14253637
Finite size effects on the galaxy number counts: Evidence for fractal behavior up to the deepest scale
We introduce and study two new concepts which are essential for the quantitative analysis of the statistical quality of the available galaxy samples. These are the dilution effect and the small scale fluctuations. We show that the various data that are considered as pointing to a homogenous distribution are all affected by these spurious effects and their interpretation should be completely changed. In particular, we show that finite size effects strongly affect the determination of the galaxy number counts, namely the number versus magnitude relation (N (< m)) as computed from the origin. When one computes N (< m) averaged over all the points of a redshift survey, one observes an exponent α = D/5 ≈ 0.4 compatible with the fractal dimension D ≈ 2 derived from the full correlation analysis. Instead the observation of an exponent α ≈ 0.6 at relatively small scales, where the distribution is certainly not homogeneous, is shown to be related to finite size effects. We conclude therefore that the observed counts correspond to a fractal distribution with dimension D ≈ 2 in the entire range 12 ⪅ m ⪅ 28, that is to say the largest scales ever probed for luminous matter. In addition our results permit to clarify various problems of the angular catalogs, and to show their compatibility with the fractal behaviour. We consider also the distribution of Radio-galaxies, Quasars and γ-ray bursts, and we show their compatibility with a fractal structure with D ≈ 1.6–1.8. Finally we have established a quantitative criterion that allows us to define and predict the statistical validity of a galaxy catalog (angular or three-dimensional).
9116544
Gravitational dynamics of an infinite shuffled lattice: Particle coarse-graining, nonlinear clustering, and the continuum limit.
We study the evolution under their self-gravity of particles evolving from infinite "shuffled lattice" initial conditions. We focus here specifically on the comparison between the evolution of such a system and that of "daughter" coarse-grained particle distributions. These are sparser (i.e., lower density) particle distributions, defined by a simple coarse-graining procedure, which share the same large-scale mass fluctuations. We consider both the case that such coarse-grainings are performed (i) on the initial conditions, and (ii) at a finite time with a specific additional prescription. In numerical simulations we observe that, to a first approximation, these coarse-grainings represent well the evolution of the two-point correlation properties over a significant range of scales. We note, in particular, that the form of the two-point correlation function in the original system, when it is evolving in the asymptotic "self-similar" regime, may be reproduced well in a daughter coarse-grained system in which the dynamics are still dominated by two-body (nearest neighbor) interactions. This provides a simple physical description of the origin of the form of part of the asymptotic nonlinear correlation function. Using analytical results on the early time evolution of these systems, however, we show that small observed differences between the evolved system and its coarse-grainings at the initial time will in fact diverge as the ratio of the coarse-graining scale to the original interparticle distance increases. The second coarse-graining studied, performed at a finite time in a specified manner, circumvents this problem. It also makes it more physically transparent why gravitational dynamics from these initial conditions tends toward a self-similar evolution. We finally discuss the precise definition of a limit in which a continuum (specifically Vlasov-type) description of the observed linear and nonlinear evolution should be applicable. This requires the introduction of an additional intrinsic length scale (e.g., a physical smoothing in the force at small scales), which is kept fixed as the particle density diverges. In this limit the different coarse-grainings are equivalent and leave the evolution of the "mother" system invariant.
119390803
Statistical Physics for Cosmic Structures
The recent observations of galaxy and dark matter clumpy distributions have provided new elements to the understanding of the problem of cosmological structure formation. The strong clumping characterizing galaxy structures seems to be present in the overall mass distribution and its relation to the highly isotropic Cosmic Microwave Background Radiation represents a fundamental problem. The extension of structures, the formation of power-law correlations characterizing the strongly clustered regime and the relation between dark and visible matter are the key problems both from an observational and a theoretical point of view. We discuss recent progresses in the studies of structure formation by using concepts and methods of statistical physics.
14466810
Basic properties of galaxy clustering in the light of recent results from the Sloan Digital Sky Survey
We discuss some of the basic implications of recent results on galaxy correlations published by the SDSS collaboration. In particular we focus on the evidence which has been recently presented for the scale and nature of the transition to homogeneity in the galaxy distribution, and results which describe the dependence of clustering on luminosity. The two questions are in fact strictly entangled, as the stability of the measure of the amplitude of the correlation function depends on the scale at which the mean density becomes well defined. We note that the recent results which indicate the convergence to well defined homogeneity in a volume equivalent to that of a sphere of radius 70 Mpc/h, place in doubt previous detections of luminosity bias from measures of the amplitude of the correlation function. We emphasize that the way to resolve these issues is to first use, in volume limited samples corresponding to different ranges of luminosity, the unnormalized two point statistics to establish the scale (and value) at which the mean density becomes well defined. We note also that the recent SDSS results for these statistics are in good agreement with those obtained by us through analyses of many previous samples, confirming in particular that the galaxy distribution is well described by a fractal dimension D 2 up to a scale of at least 20 Mpc/h. We discuss critically the agreement of this new data with current theoretical models.
15688477
Scaling and fluctuations in galaxy distribution: two tests to probe large scale structures
We present a brief introduction to the statistical properties of systems with large fluctuations. We point out that for such systems the relevant statistical quantities are scaling exponents and the nature of fluctuations is completely different from the one belonging to system with small fluctuations. We then propose two test to be performed on galaxy counts data as a function of apparent magnitude, which may clarify in a direct way the nature of the large scale galaxy clustering
14013791
Initial Conditions, Discreteness and Non-Linear Structure Formation in Cosmology
In this lecture we address three different but related aspects of the initial continuous fluctuation field in standard cosmological models. Firstly we discuss the properties of the so-called Harrison-Zeldovich like spectra. This power spectrum is a fundamental feature of all current standard cosmological models. In a simple classification of all stationary stochastic processes into three categories, we highlight with the name “super- homogeneous” the properties of the class to which models like this, with P(0) = 0, belong. In statistical physics language they are well described as glass-like. Secondly, the initial continuous density field with such small amplitude correlated Gaussian fluctuations must be discretised in order to set up the initial particle distribution used in gravitational N-body simulations. We discuss the main issues related to the effects of discretisation, particularly concerning the effect of particle induced fluctuations on the statistical properties of the initial conditions and on the dynamical evolution of gravitational clustering.
110590916
NEMO: Status of the Project
The activities towards the realisation of a km 3 Cherenkov neutrino detector, carried out by the NEMO Collaboration are described. Long term exploration of a 3500 m deep site close to the Sicilian coast has shown that it is optimal for the installation of the detector. A complete feasibility study, that has considered all the components of the detector as well as its deployment, has been carried out demonstrating that technological solutions exist for the realization of an underwater km 3 detector. The realization of a technological demonstrator (the NEMO Phase 1 project) is under way.
7619800
Latchup Topology for Pixel Readout Using Commercial Transistors
The stimulated ignition of latchup effects caused by external radiation has till now proved to be a hidden hazard. However this paper presents the effect in a new light-as a new approach for detecting particles by means of a solid-state device susceptible to latchup effects. This device can also be used as a circuit for reading a sensor's signal by leaving off-circuit sensing capabilities. Given that MOS transistors are widely used in microelectronics devices and sensors, the latchup-based cell is proposed as a new structure for future applications in particle detection, in the amplification of sensor signals and also in radiation monitoring.
120711647
A rad-hard 2D-compressor ASIC for ALICE SDD experiment
Abstract CARLOSv3 is a custom digital integrated circuit that plays a significant role in the data acquisition (DAQ) chain of the silicon drift detector (SDD) of a large ion collider experiment (ALICE). Each CARLOSv3 acts as a JTAG switch for front-end electronics and implements on-line two-dimensional (2D) data compression for two 8-bit DAQ channels handling the data streams delivered by the SDDs. The chip has been implemented using the 0.25 μm CMOS 3-metal CERN-developed digital library. It is composed of nearly 10k gates, has 88 user pads out of a total of 100 pads, has a die size of 4×4 mm 2 and is packaged in a CQFP100 ceramic package. In the paper, the chip is presented and described together with the 2D compression algorithm, the readout chain for ALICE SDD and the tests carried out.
110682003
High efficiency readout circuits for large matrices of pixels
Abstract In future collider experiments the increasing luminosity and center of mass energy are rising challenges in the research field of tracking systems where it is crucial to have very fast sensors with efficient readout in order to sustain the high particle flux. In this context we propose a high-efficiency digital readout architecture for large binary pixel matrices that is meant to cope with high hit-rates, up to 100 MHz/cm 2 , foreseen in the innermost tracker layer of the new generation particle accelerators. We modelled and designed several readout circuits to be integrated in the periphery of hybrid detectors. In this work we focus on a particular solution, to be interconnected to a 200 × 256 ‐ pixel matrix covering an area of 1.3 cm 2 . The hits are latched at pixel level, and the matrix provides a digital interface to the peripheral digital readout. The architecture is highly paralleled in order to reduce the pixels dead time introduced by readout which can be operated in data-push or triggered mode. In addition, a cluster compression algorithm, exploiting also the time sorted hit extraction, has been developed to reduce the output bandwidth. The architecture has been modelled in a hardware description language, to be synthesized in a net-list of foundry standard-cells. A Monte Carlo hit generator has been attached to simulations to evaluate the readout efficiency, both in data-push and triggered working mode. Simulation results will be presented, pointing out beside the efficiency results, the benefits introduced by the compression algorithm.
119249031
Measurement of the atmospheric muon flux with the NEMO Phase-1 detector
Abstract The NEMO Collaboration installed and operated an underwater detector including prototypes of the critical elements of a possible underwater km 3 neutrino telescope: a four-floor tower (called Mini-Tower) and a Junction Box. The detector was developed to test some of the main systems of the km 3 detector, including the data transmission, the power distribution, the timing calibration and the acoustic positioning systems as well as to verify the capabilities of a single tridimensional detection structure to reconstruct muon tracks. We present results of the analysis of the data collected with the NEMO Mini-Tower. The position of photomultiplier tubes (PMTs) is determined through the acoustic position system. Signals detected with PMTs are used to reconstruct the tracks of atmospheric muons. The angular distribution of atmospheric muons was measured and results compared to Monte Carlo simulations.