35,098 research outputs found
Real-time Finite Fault Rupture Detector (FinDer) for large earthquakes
To provide rapid estimates of fault rupture extent during large earthquakes, we have developed the Finite Fault Rupture Detector algorithm, âFinDerâ. FinDer uses image recognition techniques to detect automatically surface-projected fault ruptures in real-time (assuming a line source) by estimating their current centroid position, length L, and strike Ξ. The approach is based on a rapid high-frequency near/far-source classification of ground motion amplitudes in a dense seismic network (station spacing <50 km), and comparison with a set of pre-calculated templates using âMatching by Correlationâ. To increase computational efficiency, we perform the correlation in the wavenumber domain. FinDer keeps track of the current dimensions of a rupture in progress. Errors in L are typically on the same order as station spacing in the network. The continuously updated estimates of source geometries as provided by FinDer make predicted shaking intensities more accurate and thus more useful for earthquake early warning, ShakeMaps, and related products. The applicability of the algorithm is demonstrated for several recorded and simulated earthquakes with different focal mechanisms, including the 2009 M_w 6.3 LâAquila (Italy), the 1999 M_w 7.6 ChiChi (Taiwan) and the M_w 7.8 ShakeOut scenario earthquake on the southern San Andreas Fault (California)
Interactions of Satellite Galaxies in Cosmological Dark Matter Halos
We present a statistical analysis of the interactions between satellite
galaxies in cosmological dark matter halos taken from fully self-consistent
high-resolution simulations of galaxy clusters. We show that the number
distribution of satellite encounters has a tail that extends to as many as 3-4
encounters per orbit. On average 30% of the substructure population had at
least one encounter (per orbit) with another satellite galaxy. However, this
result depends on the age of the dark matter host halo with a clear trend for
more interactions in younger systems. We also report a correlation between the
number of encounters and the distance of the satellites to the centre of the
cluster: satellite galaxies closer to the centre experience more interactions.
However, this can be simply explained by the radial distribution of the
substructure population and merely reflects the fact that the density of
satellites is higher in those regions.
In order to find substructure galaxies we applied (and present) a new
technique based upon the N-body code MLAPM. This new halo finder MHF
(MLAPM's-Halo-Finder) acts with exactly the same accuracy as the N-body code
itself and is therefore free of any bias and spurious mismatch between
simulation data and halo finding precision related to numerical effects.Comment: 6 pages, 4 figures, accepted by PASA (refereed contribution to the
5th Galactic Chemodynamics workshop, July 2003
The DEEP2 Galaxy Redshift Survey: Clustering of Groups and Group Galaxies at z~1
We study the clustering properties of groups and of galaxies in groups in the
DEEP2 Galaxy Redshift Survey dataset at z~1. Four clustering measures are
presented: 1) the group correlation function for 460 groups with estimated
velocity dispersions of sigma>200 km/s, 2) the galaxy correlation for the full
galaxy sample, using a flux-limited sample of 9800 objects between 0.7<z<1.0,
3) the galaxy correlation for galaxies in groups, and 4) the group-galaxy
cross-correlation function. Using the observed number density and clustering
amplitude of the groups, the estimated minimum group dark matter halo mass is
M_min~6 10^12 h^-1 M_Sun for a flat LCDM cosmology. Groups are more clustered
than galaxies, with a relative bias of b=1.7 +/-0.04 on scales r_p=0.5-15
Mpc/h. Galaxies in groups are also more clustered than the full galaxy sample,
with a scale-dependent relative bias which falls from b~2.5 +/-0.3 at r_p=0.1
Mpc/h to b~1 +/-0.5 at r_p=10 Mpc/h. The correlation functions for all galaxies
and galaxies in groups can be fit by a power-law on scales r_p=0.05-20 Mpc/h.
We empirically measure the contribution to the projected correlation function
for galaxies in groups from a `one-halo' term and a `two-halo' term by counting
pairs of galaxies in the same or in different groups. The projected
cross-correlation between shows that red galaxies are more centrally
concentrated in groups than blue galaxies at z~1. DEEP2 galaxies in groups
appear to have a shallower radial distribution than that of mock galaxy
catalogs made from N-body simulations, which assume a central galaxy surrounded
by satellite galaxies with an NFW profile. We show that the clustering of
galaxies in groups can be used to place tighter constraints on the halo model
than can be gained from using the usual galaxy correlation function alone.Comment: 22 pages, 12 figures, in emulateapj format, accepted to ApJ, minor
changes made to match published versio
Cosmic voids detection without density measurements
Cosmic voids are effective cosmological probes to discriminate among
competing world models. Their identification is generally based on density or
geometry criteria that, because of their very nature, are prone to shot noise.
We propose two void finders that are based on dynamical criterion to select
voids in Lagrangian coordinates and minimise the impact of sparse sampling. The
first approach exploits the Zel'dovich approximation to trace back in time the
orbits of galaxies located in voids and their surroundings, the second uses the
observed galaxy-galaxy correlation function to relax the objects' spatial
distribution to homogeneity and isotropy. In both cases voids are defined as
regions of the negative velocity divergence, that can be regarded as sinks of
the back-in-time streamlines of the mass tracers. To assess the performance of
our methods we used a dark matter halo mock catalogue CoDECS, and compared the
results with those obtained with the ZOBOV void finder. We find that the void
divergence profiles are less scattered than the density ones and, therefore,
their stacking constitutes a more accurate cosmological probe. The significance
of the divergence signal in the central part of voids obtained from both our
finders is 60% higher than for overdensity profiles in the ZOBOV case. The
ellipticity of the stacked void measured in the divergence field is closer to
unity, as expected, than what is found when using halo positions. Therefore our
void finders are complementary to the existing methods, that should contribute
to improve the accuracy of void-based cosmological tests.Comment: 12 pages, 18 figures, accepted for publication in MNRA
The Rockstar Phase-Space Temporal Halo Finder and the Velocity Offsets of Cluster Cores
We present a new algorithm for identifying dark matter halos, substructure,
and tidal features. The approach is based on adaptive hierarchical refinement
of friends-of-friends groups in six phase-space dimensions and one time
dimension, which allows for robust (grid-independent, shape-independent, and
noise-resilient) tracking of substructure; as such, it is named Rockstar
(Robust Overdensity Calculation using K-Space Topologically Adaptive
Refinement). Our method is massively parallel (up to 10^5 CPUs) and runs on the
largest current simulations (>10^10 particles) with high efficiency (10 CPU
hours and 60 gigabytes of memory required per billion particles analyzed). A
previous paper (Knebe et al 2011) has shown Rockstar to have class-leading
recovery of halo properties; we expand on these comparisons with more tests and
higher-resolution simulations. We show a significant improvement in
substructure recovery as compared to several other halo finders and discuss the
theoretical and practical limits of simulations in this regard. Finally, we
present results which demonstrate conclusively that dark matter halo cores are
not at rest relative to the halo bulk or satellite average velocities and have
coherent velocity offsets across a wide range of halo masses and redshifts. For
massive clusters, these offsets can be up to 350 km/s at z=0 and even higher at
high redshifts. Our implementation is publicly available at
http://code.google.com/p/rockstar .Comment: 20 pages, 14 figures. Minor revisions to match accepted versio
The Characterised Noise Hi source finder: Detecting Hi galaxies using a novel implementation of matched filtering
The spectral line datacubes obtained from the Square Kilometre Array (SKA)
and its precursors, such as the Australian SKA Pathfinder (ASKAP), will be
sufficiently large to necessitate automated detection and parametrisation of
sources. Matched filtering is widely acknowledged as the best possible method
for the automated detection of sources. This paper presents the Characterised
Noise Hi (CNHI) source finder, which employs a novel implementation of matched
filtering. This implementation is optimised for the 3-D nature of the planned
Wide-field ASKAP Legacy L-band All- sky Blind surveY's (WALLABY) Hi spectral
line observations. The CNHI source finder also employs a novel sparse
representation of 3-D objects, with a high compression rate, to implement Lutz
one-pass algorithm on datacubes that are too large to process in a single pass.
WALLABY will use ASKAP's phenomenal 30 square degree field of view to image
\sim 70% of the sky. It is expected that WALLABY will find 500 000 Hi galaxies
out to z \sim 0.2.Comment: Part of the 2012 PASA Source Finding Special Issue, 10 figure
The Cross-Correlation between Galaxies and Groups: Probing the Galaxy Distribution in and around Dark Matter Haloes
We determine the cross-correlation function between galaxies and galaxy
groups, using both the Two-Degree Field Galaxy Redshift Survey (2dFGRS) and the
Sloan Digital Sky Survey (SDSS). We study the cross-correlation as a function
of group mass, and as a function of the luminosity, stellar mass, colour,
spectral type and specific star formation rate of the galaxies. All these
cross-correlation functions show a clear transition from the `1-halo' to the
`2-halo' regimes on a scale comparable to the virial radius of the groups in
consideration. On scales larger than the virial radius, all cross-correlation
functions are roughly parallel, consistent with the linear bias model. In
particular, the large scale correlation amplitudes are higher for more massive
groups, and for brighter and redder galaxies. In the `1-halo' regime, the
cross-correlation function depends strongly on the definition of the group
center. We consider both a luminosity-weighted center (LWC) and a center
defined by the location of the brightest group galaxy (BGC). With the first
definition, the bright early-type galaxies in massive groups are found to be
more centrally concentrated than the fainter, late-type galaxies. Using the
BGC, and excluding the brightest galaxy from the cross correlation analysis, we
only find significant segregation in massive groups (M \gta
10^{13}h^{-1}\msun) for galaxies of different spectral types (or colours or
specific star formation rates). In haloes with masses \la 10^{13}h^{-1}\msun,
there is a significant deficit of bright satellite galaxies. Comparing the
results from the 2dFGRS with those obtained from realistic mock samples, we
find that the distribution of galaxies in groups is much less concentrated than
dark matter haloes predicted by the current CDM model. (Abridged)Comment: 18 pages, 11 figures. Accepted for publication in MNRAS, 1 table
added, fig7 replace
Void Dynamics
Cosmic voids are becoming key players in testing the physics of our Universe.
Here we concentrate on the abundances and the dynamics of voids as these are
among the best candidates to provide information on cosmological parameters.
Cai, Padilla \& Li (2014) use the abundance of voids to tell apart Hu \&
Sawicki models from General Relativity. An interesting result is that
even though, as expected, voids in the dark matter field are emptier in
gravity due to the fifth force expelling away from the void centres, this
result is reversed when haloes are used to find voids. The abundance of voids
in this case becomes even lower in compared to GR for large voids.
Still, the differences are significant and this provides a way to tell apart
these models. The velocity field differences between and GR, on the
other hand, are the same for halo voids and for dark matter voids. Paz et al.
(2013), concentrate on the velocity profiles around voids. First they show the
necessity of four parameters to describe the density profiles around voids
given two distinct void populations, voids-in-voids and voids-in-clouds. This
profile is used to predict peculiar velocities around voids, and the
combination of the latter with void density profiles allows the construction of
model void-galaxy cross-correlation functions with redshift space distortions.
When these models are tuned to fit the measured correlation functions for voids
and galaxies in the Sloan Digital Sky Survey, small voids are found to be of
the void-in-cloud type, whereas larger ones are consistent with being
void-in-void. This is a novel result that is obtained directly from redshift
space data around voids. These profiles can be used to remove systematics on
void-galaxy Alcock-Pacinsky tests coming from redshift-space distortions.Comment: 8 pages, 4 figures, to appear in the proceedings of IAU308 Symposium
"The Zeldovich Universe
Data compression using correlations and stochastic processes in the ALICE Time Projection chamber
In this paper lossless and a quasi lossless algorithms for the online
compression of the data generated by the Time Projection Chamber (TPC) detector
of the ALICE experiment at CERN are described. The first algorithm is based on
a lossless source code modelling technique, i.e. the original TPC signal
information can be reconstructed without errors at the decompression stage. The
source model exploits the temporal correlation that is present in the TPC data
to reduce the entropy of the source. The second algorithm is based on a lossy
source code modelling technique. In order to evaluate the consequences of the
error introduced by the lossy compression, the results of the trajectory
tracking algorithms that process data offline are analyzed, in particular, with
respect to the noise introduced by the compression. The offline analysis has
two steps: cluster finder and track finder. The results on how these algorithms
are affected by the lossy compression are reported. In both compression
technique entropy coding is applied to the set of events defined by the source
model to reduce the bit rate to the corresponding source entropy. Using TPC
simulated data, the lossless and the lossy compression achieve a data reduction
to 49.2% of the original data rate and respectively in the range of 35% down to
30% depending on the desired precision.In this study we have focused on methods
which are easy to implement in the frontend TPC electronics.Comment: 8 pages, 3 figures, Talk from the 2003 Computing in High Energy and
Nuclear Physics (CHEP03), La Jolla, Ca, USA, March 2003, PSN THLT00
Bootstrapping bilinear models of robotic sensorimotor cascades
We consider the bootstrapping problem, which consists in learning a model of the agent's sensors and actuators starting from zero prior information, and we take the problem of servoing as a cross-modal task to validate the learned models. We study the class of bilinear dynamics sensors, in which the derivative of the observations are a bilinear form of the control commands and the observations themselves. This class of models is simple yet general enough to represent the main phenomena of three representative robotics sensors (field sampler, camera, and range-finder), apparently very different from one another. It also allows a bootstrapping algorithm based on hebbian learning, and that leads to a simple and bioplausible control strategy. The convergence properties of learning and control are demonstrated with extensive simulations and by analytical arguments
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