14,805 research outputs found
Kinematics in the Central Kiloparsec of Spiral Galaxies
Results from kinematic observations of the central regions of spiral galaxies
are reviewed, with particular emphasis on starburst and AGN hosts. While
morphological studies lead to important insight, a more complete understanding
of the physical processes that drive the evolution of the central regions can
be achieved with measurements of the kinematics of gas and stars. Here, a
variety of observational techniques at different wavelengths is critically
discussed, and specific areas of interest are highlighted, such as inflow in
barred galaxies and the origin of nuclear spiral arms. A brief discussion of a
number of case studies is presented to illustrate recent progress.Comment: Invited review, to appear in The Central kpc of Starbursts and AGN:
the La Palma Connection, eds. J. H. Knapen, J. E. Beckman, I. Shlosman, and
T. J. Mahoney (ASP: San Francisco), Vol. 249. 18 pages latex, includes 9
postscript figures. Uses newpasp.sty (included
A method of moments estimator of tail dependence
In the world of multivariate extremes, estimation of the dependence structure
still presents a challenge and an interesting problem. A procedure for the
bivariate case is presented that opens the road to a similar way of handling
the problem in a truly multivariate setting. We consider a semi-parametric
model in which the stable tail dependence function is parametrically modeled.
Given a random sample from a bivariate distribution function, the problem is to
estimate the unknown parameter. A method of moments estimator is proposed where
a certain integral of a nonparametric, rank-based estimator of the stable tail
dependence function is matched with the corresponding parametric version. Under
very weak conditions, the estimator is shown to be consistent and
asymptotically normal. Moreover, a comparison between the parametric and
nonparametric estimators leads to a goodness-of-fit test for the semiparametric
model. The performance of the estimator is illustrated for a discrete spectral
measure that arises in a factor-type model and for which likelihood-based
methods break down. A second example is that of a family of stable tail
dependence functions of certain meta-elliptical distributions.Comment: Published in at http://dx.doi.org/10.3150/08-BEJ130 the Bernoulli
(http://isi.cbs.nl/bernoulli/) by the International Statistical
Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm
High-resolution UKIRT observations of circumnuclear star formation in M100
We present high-resolution, near-infrared imaging of the circumnuclear region
of the barred spiral galaxy M100 (=NGC 4321), accompanied by near-infrared
spectroscopy. We identify a total of 43 distinct regions in the K-band image,
and determine magnitudes and colours for 41 of them. By comparison with other
near-infrared maps we also derive colour excesses and K-band extinctions for
the knots. Combining the imaging and spectroscopic results, we conclude that
the knots are the result of bursts of star formation within the last 15-25 Myr.
We discuss the implications of these new results for our dynamical and
evolutionary understanding of this galaxy.Comment: 10 pages, 4 figures, uses mn-1.4.sty. Accepted for publication in
Monthly Notices of the Royal Astronomical Societ
Spectral gap of the Erlang A model in the Halfin-Whitt regime
We consider a hybrid diffusion process that is a combination of two
Ornstein-Uhlenbeck processes with different restraining forces. This process
serves as the heavy-traffic approximation to the Markovian many-server queue
with abandonments in the critical Halfin-Whitt regime. We obtain an expression
for the Laplace transform of the time-dependent probability distribution, from
which the spectral gap is explicitly characterized. The spectral gap gives the
exponential rate of convergence to equilibrium. We further give various
asymptotic results for the spectral gap, in the limits of small and large
abandonment effects. It turns out that convergence to equilibrium becomes
extremely slow for overloaded systems with small abandonment effects.Comment: 48 page
Measuring the 3D shape of X-ray clusters
Observations and numerical simulations of galaxy clusters strongly indicate
that the hot intracluster x-ray emitting gas is not spherically symmetric. In
many earlier studies spherical symmetry has been assumed partly because of
limited data quality, however new deep observations and instrumental designs
will make it possible to go beyond that assumption. Measuring the temperature
and density profiles are of interest when observing the x-ray gas, however the
spatial shape of the gas itself also carries very useful information. For
example, it is believed that the x-ray gas shape in the inner parts of galaxy
clusters is greatly affected by feedback mechanisms, cooling and rotation, and
measuring this shape can therefore indirectly provide information on these
mechanisms. In this paper we present a novel method to measure the
three-dimensional shape of the intracluster x-ray emitting gas. We can measure
the shape from the x-ray observations only, i.e. the method does not require
combination with independent measurements of e.g. the cluster mass or density
profile. This is possible when one uses the full spectral information contained
in the observed spectra. We demonstrate the method by measuring radial
dependent shapes along the line of sight for CHANDRA mock data. We find that at
least 10^6 photons are required to get a 5-{\sigma} detection of shape for an
x-ray gas having realistic features such as a cool core and a double powerlaw
for the density profile. We illustrate how Bayes' theorem is used to find the
best fitting model of the x-ray gas, an analysis that is very important in a
real observational scenario where the true spatial shape is unknown. Not
including a shape in the fit may propagate to a mass bias if the x-ray is used
to estimate the total cluster mass. We discuss this mass bias for a class of
spacial shapes.Comment: 29 pages, 16 figure
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