Color bimodality: Implications for galaxy evolution

We use a sample of 69726 galaxies from the SDSS to study the variation of the bimodal color-magnitude (CM) distribution with environment. Dividing the galaxy population by environment (Sigma_5) and luminosity (-23<M_r<-17), the u-r color functions are modeled using double-Gaussian functions. This enables a deconvolution of the CM distributions into two populations: red and blue sequences. The changes with increasing environmental density can be separated into two effects: a large increase in the fraction of galaxies in the red distribution, and a small color shift in the CM relations of each distribution. The average color shifts are 0.05+-0.01 and 0.11+-0.02 for the red and blue distributions, respectively, over a factor of 100 in projected neighbor density. The red fraction varies between about 0% and 70% for low-luminosity galaxies and between about 50% and 90% for high-luminosity galaxies. This difference is also shown by the variation of the luminosity functions with environment. We demonstrate that the effects of environment and luminosity can be unified. A combined quantity, Sigma_mod = Sigma_5/Mpc^{-2} + L_r/L_{-20.2}, predicts the fraction of red galaxies, which may be related to the probability of transformation events. Our results are consistent with major interactions (mergers and/or harassment) causing galaxies to transform from the blue to the red distribution. We discuss this and other implications for galaxy evolution from earlier results and model the effect of slow transformations on the color functions.Comment: 14 pages, 8 figures, in AIP Conf. Proc., The New Cosmology, eds. R. E. Allen et al. (aka. The Mitchell Symposium), see http://proceedings.aip.org/proceedings/confproceed/743.jsp ; v2: replaced Figure 5 which was incomplete in original submissio

Galaxy bimodality versus stellar mass and environment

We analyse a z<0.1 galaxy sample from the Sloan Digital Sky Survey focusing on the variation of the galaxy colour bimodality with stellar mass and projected neighbour density Sigma, and on measurements of the galaxy stellar mass functions. The characteristic mass increases with environmental density from about 10^10.6 Msun to 10^10.9 Msun (Kroupa IMF, H_0=70) for Sigma in the range 0.1--10 per Mpc^2. The galaxy population naturally divides into a red and blue sequence with the locus of the sequences in colour-mass and colour-concentration index not varying strongly with environment. The fraction of galaxies on the red sequence is determined in bins of 0.2 in log Sigma and log mass (12 x 13 bins). The red fraction f_r generally increases continuously in both Sigma and mass such that there is a unified relation: f_r = F(Sigma,mass). Two simple functions are proposed which provide good fits to the data. These data are compared with analogous quantities in semi-analytical models based on the Millennium N-body simulation: the Bower et al. (2006) and Croton et al. (2006) models that incorporate AGN feedback. Both models predict a strong dependence of the red fraction on stellar mass and environment that is qualitatively similar to the observations. However, a quantitative comparison shows that the Bower et al. model is a significantly better match; this appears to be due to the different treatment of feedback in central galaxies.Comment: 19 pages, 17 figures; accepted by MNRAS, minor change

Measuring the relativistic perigee advance with Satellite Laser Ranging

One of the most famous classical tests of General Relativity is the gravitoelectric secular advance of the pericenter of a test body in the gravitational field of a central mass. In this paper we explore the possibility of performing a measurement of the gravitoelectric pericenter advance in the gravitational field of the Earth by analyzing the laser-ranged data to some existing, or proposed, laser-ranged geodetic satellites. At the present level of knowledge of various error sources, the relative precision obtainable with the data from LAGEOS and LAGEOS II, suitably combined, is of the order of $10^{\rm -3}$. Nevertheless, these accuracies could sensibly be improved in the near future when the new data on the terrestrial gravitational field from the CHAMP and GRACE missions will be available. The use of the perigee of LARES (LAser RElativity Satellite), in the context of a suitable combination of orbital residuals including also LAGEOS II, should further raise the precision of the measurement. As a secondary outcome of the proposed experiment, with the so obtained value of \ppn and with \et=4\beta-\gamma-3 from Lunar Laser Ranging it could be possible to obtain an estimate of the PPN parameters $\gamma$ and $\beta$ at the $10^{-2}-10^{-3}$ level.Comment: LaTex2e, 14 pages, no figures, 2 tables. To appear in Classical and Quantum Gravit

OP2-Clang : a source-to-source translator using Clang/LLVM LibTooling

Domain Specific Languages or Active Library frameworks have recently emerged as an important method for gaining performance portability, where an application can be efficiently executed on a wide range of HPC architectures without significant manual modifications. Embedded DSLs such as OP2, provides an API embedded in general purpose languages such as C/C++/Fortran. They rely on source-to-source translation and code refactorization to translate the higher-level API calls to platform specific parallel implementations. OP2 targets the solution of unstructured-mesh computations, where it can generate a variety of parallel implementations for execution on architectures such as CPUs, GPUs, distributed memory clusters and heterogeneous processors making use of a wide range of platform specific optimizations. Compiler tool-chains supporting source-to-source translation of code written in mainstream languages currently lack the capabilities to carry out such wide-ranging code transformations. Clang/LLVM’s Tooling library (LibTooling) has long been touted as having such capabilities but have only demonstrated its use in simple source refactoring tasks. In this paper we introduce OP2-Clang, a source-to-source translator based on LibTooling, for OP2’s C/C++ API, capable of generating target parallel code based on SIMD, OpenMP, CUDA and their combinations with MPI. OP2-Clang is designed to significantly reduce maintenance, particularly making it easy to be extended to generate new parallelizations and optimizations for hardware platforms. In this research, we demonstrate its capabilities including (1) the use of LibTooling’s AST matchers together with a simple strategy that use parallelization templates or skeletons to significantly reduce the complexity of generating radically different and transformed target code and (2) chart the challenges and solution to generating optimized parallelizations for OpenMP, SIMD and CUDA. Results indicate that OP2-Clang produces near-identical parallel code to that of OP2’s current source-to-source translator. We believe that the lessons learnt in OP2-Clang can be readily applied to developing other similar source-to-source translators, particularly for DSLs

OP2-Clang : a source-to-source translator using Clang/LLVM LibTooling

Domain Specific Languages or Active Library frameworks have recently emerged as an important method for gaining performance portability, where an application can be efficiently executed on a wide range of HPC architectures without significant manual modifications. Embedded DSLs such as OP2, provides an API embedded in general purpose languages such as C/C++/Fortran. They rely on source-to-source translation and code refactorization to translate the higher-level API calls to platform specific parallel implementations. OP2 targets the solution of unstructured-mesh computations, where it can generate a variety of parallel implementations for execution on architectures such as CPUs, GPUs, distributed memory clusters and heterogeneous processors making use of a wide range of platform specific optimizations. Compiler tool-chains supporting source-to-source translation of code written in mainstream languages currently lack the capabilities to carry out such wide-ranging code transformations. Clang/LLVM’s Tooling library (LibTooling) has long been touted as having such capabilities but have only demonstrated its use in simple source refactoring tasks. In this paper we introduce OP2-Clang, a source-to-source translator based on LibTooling, for OP2’s C/C++ API, capable of generating target parallel code based on SIMD, OpenMP, CUDA and their combinations with MPI. OP2-Clang is designed to significantly reduce maintenance, particularly making it easy to be extended to generate new parallelizations and optimizations for hardware platforms. In this research, we demonstrate its capabilities including (1) the use of LibTooling’s AST matchers together with a simple strategy that use parallelization templates or skeletons to significantly reduce the complexity of generating radically different and transformed target code and (2) chart the challenges and solution to generating optimized parallelizations for OpenMP, SIMD and CUDA. Results indicate that OP2-Clang produces near-identical parallel code to that of OP2’s current source-to-source translator. We believe that the lessons learnt in OP2-Clang can be readily applied to developing other similar source-to-source translators, particularly for DSLs

Tracking Down a Critical Halo Mass for Killing Galaxies through the Growth of the Red-Sequence

Red-sequence galaxies record the history of terminated star-formation in the Universe and can thus provide important clues to the mechanisms responsible for this termination. We construct composite samples of published cluster and field galaxy photometry in order to study the build-up of galaxies on the red-sequence, as parameterised by the dwarf-to-giant ratio (DGR). We find that the DGR in clusters is higher than that of the field at all redshifts, implying that the faint end of the red-sequence was established first in clusters. We find that the DGR evolves with redshift for both samples, consistent with the ``down-sizing'' picture of star formation. We examine the predictions of semi-analytic models for the DGR and find that neither the magnitude of its environmental dependence nor its evolution is correctly predicted in the models. Red-sequence DGRs are consistently too high in the models, the most likely explanation being that the strangulation mechanism used to remove hot gas from satellite galaxies is too efficient. Finally we present a simple toy model including a threshold mass, below which galaxies are not strangled, and show that this can predict the observed evolution of the field DGR.Comment: MNRAS letters accepted. 5 pages, 1 figur

The intermediate-redshift galaxy cluster CL 0048-2942. Stellar populations

We present a detailed study of the cluster CL 0048-2942, located at z~0.64, based on a photometric and spectroscopic catalogue of 54 galaxies in a 5 x 5 square arcmin region centred in that cluster. Of these, 23 galaxies were found to belong to the cluster. Based on this sample, the line-of-sight velocity dispersion of the cluster is approximately 680 +- 140 km/s. We have performed stellar population synthesis in the cluster members as well as in the field galaxies of the sample and found that there are population gradients in the cluster with central galaxies hosting mainly intermediate/old populations whereas galaxies in the cluster outskirts show clearly an increase of younger populations, meaning that star formation is predominantly taking place in the outer regions of the cluster. In a general way, field galaxies seem to host less evolved stellar populations than cluster members. In fact, in terms of ages, young supergiant stars dominate the spectra of field galaxies whereas cluster galaxies display a dominant number of old and intermediate age stars. Following the work of other authors (e.g. Dressler et al. 1999) we have estimated the percentage of K+A galaxies in our sample and found around 13% in the cluster and 10% in the field. These values were estimated through means of a new method, based on stellar population synthesis results, that takes into account all possible absorption features in the spectrum and thus makes optimal use of the data.Comment: Accepted by Astronomy & Astrophysics. 24 pages, 10 figures, 10 tables (figures 3, 4, 5 and tables 1, 3, 4, 5, 6, 7, 8 will be available in electronic format only in the A&A published version

On Bootstrap Percolation in Living Neural Networks

Recent experimental studies of living neural networks reveal that their global activation induced by electrical stimulation can be explained using the concept of bootstrap percolation on a directed random network. The experiment consists in activating externally an initial random fraction of the neurons and observe the process of firing until its equilibrium. The final portion of neurons that are active depends in a non linear way on the initial fraction. The main result of this paper is a theorem which enables us to find the asymptotic of final proportion of the fired neurons in the case of random directed graphs with given node degrees as the model for interacting network. This gives a rigorous mathematical proof of a phenomena observed by physicists in neural networks

The Las Campanas/AAT Rich Cluster Survey III: Spectroscopic Studies of X-ray Bright Galaxy Clusters at z~0.1

[abridged] We present the analysis of the spectroscopic and photometric catalogues of 11 X-ray luminous clusters at z=0.07-0.16 from the Las Campanas / Anglo-Australian Telescope Rich Cluster Survey. Our spectroscopic dataset consists of over 1600 galaxy cluster members, of which two thirds are outside r_200. We assign cluster membership using a detailed mass model and expand on our previous work on the cluster colour-magnitude relation where membership was inferred statistically. We confirm that the modal colours of galaxies on the colour magnitude relation become progressively bluer with increasing radius and decreasing local galaxy density. Interpreted as an age effect, we hypothesize that these trends in galaxy colour should be reflected in mean Hdelta equivalent width. We confirm that passive galaxies in the cluster increase in Hdelta line strength as dHdelta / d r_p = 0.35 +/- 0.06. A variation of star formation rate, as measured by [OII], with increasing local density of the environment is discernible and is shown to be in broad agreement with previous studies from 2dFGRS and SDSS. We find that clusters at z~0.1 are less active than their higher redshift analogues. We also investigate unusual populations of blue and very red nonstarforming galaxies and we suggest that the former are likely to be the progenitors of galaxies which will lie on the colour-magnitude relation, while the colours of the latter possibly reflect dust reddening. The cluster galaxies at large radii consist of both backsplash ones and those that are infalling to the cluster for the first time. We make a comparison to the field population at z~0.1 and examine broad differences between the two populations. Individually, the clusters show significant variation in their galaxy populations which reflects their recent infall histories.Comment: 25 pages, 16 figures. Accepted for publication in MNRA