6,133 research outputs found
Sensitivity of the limit shape of sample clouds from meta densities
The paper focuses on a class of light-tailed multivariate probability
distributions. These are obtained via a transformation of the margins from a
heavy-tailed original distribution. This class was introduced in Balkema et al.
(J. Multivariate Anal. 101 (2010) 1738-1754). As shown there, for the
light-tailed meta distribution the sample clouds, properly scaled, converge
onto a deterministic set. The shape of the limit set gives a good description
of the relation between extreme observations in different directions. This
paper investigates how sensitive the limit shape is to changes in the
underlying heavy-tailed distribution. Copulas fit in well with multivariate
extremes. By Galambos's theorem, existence of directional derivatives in the
upper endpoint of the copula is necessary and sufficient for convergence of the
multivariate extremes provided the marginal maxima converge. The copula of the
max-stable limit distribution does not depend on the margins. So margins seem
to play a subsidiary role in multivariate extremes. The theory and examples
presented in this paper cast a different light on the significance of margins.
For light-tailed meta distributions, the asymptotic behaviour is very sensitive
to perturbations of the underlying heavy-tailed original distribution, it may
change drastically even when the asymptotic behaviour of the heavy-tailed
density is not affected.Comment: Published in at http://dx.doi.org/10.3150/11-BEJ370 the Bernoulli
(http://isi.cbs.nl/bernoulli/) by the International Statistical
Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm
Asymptotic independence for unimodal densities
Asymptotic independence of the components of random vectors is a concept used
in many applications. The standard criteria for checking asymptotic
independence are given in terms of distribution functions (dfs). Dfs are rarely
available in an explicit form, especially in the multivariate case. Often we
are given the form of the density or, via the shape of the data clouds, one can
obtain a good geometric image of the asymptotic shape of the level sets of the
density. This paper establishes a simple sufficient condition for asymptotic
independence for light-tailed densities in terms of this asymptotic shape. This
condition extends Sibuya's classic result on asymptotic independence for
Gaussian densities.Comment: 33 pages, 4 figure
An HI survey of the Centaurus and Sculptor Groups - Constraints on the space density of low mass galaxies
We present results of two 21-cm HI surveys performed with the Australia
Telescope Compact Array in the nearby Centaurus A and Sculptor galaxy groups.
These surveys are sensitive to compact HI clouds and galaxies with HI masses as
low as 3E+06 Msun, and are therefore among the most sensitive extragalactic HI
surveys to date. The surveys consist of sparsely spaced pointings that sample
approximately 2% of the groups' area on the sky. We detected previously known
group members, but we found no new HI clouds or galaxies down to the
sensitivity limit of the surveys. If the HI mass function had a faint end slope
of alpha = 1.5 below M_{HI} = 10^{7.5} Msun in these groups, we would have
expected ~3 new objects. Cold dark matter theories of galaxy formation predict
the existence of a large number low mass DM sub-halos that might appear as tiny
satellites in galaxy groups. Our results support and extend similar conclusions
derived from previous HI surveys that a HI rich population of these satellites
does not exist.Comment: Accepted for publication in A&
High Metallicity Mg II Absorbers in the z < 1 Lyman alpha Forest of PKS 0454+039: Giant LSB Galaxies?
We report the discovery of two iron-group enhanced high-metallicity Mg II
absorbers in a search through 28 Lyman Alpha forest clouds along the PKS
0454+039 sight line. Based upon our survey and the measured redshift number
densities of W_r(MgII) <= 0.3 A absorbers and Lyman Alpha absorbers at z ~ 1,
we suggest that roughly 5% of Lyman Alpha absorbers at z < 1 will exhibit
"weak" Mg II absorption to a 5-sigma W_r(2796) detection limit of 0.02 A. The
two discovered absorbers, at redshifts z = 0.6248 and z = 0.9315, have W_r(Lya)
= 0.33 and 0.15 A, respectively. Based upon photoionization modeling, the H I
column densities are inferred to be in the range 15.8 <= log N(HI) <= 16.8
cm^-2. For the z = 0.6428 absorber, if the abundance pattern is solar, then the
cloud has [Fe/H] > -1; if its gas-phase abundance follows that of depleted
clouds in our Galaxy, then [Fe/H] > 0 is inferred. For the z = 0.9315 absorber,
the metallicity is [Fe/H] > 0, whether the abundance pattern is solar or
suffers depletion. Imaging and spectroscopic studies of the PKS 0454+039 field
reveal no candidate luminous objects at these redshifts. We discuss the
possibility that these Mg II absorbers may arise in the class of "giant" low
surface brightness galaxies, which have [Fe/H] >= -1, and even [Fe/H] >= 0, in
their extended disks. We tentatively suggest that a substantial fraction of
these "weak" Mg II absorbers may select low surface brightness galaxies out to
z ~ 1.Comment: Accepted The Astrophysical Journal; 25 pages; 6 encapsulated figure
Automated Classification of Airborne Laser Scanning Point Clouds
Making sense of the physical world has always been at the core of mapping. Up
until recently, this has always dependent on using the human eye. Using
airborne lasers, it has become possible to quickly "see" more of the world in
many more dimensions. The resulting enormous point clouds serve as data sources
for applications far beyond the original mapping purposes ranging from flooding
protection and forestry to threat mitigation. In order to process these large
quantities of data, novel methods are required. In this contribution, we
develop models to automatically classify ground cover and soil types. Using the
logic of machine learning, we critically review the advantages of supervised
and unsupervised methods. Focusing on decision trees, we improve accuracy by
including beam vector components and using a genetic algorithm. We find that
our approach delivers consistently high quality classifications, surpassing
classical methods
The impact of local stellar radiation on the HI column density distribution
It is often assumed that local sources of ionizing radiation have little
impact on the distribution of HI in the post-reionization Universe. While this
is a good assumption for the IGM, analytic arguments suggest that local sources
may typically be more important than the background radiation for high column
density absorbers (N_HI > 10^17 /cm^2). We post-process cosmological
simulations with accurate radiation transport to investigate the impact of
local stellar sources on the HI distribution. We demonstrate that the limited
numerical resolution and the simplified treatment of the ISM that are typical
of current cosmological simulations provide significant challenges, but that
many of the problems can be overcome by taking two steps. First, using ISM
particles rather than stellar particles as sources results in a much better
sampling of the source distribution. Second, by rescaling the source
luminosities so that the amount of radiation escaping into the IGM agrees with
that required to produce the observed background, many of the results become
insensitive to errors in the predicted fraction of the radiation that escapes
the immediate vicinity of the sources. By adopting this strategy and by varying
the assumptions about the structure of the unresolved ISM, we conclude that we
can robustly estimate the effect of local sources for column densities N_HI <<
10^21 /cm^2. However, neither the escape fraction of ionizing radiation nor the
effect of local sources on the abundance of N_HI >~ 10^21 systems can be
predicted with confidence. We find that local stellar radiation is unimportant
for N_HI << 10^17, but that it can affect Lyman Limit and DLA systems. For
10^18 < N_HI < 10^21 the impact of local sources increases with redshift. At z
= 5 the abundance of absorbers with N_HI >> 10^17 is substantially reduced, but
at z = 0 the reduction only becomes significant for N_HI >~ 10^21 /cm^2.Comment: 19 pages, 12 figures; Accepted for publication in MNRA
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