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Now showing items 1 - 16 of 203587

  • TREVR: A general N log(2) N radiative transfer algorithm

    Grond, J. J.   Woods, R. M.   Wadsley, J. W.   Couchman, H. M. P.  

    We present Tree-based REVerse Ray Tracing (TREVR), a general algorithm for computing the radiation field, including absorption, in astrophysical simulations. TREVR is designed to handle large numbers of sources and absorbers; it is based on a tree data structure and is thus suited to codes that use trees for their gravity or hydrodynamics solvers (e.g. adaptivemesh refinement). It achieves computational speed while maintaining a specified accuracy via controlled lowering of the resolution of both sources and rays from each source. TREVR computes the radiation field in order N log N-source time without absorption and order N log N(source)logN time with absorption. These scalings arise from merging sources of radiation according to an opening angle criterion and walking the tree structure to trace a ray to a depth that gives the chosen accuracy for absorption. The absorption-depth refinement criterion is unique to TREVR. We provide a suite of tests demonstrating the algorithm's ability to accurately compute fluxes, ionization fronts, and shadows.
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  • On the estimation and detection of the Rees-Sciama effect

    Fullana, M. J.   Arnau, J. V.   Thacker, R. J.   Couchman, H. M. P.   Saez, D.  

    Maps of the Rees-Sciama (RS) effect are simulated using the parallel N-body code, HYDRA, and a run-time ray-tracing procedure. A method designed for the analysis of small, square cosmic microwave background (CMB) maps is applied to our RS maps. Each of these techniques has been tested and successfully applied in previous papers. Within a range of angular scales, our estimate of the RS angular power spectrum due to variations in the peculiar gravitational potential on scales smaller than 42/h megaparsecs is shown to be robust. An exhaustive study of the redshifts and spatial scales relevant for the production of RS anisotropy is developed for the first time. Results from this study demonstrate that (i) to estimate the full integrated RS effect, the initial redshift for the calculations (integration) must be greater than 25, (ii) the effect produced by strongly non-linear structures is very small and peaks at angular scales close to 4.3 arcmin, and (iii) the RS anisotropy cannot be detected either directly-in temperature CMB maps-or by looking for cross-correlations between these maps and tracers of the dark matter distribution. To estimate the RS effect produced by scales larger than 42/h megaparsecs, where the density contrast is not strongly non-linear, high accuracy N-body simulations appear unnecessary. Simulations based on approximations such as the Zel'dovich approximation and adhesion prescriptions, for example, may be adequate. These results can be used to guide the design of future RS simulations.
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  • Accurate estimators of correlation functions in Fourier space

    Sefusatti, E.   Crocce, M.   Scoccimarro, R.   Couchman, H. M. P.  

    Efficient estimators of Fourier-space statistics for large number of objects rely on fast Fourier transforms (FFTs), which are affected by aliasing from unresolved small-scale modes due to the finite FFT grid. Aliasing takes the form of a sum over images, each of them corresponding to the Fourier content displaced by increasing multiples of the sampling frequency of the grid. These spurious contributions limit the accuracy in the estimation of Fourier-space statistics, and are typically ameliorated by simultaneously increasing grid size and discarding high-frequency modes. This results in inefficient estimates for e.g. the power spectrum when desired systematic biases are well under per cent level. We show that using interlaced grids removes odd images, which include the dominant contribution to aliasing. In addition, we discuss the choice of interpolation kernel used to define density perturbations on the FFT grid and demonstrate that using higher order interpolation kernels than the standard Cloud-In-Cell algorithm results in significant reduction of the remaining images. We show that combining fourth-order interpolation with interlacing gives very accurate Fourier amplitudes and phases of density perturbations. This results in power spectrum and bispectrum estimates that have systematic biases below 0.01 per cent all the way to the Nyquist frequency of the grid, thus maximizing the use of unbiased Fourier coefficients for a given grid size and greatly reducing systematics for applications to large cosmological data sets.
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    Maxwell, Aaron J.   Wadsley, James   Couchman, H. M. P.   Mashchenko, Sergey  

    We present a new model for the formation of stellar halos in dwarf galaxies. We demonstrate that the stars and star clusters that form naturally in the inner regions of dwarfs are expected to migrate from the gas-rich, star-forming center to join the stellar spheroid. For dwarf galaxies, this process could be the dominant source of halo stars. The effect is caused by stellar-feedback-driven bulk motions of dense gas which, by causing potential fluctuations in the inner regions of the halo, couple to all collisionless components. This effect has been demonstrated to generate cores in otherwise cuspy cold dark matter profiles and is particularly effective in dwarf galaxy halos. It can build a stellar spheroid with larger ages and lower metallicities at greater radii without requiring an outside-in formation model. Globular-cluster-type star clusters can be created in the galactic ISM and then migrate to the spheroid on 100 Myr timescales. Once outside the inner regions, they are less susceptible to tidal disruption and are thus long lived; clusters on wider orbits may be easily unbound from the dwarf to join the halo of a larger galaxy during a merger. A simulated dwarf galaxy (M-vir similar or equal to 10(9) M-circle dot at z = 5) is used to examine this gravitational coupling to dark matter and stars.
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    McNally, C. P.   Wadsley, J.   Couchman, H. M. P.  

    We demonstrate a significant difference in the angular momentum transport properties of galactic disks between regions in which the interstellar medium is single phase or two phase. Our study is motivated by observations of Hi in extended galactic disks which indicate velocity dispersions of nonthermal origin, suggesting that turbulence in the gas may be contributing significantly to the observed dispersion. To address this, we have implemented a shearing-box framework within the FLASH code. The new code was used to perform local simulations of galactic disks that incorporate differential rotation, self-gravity, vertical stratification, hydrodynamics, and cooling. These simulations explore plausible mechanisms for driving turbulent motions via the thermal and self-gravitational instabilities coupling to differential rotation. Where a two-phase medium develops, gravitational angular momentum transporting stresses are much greater, creating a possible mechanism for transferring energy from galactic rotation to turbulence. In simulations where the disk conditions do not trigger the formation of a two-phase medium, it is found that perturbations to the flow damp without leading to a sustained mechanism for driving turbulence. The differing angular momentum transport properties of the single- and two-phase regimes of the disk suggest that a significant, dynamically motivated division can be drawn between the two, even when this division occurs far outside the star formation cutoff in a galactic disk.
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  • Cosmological galaxy evolution with superbubble feedback - II. The limits of supernovae

    Keller, B. W.   Wadsley, J.   Couchman, H. M. P.  

    We explore when supernovae can (and cannot) regulate the star formation and bulge growth in galaxies based on a sample of 18 simulated galaxies. The simulations are the first to model feedback superbubbles including evaporation and conduction. These processes determine the mass loadings and wind speeds of galactic outflows. We show that for galaxies with virial masses >10(12) M-circle dot, supernovae alone cannot prevent excessive star formation. This occurs due to a shutdown of galactic winds, with wind mass loadings falling from eta similar to 10 to eta < 1. In more massive systems, the ejection of baryons to the circumgalactic medium falters earlier on and the galaxies diverge significantly from observed galaxy scaling relations and morphologies. The decreasing efficiency is due to a deepening potential well preventing gas escape, and is unavoidable if mass-loaded outflows regulate star formation on galactic scales. This implies that non-supernova feedback mechanisms must become dominant for galaxies with stellar masses greater than similar to 4 x 10(10) M-circle dot. The runaway growth of the central stellar bulge, strongly linked to black hole growth, suggests that feedback from active galactic nuclei is the likely mechanism. Below this mass, supernovae alone are able to produce a realistic stellar mass fraction, star formation history and disc morphology.
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  • On the treatment of entropy mixing in numerical cosmology

    Wadsley, J. W.   Veeravalli, G.   Couchman, H. M. P.  

    For simulations of fluid dynamics in astrophysics, physical viscosity and diffusion are typically neglected. However, in this high Reynolds number regime, real fluids become highly turbulent and turbulent processes mediate substantial transport of momentum and heat that is diffusive in nature. In the absence of models for these processes, code-dependent numerical effects dominate how diffusion operates and may lead to physically incorrect simulation results. We highlight the qualitative difference in these numerical effects for smooth particle hydrodynamics (SPH) and grid-based Eulerian codes using two test problems: a buoyant gas bubble and gas in a galaxy cluster. Grid codes suffer from numerical diffusion in the absence of explicit terms, and small-scale diffusion of heat is completely absent in the Lagrangian SPH method. We find that SPH with heat diffusion added at a level similar to that expected from turbulence diffusion generates more physically appealing results. These results suggest, but do not confirm, that a flat entropy core is to be expected for gas in an idealized galaxy cluster (i.e. one without physics beyond that of a non-radiating gas). A goal of this work is thus to draw attention to the as yet unfulfilled need for models of turbulent diffusive processes in compressible gases in astrophysics.
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  • Measuring AGN feedback with the Sunyaev-Zel'dovich effect

    Scannapieco, Evan   Thacker, Robert J.   Couchman, H. M. P.  

    One of the most important and poorly understood issues in structure formation is the role of outflows driven by active galactic nuclei (AGNs). Using large-scale cosmological simulations, we compute the impact of such outflows on the small-scale distribution of the cosmic microwave background (CMB). Like gravitationally heated structures, AGN outflows induce CMB distortions through both thermal motions and peculiar velocities, by processes known as the thermal and kinetic Sunyaev-Zel'dovich (SZ) effects, respectively. For AGN outflows the thermal SZ effect is dominant, doubling the angular power spectrum on arcminute scales. But the most distinct imprint of AGN feedback is a substantial increase in the thermal SZ distortions around elliptical galaxies, poststarburst elliptical galaxies, and quasars that is linearly proportional to the outflow energy. While point-source subtraction is difficult for quasars, we show that by appropriately stacking microwave measurements around early-type galaxies, the new generation of small-scale microwave telescopes will be able to directly measure AGN feedback at the level that is important for current theoretical models.
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  • Stellar feedback in dwarf galaxy formation

    Mashchenko, Sergey   Wadsley, James   Couchman, H. M. P.  

    Dwarf galaxies pose substantial challenges for cosmological models. In particular, current models predict a dark- matter density that is divergent at the center, which is in sharp contrast with observations that indicate a core of roughly constant density. Energy feedback, from supernova explosions and stellar winds, has been proposed as a major factor shaping the evolution of dwarf galaxies. We present detailed cosmological simulations with sufficient resolution both to model the relevant physical processes and to directly assess the impact of stellar feedback on observable properties of dwarf galaxies. We show that feedback drives large- scale, bulk motions of the interstellar gas, resulting in substantial gravitational potential fluctuations and a consequent reduction in the central matter density, bringing the theoretical predictions in agreement with observations.
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  • Quasars: What turns them off?

    Thacker, Robert J.   Scannapieco, Evan   Couchman, H. M. P.  

    While the high-redshift quasar luminosity function closely parallels the hierarchical growth of dark matter halos, at lower redshifts quasars exhibit an antihierarchical turnoff, which moves from the most luminous objects to the faintest. We explore the idea that this may arise from self-regulating feedback, caused by quasar outflows. Using a hybrid approach that combines a detailed hydrodynamic simulation with observationally derived relationships, we calculate the luminosity function of quasars down to a redshift of z = 1 in a large, cosmologically representative volume. Outflows are included explicitly by tracking halo mergers and driving shocks into the surrounding intergalactic medium, with an energy output equal to a fixed 5% fraction of the bolometric luminosity. Our results are in excellent agreement with measurements of the spatial distribution of quasars on both small and large scales, and we detect an intriguing excess of galaxy-quasar pairs at very short separations. Our results also reproduce an antihierarchical turnoff in the quasar luminosity function; however, this falls short of that observed, as well as that predicted by analogous semianalytic models. The difference can be traced to the treatment of gas heating within galaxies and the presence of in-shock cooling. The simulated galaxy cluster L-X-T relationship is close to that observed for z z I clusters, but the simulated galaxy groups at z = 1 are significantly perturbed by quasar outflows. Measurements of anomalously high X-ray emission in high-redshift groups, along with detections of 1000 km s(-1) winds in poststarburst ellipticals, would provide definitive evidence for the AGN-heating hypothesis.
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  • The Celestial Buffet: multiple populations and globular cluster formation in dwarf galaxies

    Maxwell, Aaron J.   Wadsley, James   Couchman, H. M. P.   Sills, Alison  

    We present a framework that explains the commonly observed variation in light element abundances in globular clusters. If globular clusters form in the centres of dwarf galaxies, they will be pumped on to larger orbits as star formation progresses. The potential well will only retain the moderate velocity asymptotic giant branch (AGB) ejecta, the expected source of enrichment, but not supernova ejecta. There is no need to increase the initial cluster mass, a requirement of self-enrichment scenarios, as all the stars within the dwarf can contribute. As the clusters move through the dwarf centre they sweep up a mix of AGB ejecta and in-falling pristine gas to form a second generation of stars. The specific mix will vary in time and is thus able to explain the spread in second generation abundances observed in different clusters. The globular clusters will survive to the present day or be stripped as part of the hierarchical merging process of larger galaxies. We illustrate how this process may operate using a high-resolution simulation of a dwarf galaxy at high redshift.
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  • The removal of cusps from galaxy centres by stellar feedback in the early Universe

    Mashchenko, Sergey   Couchman, H. M. P.   Wadsley, James  

    The standard cosmological model, now strongly constrained by direct observations of the Universe at early epochs, is very successful in describing the evolution of structure on large and intermediate scales(1). Unfortunately, serious contradictions remain on smaller, galactic scales(1,2). Among the main small-scale problems is a significant and persistent discrepancy between observations of nearby galaxies, which imply that galactic dark matter haloes have a density profile with a flat core(3-6), and the cosmological model, which predicts that the haloes should have divergent density ( a cusp) at the centre(7,8). Here we report numerical simulations that show that random bulk motions of gas in small primordial galaxies, of the magnitude expected in these systems, will result in a flattening of the central dark matter cusp on relatively short timescales (similar to 10(8) years). Gas bulk motions in early galaxies are driven by supernova explosions that result from ongoing star formation. Our mechanism is general, and would have operated in all star-forming galaxies at redshifts z >= 10. Once removed, the cusp cannot be reintroduced during the subsequent mergers involved in the build-up of larger galaxies(9,10). As a consequence, in the present Universe both small and large galaxies would have flat dark matter core density profiles, in agreement with observations.
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  • MAGICC haloes: confronting simulations with observations of the circumgalactic medium at z=0

    Stinson, G. S.   Brook, C.   Prochaska, J. Xavier   Hennawi, Joe   Shen, Sijing   Wadsley, J.   Pontzen, Andrew   Couchman, H. M. P.   Quinn, T.   Maccio, Andrea V.   Gibson, Brad K.  

    We explore the circumgalactic medium (CGM) of two simulated star-forming galaxies with luminosities L 0.1 and 1?L? generated using the smooth particle hydrodynamic code gasoline. These simulations are part of the Making Galaxies In a Cosmological Context (magicc) program in which the stellar feedback is tuned to match the stellar masshalo mass relationship. For comparison, each galaxy was also simulated using a lower feedback (LF) model which has strength comparable to other implementations in the literature. The magicc feedback (MF) model has a higher incidence of massive stars and an approximately two times higher energy input per supernova. Apart from the low-mass halo using LF, each galaxy exhibits a metal-enriched CGM that extends to approximately the virial radius. A significant fraction of this gas has been heated in supernova explosions in the disc and subsequently ejected into the CGM where it is predicted to give rise to substantial O?vi absorption. The simulations do not yet address the question of what happens to the O?vi when the galaxies stop forming stars. Our models also predict a reservoir of cool H?i clouds that show strong Lya absorption to several hundred kpc. Comparing these models to recent surveys with the Hubble Space Telescope, we find that only the MF models have sufficient O?vi and H?i gas in the CGM to reproduce the observed distributions. In separate analyses, these same MF models also show better agreement with other galaxy observables (e.g. rotation curves, surface brightness profiles and H?i gas distribution). We infer that the CGM is the dominant reservoir of baryons for galaxy haloes.
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  • The distribution of metals in cosmological hydrodynamical simulations of dwarf disc galaxies

    Pilkington, K.   Gibson, B. K.   Brook, C. B.   Calura, F.   Stinson, G. S.   Thacker, R. J.   Michel-Dansac, L.   Bailin, J.   Couchman, H. M. P.   Wadsley, J.   Quinn, T. R.   Maccio, A.  

    We examine the chemical properties of five cosmological hydrodynamical simulations of an M33-like disc galaxy which have been shown previously to be consistent with the morphological characteristics and bulk scaling relations expected of late-type spirals. These simulations are part of the Making Galaxies in a Cosmological Context Project, in which stellar feedback is tuned to match the stellar masshalo mass relationship. Each realization employed identical initial conditions and assembly histories, but differed from one another in their underlying baryonic physics prescriptions, including (a) the efficiency with which each supernova energy couples to the surrounding interstellar medium, (b) the impact of feedback associated with massive star radiation pressure, (c) the role of the minimum shut-off time for radiative cooling of Type II supernovae remnants, (d) the treatment of metal diffusion and (e) varying the initial mass function. Our analysis focusses on the resulting stellar metallicity distribution functions (MDFs) in each simulated (analogous) solar neighbourhood (23 disc scalelengths from the galactic centre) and central bulge region. We compare and contrast the simulated MDFs skewness, kurtosis and dispersion (inter-quartile, inter-decile, inter-centile and inter-tenth-percentile regions) with that of the empirical solar neighbourhood MDF and Local Group dwarf galxies. We find that the MDFs of the simulated discs are more negatively skewed, with higher kurtosis, than those observed locally in the Milky Way and Local Group dwarfs. We can trace this difference to the simulations very tight and correlated agemetallicity relations (compared with that of the Milky Way's solar neighbourhood), suggesting that these relations within dwarf discs might be steeper than in L? discs (consistent with the simulations star formation histories and extant empirical data), and/or the degree of stellar orbital redistribution and migration inferred locally has not been captured in their entirety, at the resolution of our simulations. The important role of metal diffusion in ameliorating the overproduction of extremely metal-poor stars is highlighted.
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  • The properties of bound and unbound molecular cloud populations formed in galactic disc simulations

    Ward, Rachel L.   Benincasa, Samantha M.   Wadsley, James   Sills, Alison   Couchman, H. M. P.  

    We explore the effect of galactic environment on properties of molecular clouds. Using clouds formed in a large-scale galactic disc simulation, we measure the observable properties from synthetic column density maps. We confirm that a significant fraction of unbound clouds forms naturally in a galactic disc environment and that a mixed population of bound and unbound clouds can match observed scaling relations and distributions for extragalactic molecular clouds. By dividing the clouds into inner and outer disc populations, we compare their distributions of properties and test whether there are statistically significant differences between them. We find that clouds in the outer disc have lower masses, sizes, and velocity dispersions as compared to those in the inner disc for reasonable choices of the inner/outer boundary. We attribute the differences to the strong impact of galactic shear on the disc stability at large galactocentric radii. In particular, our Toomre analysis of the disc shows a narrowing envelope of unstable masses as a function of radius, resulting in the formation of smaller, lower mass fragments in the outer disc. We also show that the star formation rate is affected by the environment of the parent cloud, and is particularly influenced by the underlying surface density profile of the gas throughout the disc. Our work highlights the strengths of using galaxy-scale simulations to understand the formation and evolution of cloud properties - and the star formation within them - in the context of their environment.
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  • A Numerical Renormalization Solution for Self‐similar Cosmic Structure Formation

    Couchman, H. M. P.   Peebles, P. J. E.  

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