41,267 research outputs found
The stellar host in star-forming low-mass galaxies: Evidence for two classes
The morphological evolution of star-forming galaxies provides important clues
to understand their physical properties, as well as the triggering and
quenching mechanisms of star formation. We aim at connecting morphology and
star-formation properties of low-mass galaxies (median stellar mass
10 M) at low redshift ().
We use a sample of medium-band selected star-forming galaxies from the
GOODS-North field. H images for the sample are created combining both
spectral energy distribution fits and HST data. Using them, we mask the star
forming regions to obtain an unbiased two-dimensional model of the light
distribution of the host galaxies. For this purpose we use , a
new Bayesian photometric decomposition code. We apply it independently to 7 HST
bands assuming a S\'ersic surface brightness model.
Star-forming galaxy hosts show low S\'ersic index (with median
0.9), as well as small sizes (median 1.6 kpc), and negligible
change of the parameters with wavelength (except for the axis ratio, which
grows with wavelength). Using a clustering algorithm, we find two different
classes of star-forming galaxies: A more compact, redder, and high- (class
A) and a more extended, bluer and lower- one (class B). We also find
evidence that the first class is more spheroidal-like. In addition, we find
that 48% of the analyzed galaxies present negative color gradients (only 5% are
positive).
The host component of low-mass star-forming galaxies at separates
into two different classes, similar to what has been found for their higher
mass counterparts. The results are consistent with an evolution from class B to
class A. Several mechanisms from the literature, like minor and major mergers,
and violent disk instability, can explain the physical process behind the
likely transition between the classes. [abridged]Comment: Accepted for publication in Astronomy & Astrophysics. 13 pages, 11
figure
Mining the UKIDSS GPS: star formation and embedded clusters
Data mining techniques must be developed and applied to analyse the large
public data bases containing hundreds to thousands of millions entries. The aim
of this study is to develop methods for locating previously unknown stellar
clusters from the UKIDSS Galactic Plane Survey catalogue data. The cluster
candidates are computationally searched from pre-filtered catalogue data using
a method that fits a mixture model of Gaussian densities and background noise
using the Expectation Maximization algorithm. The catalogue data contains a
significant number of false sources clustered around bright stars. A large
fraction of these artefacts were automatically filtered out before or during
the cluster search. The UKIDSS data reduction pipeline tends to classify
marginally resolved stellar pairs and objects seen against variable surface
brightness as extended objects (or "galaxies" in the archive parlance). 10% or
66 x 10^6 of the sources in the UKIDSS GPS catalogue brighter than 17
magnitudes in the K band are classified as "galaxies". Young embedded clusters
create variable NIR surface brightness because the gas/dust clouds in which
they were formed scatters the light from the cluster members. Such clusters
appear therefore as clusters of "galaxies" in the catalogue and can be found
using only a subset of the catalogue data. The detected "galaxy clusters" were
finally screened visually to eliminate the remaining false detections due to
data artefacts. Besides the embedded clusters the search also located locations
of non clustered embedded star formation. The search covered an area of 1302
square degrees and 137 previously unknown cluster candidates and 30 previously
unknown sites of star formation were found
Multivariate Approaches to Classification in Extragalactic Astronomy
Clustering objects into synthetic groups is a natural activity of any
science. Astrophysics is not an exception and is now facing a deluge of data.
For galaxies, the one-century old Hubble classification and the Hubble tuning
fork are still largely in use, together with numerous mono-or bivariate
classifications most often made by eye. However, a classification must be
driven by the data, and sophisticated multivariate statistical tools are used
more and more often. In this paper we review these different approaches in
order to situate them in the general context of unsupervised and supervised
learning. We insist on the astrophysical outcomes of these studies to show that
multivariate analyses provide an obvious path toward a renewal of our
classification of galaxies and are invaluable tools to investigate the physics
and evolution of galaxies.Comment: Open Access paper.
http://www.frontiersin.org/milky\_way\_and\_galaxies/10.3389/fspas.2015.00003/abstract\>.
\<10.3389/fspas.2015.00003 \&g
Constant Approximation for -Median and -Means with Outliers via Iterative Rounding
In this paper, we present a new iterative rounding framework for many
clustering problems. Using this, we obtain an -approximation algorithm for -median with outliers, greatly
improving upon the large implicit constant approximation ratio of Chen [Chen,
SODA 2018]. For -means with outliers, we give an -approximation, which is the first -approximation for
this problem. The iterative algorithm framework is very versatile; we show how
it can be used to give - and -approximation
algorithms for matroid and knapsack median problems respectively, improving
upon the previous best approximations ratios of [Swamy, ACM Trans.
Algorithms] and [Byrka et al, ESA 2015].
The natural LP relaxation for the -median/-means with outliers problem
has an unbounded integrality gap. In spite of this negative result, our
iterative rounding framework shows that we can round an LP solution to an
almost-integral solution of small cost, in which we have at most two
fractionally open facilities. Thus, the LP integrality gap arises due to the
gap between almost-integral and fully-integral solutions. Then, using a
pre-processing procedure, we show how to convert an almost-integral solution to
a fully-integral solution losing only a constant-factor in the approximation
ratio. By further using a sparsification technique, the additive factor loss
incurred by the conversion can be reduced to any
The Phoenix Deep Survey: Extremely Red Galaxies and Cluster Candidates
We present the results of a study of a sample of 375 Extremely Red Galaxies
(ERGs) in the Phoenix Deep Survey, 273 of which constitute a subsample which is
80% complete to K_s = 18.5 over an area of 1160 arcmin^2. The angular
correlation function for ERGs is estimated, and the association of ERGs with
faint radio sources explored. We find tentative evidence that ERGs and faint
radio sources are associated at z > 0.5. A new overdensity-mapping algorithm
has been used to characterize the ERG distribution, and identify a number of
cluster candidates, including a likely cluster containing ERGs at 0.5 < z < 1.
Our algorithm is also used in an attempt to probe the environments in which
faint radio sources and ERGs are associated. We find limited evidence that the
I - K_s > 4 criterion is more efficient than R - K_s > 5 at selecting dusty
star-forming galaxies, rather than passively evolving ERGs.Comment: 14 emulateapj pages, 15 figures, 1 table, accepted for publication in
Astronomical Journal. A version with full resolution figures is available at
http://www.physics.usyd.edu.au/~asmith/research/ERGpaper.pd
Seedless clustering in all-sky searches for gravitational-wave transients
The problem of searching for unmodeled gravitational-wave bursts can be
thought of as a pattern recognition problem: how to find statistically
significant clusters in spectrograms of strain power when the precise signal
morphology is unknown. In a previous publication, we showed how "seedless
clustering" can be used to dramatically improve the sensitivity of searches for
long-lived gravitational-wave transients. In order to manage the computational
costs, this initial analysis focused on externally triggered searches where the
source location and emission time are both known to some degree of precision.
In this paper, we show how the principle of seedless clustering can be extended
to facilitate computationally-feasible, all-sky searches where the direction
and emission time of the source are entirely unknown. We further demonstrate
that it is possible to achieve a considerable reduction in computation time by
using graphical processor units (GPUs), thereby facilitating more sensitive
searches.Comment: 9 pages, 2 figure
Extended Object Tracking: Introduction, Overview and Applications
This article provides an elaborate overview of current research in extended
object tracking. We provide a clear definition of the extended object tracking
problem and discuss its delimitation to other types of object tracking. Next,
different aspects of extended object modelling are extensively discussed.
Subsequently, we give a tutorial introduction to two basic and well used
extended object tracking approaches - the random matrix approach and the Kalman
filter-based approach for star-convex shapes. The next part treats the tracking
of multiple extended objects and elaborates how the large number of feasible
association hypotheses can be tackled using both Random Finite Set (RFS) and
Non-RFS multi-object trackers. The article concludes with a summary of current
applications, where four example applications involving camera, X-band radar,
light detection and ranging (lidar), red-green-blue-depth (RGB-D) sensors are
highlighted.Comment: 30 pages, 19 figure
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