8,460 research outputs found
The twilight zone in the parametric evolution of eigenstates: beyond perturbation theory and semiclassics
Considering a quantized chaotic system, we analyze the evolution of its
eigenstates as a result of varying a control parameter. As the induced
perturbation becomes larger, there is a crossover from a perturbative to a
non-perturbative regime, which is reflected in the structural changes of the
local density of states. For the first time the {\em full} scenario is explored
for a physical system: an Aharonov-Bohm cylindrical billiard. As we vary the
magnetic flux, we discover an intermediate twilight regime where perturbative
and semiclassical features co-exist. This is in contrast with the {\em simple}
crossover from a Lorentzian to a semicircle line-shape which is found in
random-matrix models.Comment: 4 pages, 4 figures, improved versio
On the lack of stellar bars in Coma dwarf galaxies
We present a study of the bar fraction in the Coma cluster galaxies based on
a sample of ~190 galaxies selected from the SDSS-DR6 and observed with the
Hubble Space Telescope (HST) Advanced Camera for Survey (ACS). The
unprecedented resolution of the HST-ACS images allows us to explore the
presence of bars, detected by visual classification, throughout a luminosity
range of 9 mag (-23 < M_r < -14), permitting us to study the poor known region
of dwarf galaxies. We find that bars are hosted by galaxies in a tight range of
both luminosities (-22 < M_r < -17) and masses (10^9 < M*/Msun < 10^11). In
addition, we find that the bar fraction does not vary significantly when going
from the center to the cluster outskirts, implying that cluster environment
plays a second-order role in bar formation/evolution. The shape of the bar
fraction distribution with respect to both luminosity and mass is well matched
by the luminosity distribution of disk galaxies in Coma, indicating that bars
are good tracers of cold stellar disks.Comment: 2 pages, 1 figure, to appear in the proceedings of the conference "A
Universe of Dwarf Galaxies" (Lyon, June 14-18, 2010
Properties of bars in the local universe
We studied the fraction and properties of bars in a sample of about 3000
galaxies extracted from SDSS-DR5. This represents a volume limited sample with
galaxies located between redshift 0.01-20, and
inclination i < 60. Interacting galaxies were excluded from the sample. The
fraction of barred galaxies in our sample is 45%. We found that 32% of S0s, 55%
of early-type spirals, and 52% of late-type spirals are barred galaxies. The
bars in S0s galaxies are weaker than those in later-type galaxies. The bar
length and galaxy size are correlated, being larger bars located in larger
galaxies. Neither the bar strength nor bar length correlate with the local
galaxy density. On the contrary, the bar properties correlate with the
properties of their host galaxies. Galaxies with higher central light
concentration host less and weaker bars.Comment: 2 pages, 1 figure to appear in the proceedings of "Formation and
Evolution of Galaxy Disks", Rome, October 2007, Eds. J. Funes and E. M.
Corsin
An efficient, parametric fixpoint algorithm for analysis of java bytecode
Abstract interpretation has been widely used for the analysis of object-oriented languages and, in particular, Java source and bytecode. However, while most existing work deals with the problem of flnding expressive abstract domains that track accurately the characteristics of a particular concrete property, the underlying flxpoint algorithms have received comparatively less attention. In fact, many existing (abstract interpretation based—) flxpoint algorithms rely on relatively inefHcient techniques for solving inter-procedural caligraphs or are speciflc and tied to particular analyses. We also argüe that the design of an efficient fixpoint algorithm is pivotal to supporting the analysis of large programs. In this paper we introduce a novel algorithm for analysis of Java bytecode which includes a number of optimizations in order to reduce the number of iterations. The algorithm is parametric -in the sense that it is independent of the abstract domain used and it can be applied to different domains as "plug-ins"-, multivariant, and flow-sensitive. Also, is based on a program transformation, prior to the analysis, that results in a highly uniform representation of all the features in the language and therefore simplifies analysis. Detailed descriptions of decompilation solutions are given and discussed with an example. We also provide some performance data from a preliminary implementation of the analysis
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
- …