We present the public data release of halo and galaxy catalogues extracted from the eagle suite of cosmological hydrodynamical simulations of galaxy formation. These simulations were performed with ...an enhanced version of the gadget code that includes a modified hydrodynamics solver, time-step limiter and subgrid treatments of baryonic physics, such as stellar mass loss, element-by-element radiative cooling, star formation and feedback from star formation and black hole accretion. The simulation suite includes runs performed in volumes ranging from 25 to 100 comoving megaparsecs per side, with numerical resolution chosen to marginally resolve the Jeans mass of the gas at the star formation threshold. The free parameters of the subgrid models for feedback are calibrated to the redshift z=0 galaxy stellar mass function, galaxy sizes and black hole mass–stellar mass relation. The simulations have been shown to match a wide range of observations for present-day and higher-redshift galaxies. The raw particle data have been used to link galaxies across redshifts by creating merger trees. The indexing of the tree produces a simple way to connect a galaxy at one redshift to its progenitors at higher redshift and to identify its descendants at lower redshift. In this paper we present a relational database which we are making available for general use. A large number of properties of haloes and galaxies and their merger trees are stored in the database, including stellar masses, star formation rates, metallicities, photometric measurements and mock gri images. Complex queries can be created to explore the evolution of more than 105 galaxies, examples of which are provided in the Appendix. The relatively good and broad agreement of the simulations with a wide range of observational datasets makes the database an ideal resource for the analysis of model galaxies through time, and for connecting and interpreting observational datasets.
We adapt the L-Galaxies semi-analytic model to follow the star formation histories (SFHs) of galaxies – by which we mean a record of the formation time and metallicities of the stars that are present ...in each galaxy at a given time. We use these to construct stellar spectra in post-processing, which offers large efficiency savings and allows user-defined spectral bands and dust models to be applied to data stored in the Millennium data repository. We contrast model SFHs from the Millennium Simulation with observed ones from the VESPA algorithm as applied to the Sloan Digital Sky Survey 7 (SDSS-7) catalogue. The overall agreement is good, with both simulated and SDSS galaxies showing a steeper SFH with increased stellar mass. The SFHs of blue and red galaxies, however, show poor agreement between data and simulations, which may indicate that the termination of star formation is too abrupt in the models. The mean star formation rate (SFR) of model galaxies is well defined and is accurately modelled by a double power law at all redshifts: SFR ∝ 1/(x
−1.39 + x
1.33), where x = (t
a − t)/3.0 Gyr, t is the age of the stars and t
a is the lookback time to the onset of galaxy formation; above a redshift of unity, this is well approximated by a gamma function: SFR ∝ x
1.5e−x
, where x = (t
a − t)/2.0 Gyr. Individual galaxies, however, show a wide dispersion about this mean. When split by mass, the SFR peaks earlier for high-mass galaxies than for lower mass ones, and we interpret this downsizing as a mass-dependence in the evolution of the quenched fraction: the SFHs of star-forming galaxies show only a weak mass-dependence.
We present SciServer, a science platform built and supported by the Institute for Data Intensive Engineering and Science at the Johns Hopkins University. SciServer builds upon and extends the ...SkyServer system of server-side tools that introduced the astronomical community to SQL (Structured Query Language) and has been serving the Sloan Digital Sky Survey catalog data to the public. SciServer uses a Docker/VM based architecture to provide interactive and batch mode server-side analysis with scripting languages like Python and R in various environments including Jupyter (notebooks), RStudio and command-line in addition to traditional SQL-based data analysis. Users have access to private file storage as well as personal SQL database space. A flexible resource access control system allows users to share their resources with collaborators, a feature that has also been very useful in classroom environments. All these services, wrapped in a layer of REST APIs, constitute a scalable collaborative data-driven science platform that is attractive to science disciplines beyond astronomy.
Probabilistic cross-identification has been successfully applied to a number of problems in astronomy from matching simple point sources to associating stars with unknown proper motions and even ...radio observations with realistic morphology. Here we study the Bayes factor for clustered objects and focus in particular on galaxies to assess the effect of typical angular correlations. Numerical calculations provide the modified relationship, which (as expected) suppresses the evidence for the associations at the shortest separations where the 2-point auto-correlation function is large. Ultimately this means that the matching probability drops at somewhat shorter scales than in previous models.
I investigate the influence of a non-zero cosmological constant on the evolution of anisotropy in overdensities that grow by gravitational collapse. The claim that a positive value of Λ might produce ...stronger asphericities is considered by following the collapse of homogeneous spheroids imbedded in Friedman–Lemaître backgrounds. It is shown that, although the calculations indeed show this effect, it is small for the values of Λ allowed by the classical cosmological tests. When we do not limit ourselves by constraints arising from the choice of an initial fluctuation spectrum, structures in an open universe (Ω0 = 0.1, λ0 = 0), including the peculiar velocity structure, can be reproduced in a flat Lemaître universe (Ω0 = 0.1, λ0 = 0.9) for a large part of their evolution. From initial conditions that are not too extreme, these two world models are both able to produce strong anisotropies that easily persist for a Hubble time. This stability of flattening is the only aspect in which these two models differ significantly from the Einstein–de Sitter model. Taking into account the crudeness of the model, together with the fact that observed large-scale structures are not isolated, we conclude that use of this aspect of the dynamics of structure formation is not a promising method of placing tighter constraints on the value of Λ.
Cosmological N-body simulations play a vital role in studying models for the evolution of the Universe. To compare to observations and make a scientific inference, statistic analysis on large ...simulation datasets, e.g., finding halos, obtaining multi-point correlation functions, is crucial. However, traditional in-memory methods for these tasks do not scale to the datasets that are forbiddingly large in modern simulations. Our prior paper (Liu et al., 2015) proposes memory-efficient streaming algorithms that can find the largest halos in a simulation with up to 109 particles on a small server or desktop. However, this approach fails when directly scaling to larger datasets. This paper presents a robust streaming tool that leverages state-of-the-art techniques on GPU boosting, sampling, and parallel I/O, to significantly improve performance and scalability. Our rigorous analysis of the sketch parameters improves the previous results from finding the centers of the 103 largest halos (Liu et al., 2015) to ∼104−105, and reveals the trade-offs between memory, running time and number of halos. Our experiments show that our tool can scale to datasets with up to ∼1012 particles while using less than an hour of running time on a single GPU Nvidia GTX 1080.
We describe a simple, efficient algorithm that allows one to construct Monte Carlo realizations of merger histories of dark matter haloes. The algorithm is motivated by the excursion set model for ...the conditional and unconditional halo mass functions. The forest of trees constructed using this algorithm depends on the underlying power spectrum. For Poisson or white-noise initial power spectra, the forest has exactly the same properties as the ensemble of trees described by Sheth. In this case, many ensemble-averaged higher order statistics of the tree distribution can be computed analytically. For Gaussian initial conditions with more general power spectra, mean properties of our forests closely resemble the mean properties expected from the excursion set approach. For these more general initial conditions, our algorithm shows how to write down simple, analytic approximations to some higher order statistical quantities associated with the forest. These higher order statistics generated using our algorithm, and the associated analytic approximations, are in good agreement with what is measured in numerical simulations of hierarchical gravitational clustering.
Evaluating approximations for halo merging histories Somerville, Rachel S.; Lemson, Gerard; Kolatt, Tsafrir S. ...
Monthly Notices of the Royal Astronomical Society,
08/2000, Letnik:
316, Številka:
3
Journal Article
Recenzirano
Odprti dostop
We study the merging history of dark matter haloes in N-body simulations and semi-analytical ‘merger trees’ based on the extended Press-Schechter (EPS) formalism. The main focus of our study is the ...joint distribution of progenitor number and mass as a function of redshift and parent halo mass. We begin by investigating the mean quantities predicted directly by the Press-Schechter (PS) and EPS formalism, such as the halo mass and conditional mass functions, and compare these predictions with the results of the simulations. The higher moments of this distribution are not predicted by the EPS formalism alone and must be obtained from the merger trees. We find that the Press-Schechter model deviates from the simulations at the level of 30–50 per cent on certain mass scales, and that the sense of the discrepancy changes as a function of redshift. We show that this discrepancy is reflected in the higher moments of the distribution of progenitor mass and number. We investigate some related statistics such as the accretion rate and the mass ratio of the largest two progenitors. For galaxy sized haloes (M∼1012 M⊙), we find that the merging history of haloes, as represented by these statistics, is well reproduced in the merger trees compared with the simulations. The agreement deteriorates for larger mass haloes. We conclude that merger trees based on the extended Press-Schechter formalism provide a reasonably reliable framework for semi-analytical models of galaxy formation.
We identify 42 “candidate groups” lying between 1.8\textlessz\textless3.0 from a sample of 3502 galaxies with spectroscopic redshifts in the zCOSMOS-deep redshift survey within this same redshift ...interval. These systems contain three to five spectroscopic galaxies that lie within 500 kpc in projected distance (in physical space) and within 700 km s(-1) in velocity. Based on extensive analysis of mock catalogs that have been generated from the Millennium simulation, we examine the likely nature of these systems at the time of observation, and what they will evolve into down to the present epoch. Although few of the “member” galaxies are likely to reside in the same halo at the epoch we observe them, 50% of the systems will have, by the present epoch, all of the member galaxies in the same halo, and almost all (93%) will have at least some of the potential members in the same halo. Most of the candidate groups can therefore be described as “proto-groups.” A crude estimate of the overdensities of these structures is also consistent with the idea that these systems are being seen as they assemble. We also examine present-day halos and ask whether their progenitors would have been seen among our candidate groups. For present-day halos between 10(14) and 10(15) M(circle dot)h(-1), 35% should have appeared among our candidate groups, and this would have risen to 70% if our survey had been fully sampled, so we can conclude that our sample can be taken as representative of a large fraction of such systems. There is a clear excess of massive galaxies above 10(10)M(circle dot) around the locations of the candidate groups in a large independent COSMOS photo-z sample, but we see no evidence in this latter data for any color differentiation with respect to the field. This is, however, consistent with the idea that such differentiation arises in satellite galaxies, as indicated at z \textless1, if the candidate groups are indeed only starting to be assembled.
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