Multiplicity Fluctuations in Au+Au Collisions at RHIC

In the PHOBOS experiment, charged particles are measured in almost the full solid angle. This enables the study of fluctuations and correlations in the particle production over a very wide kinematic range. In this paper, we show results of a direct search for fluctuations identified by an unusual shape of the pseudorapidity distribution. In addition, we use analysis of correlations of the multiplicity in similar pseudorapidity bins, placed symmetrically in the forward and backward hemispheres, to test the hypothesis of production of particles in clusters.Comment: presented as a poster at the Quark Matter 2006 conferenc

How can double-barred galaxies be long-lived?

Double-barred galaxies account for almost one third of all barred galaxies, suggesting that secondary stellar bars, which are embedded in large-scale primary bars, are long-lived structures. However, up to now it has been hard to self-consistently simulate a disc galaxy that sustains two nested stellar bars for longer than a few rotation periods. N-body/hydrodynamical simulations including star formation recipes have been performed. Their properties have been compared with the most recent observational data in order to prove that they are representative of double-barred galaxies, even SB0. Overlaps in dynamical resonances and bar modes have been looked for using Fourier spectrograms. Double-barred galaxies have been successfully simulated with lifetimes as long as 7 Gyr. The stellar population of the secondary bar is younger on average than for the primary large-scale bar. An important feature of these simulations is the absence of any resonance overlap for several Gyr. In particular, there is no overlap between the primary bar ILR and the secondary bar corotation. Therefore, mode coupling cannot sustain the secondary bar mode. Star formation is identified here as possibly being responsible for bringing energy to the nuclear mode. Star formation is also responsible for limiting the amount of gas in the central region which prevents the orbits sustaining the secondary bar from being destroyed. Therefore, the secondary bar can dissolve but reappear after approx. 1 Gyr. When star formation is switched off the dynamical perturbation associated with the secondary bar needs several Gyr to fully vanish. Double-bars can be long-lived in numerical simulations with a gaseous component, even in the absence of overlap of resonances or mode coupling, provided that star formation remains active in the central region where the nuclear bar lies.Comment: 14 pages, 14 low resolution figures ; high resolution paper available at http://herve.wozniak.fr/DB.pdf (27 Mb

A New Algorithm For Difference Image Analysis

In the context of difference image analysis (DIA), we present a new method for determining the convolution kernel matching a pair of images of the same field. Unlike the standard DIA technique which involves modelling the kernel as a linear combination of basis functions, we consider the kernel as a discrete pixel array and solve for the kernel pixel values directly using linear least-squares. The removal of basis functions from the kernel model is advantageous for a number of compelling reasons. Firstly, it removes the need for the user to specify such functions, which makes for a much simpler user application and avoids the risk of an inappropriate choice. Secondly, basis functions are constructed around the origin of the kernel coordinate system, which requires that the two images are perfectly aligned for an optimal result. The pixel kernel model is sufficiently flexible to correct for image misalignments, and in the case of a simple translation between images, image resampling becomes unnecessary. Our new algorithm can be extended to spatially varying kernels by solving for individual pixel kernels in a grid of image sub-regions and interpolating the solutions to obtain the kernel at any one pixel.Comment: MNRAS Letters Accepte

New results on collectivity with ATLAS

The collective phenomena are observed not only in heavy ion collisions, but also in the proton-nucleus and in high-multiplicity $pp$ collisions. The latest results from this area obtained in ATLAS are presented. In $p$+Pb collisions the emission source of particles is measured using the HBT method. The analysis of $p$+Pb data collected in 2016 provides information on the elliptic flow of charged hadrons and muons. Low multiplicity events from $pp$, $p$+Pb and peripheral Pb+Pb collisions are studied with the cumulant methods. A deeper understanding of Pb+Pb collisions is provided by the analysis of longitudinal fluctuations of the collective flow parameters.Comment: 6 pages, 8 figure

A new chemodynamical tool to study the evolution of galaxies in the local Universe: a quick and accurate numerical technique to compute gas cooling rate for any chemical composition

We have developed a quick and accurate numerical tool to compute gas cooling whichever its chemical composition.Comment: 3 pages, 2 figures, to appear in the Proceedings of "Heating vs. Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching (Germany), Eds. Boehringer, Schuecker, Pratt, Finoguenov, Springer-Verlag series "ESO Astrophysics Symposia