contains 32 LAEs with
Ly luminosities higher than 10 erg s. A few of them reach
erg s, comparable to the two most luminous LAEs
known at , `CR7' and `COLA1'. These LAEs provide ideal targets to study
extreme galaxies in the distant universe.Comment: 18 pages, 11 figures, Accepted for publication in The Astrophysical
Journa
Semantic Cross-View Matching
Matching cross-view images is challenging because the appearance and
viewpoints are significantly different. While low-level features based on
gradient orientations or filter responses can drastically vary with such
changes in viewpoint, semantic information of images however shows an invariant
characteristic in this respect. Consequently, semantically labeled regions can
be used for performing cross-view matching. In this paper, we therefore explore
this idea and propose an automatic method for detecting and representing the
semantic information of an RGB image with the goal of performing cross-view
matching with a (non-RGB) geographic information system (GIS). A segmented
image forms the input to our system with segments assigned to semantic concepts
such as traffic signs, lakes, roads, foliage, etc. We design a descriptor to
robustly capture both, the presence of semantic concepts and the spatial layout
of those segments. Pairwise distances between the descriptors extracted from
the GIS map and the query image are then used to generate a shortlist of the
most promising locations with similar semantic concepts in a consistent spatial
layout. An experimental evaluation with challenging query images and a large
urban area shows promising results
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