33 research outputs found
Improved Performances in Subsonic Flows of an SPH Scheme with Gradients Estimated using an Integral Approach
In this paper, we present results from a series of hydrodynamical tests aimed at validating the performance of a smoothed particle hydrodynamics (SPH) formulation in which gradients are derived from an integral approach. We specifically investigate the code behavior with subsonic flows, where it is well known that zeroth-order inconsistencies present in standard SPH make it particularly problematic to correctly model the fluid dynamics. In particular, we consider the Gresho-Chan vortex problem, the growth of Kelvin-Helmholtz instabilities, the statistics of driven subsonic turbulence and the cold Keplerian disk problem. We compare simulation results for the different tests with those obtained, for the same initial conditions, using standard SPH. We also compare the results with the corresponding ones obtained previously with other numerical methods, such as codes based on a moving-mesh scheme or Godunov-type Lagrangian meshless methods. We quantify code performances by introducing error norms and spectral properties of the particle distribution, in a way similar to what was done in other works. We find that the new SPH formulation exhibits strongly reduced gradient errors and outperforms standard SPH in all of the tests considered. In fact, in terms of accuracy, we find good agreement between the simulation results of the new scheme and those produced using other recently proposed numerical schemes. These findings suggest that the proposed method can be successfully applied for many astrophysical problems in which the presence of subsonic flows previously limited the use of SPH, with the new scheme now being competitive in these regimes with other numerical methods
Modelling the Power Spectrum of Density Fluctuations: A Phenomenological Approach
We show how, based on considerations on the observed form of the galaxy
2-point spatial correlation function xi(r), a very simplified -- yet
surprisingly effective -- model for the linear density fluctuations power
spectrum can be constructed. We first relate the observed large-scale shape of
xi(r) to a power-law form for the power spectrum, P(k)\propto k^{-2.2}. For a
plausible value of the bias parameter b = 1/sigma_8 ~ 1.8, one has (delta_rho /
rho)_{rms} ~ 1 r ~ 3.5/h Mpc, suggesting that the change of slope observed in
xi(r) around this scale marks the transition between the linear and nonlinear
gravitational regimes. Under this working hypothesis, we use a simple
analytical form to fit the large-scale correlations constraints together with
the COBE CMB anisotropy measurement, thus constructing a simple
phenomenological model for the linear power spectrum. Despite its simplicity,
the model fits remarkably well directly estimated power spectra from different
optical galaxy samples, and when evolved through an N-body simulation it
provides a good match to the observed galaxy correlations. One of the most
interesting features of the model is the small-scale one-dimensional velocity
dispersion produced: sigma_{1d} = 450 Km s^{-1} at 0.5/h Mpc and sigma_{1d} =
350 Km s^{-1} for separations larger than ~ 2/h Mpc.Comment: ApJL in press, 10 pages in plain TeX, 3 figures available from
[email protected], SISSA 110/93/
Overlapping inflows as catalysts of AGN activity - II. Relative importance of turbulence and inflow-disc interaction
The main challenge for understanding the fuelling of supermassive black holes in active galactic nuclei is not to account for the source of fuel, but rather to explain its delivery from the boundaries of the black hole sphere of influence (10-100 pc) down to sub-parsec scales. In this work, we report on a series of numerical experiments aimed at exploring in further depth our model of 'overlapping inflow events' as catalysts for rapid accretion, seeding a turbulent field in the infalling gas. We initially set a gaseous shell in non-equilibrium rotation around a supermassive black hole. After infall, the shell stalls in a disc-like structure. A second shell is then set in either corotation or counterrotation with respect to the first and is let to impinge on the previously formed disc. We find that combined turbulence and overlap significantly enhance accretion in counterrotating inflows, while turbulence dominates for corotating inflows. The leftovers of overlapping inflows are warped nuclear discs, whose morphology depend on the relative orientation and angular momentum of the disc and the shell. Overlapping inflows leave observational signatures in the gas rotation curves
Measurement of the dark matter velocity anisotropy in galaxy clusters
The internal dynamics of a dark matter structure may have the remarkable
property that the local temperature in the structure depends on direction. This
is parametrized by the velocity anisotropy beta which must be zero for relaxed
collisional structures, but has been shown to be non-zero in numerical
simulations of dark matter structures. Here we present a method to infer the
radial profile of the velocity anisotropy of the dark matter halo in a galaxy
cluster from X-ray observables of the intracluster gas. This non-parametric
method is based on a universal relation between the dark matter temperature and
the gas temperature which is confirmed through numerical simulations. We apply
this method to observational data and we find that beta is significantly
different from zero at intermediate radii. Thus we find a strong indication
that dark matter is effectively collisionless on the dynamical time-scale of
clusters, which implies an upper limit on the self-interaction cross-section
per unit mass sigma/m < 1 cm2/g. Our results may provide an independent way to
determine the stellar mass density in the central regions of a relaxed cluster,
as well as a test of whether a cluster is in fact relaxed.Comment: 10 pages, 8 figures, submitted to Ap
X-ray temperature spectroscopy of simulated cooling clusters
Results from a large sample of hydrodynamical/N-body simulations of galaxy
clusters in a LCDM cosmology are used to simulate cluster X-ray observations as
expected from Chandra observations. The physical modeling of the gas includes
radiative cooling, star formation, energy feedback and metal enrichment. The
biasing of spectral temperatures with respect to mass-weighted temperatures is
found to be influenced by two independent processes. The first scale dependency
is absent in adiabatic runs and is due to cooling, whose efficiency to
transform cold gas into stars is higher for cool clusters and this in turn
implies a strong dependency of the spectral versus mass-weighted temperature
relation on the cluster mass. The second dependency is due to photon emission
because of cool gas which is accreted during merging events and biases the
spectral fits. These events have been quantified according to the power ratio
method and a robust correlation is found to exist between the spectral bias and
the amount of cluster substructure. The shape of the simulated temperature
profiles is not universal and it is steeper at the cluster center for cool
clusters than for the massive ones. The profiles are in good agreement with
data in the radial range between and ; at
small radii () the cooling runs fail to reproduce the shape of
the observed profiles. The fit is improved if one considers a hierarchical
merging scenario in which cluster cores can accrete cooler gas through merging
with cluster subclumps, though the shape of the temperature profiles is
modified in a significant way only in the regime where the mass of the
substructure is a large fraction of the cluster mass.Comment: 46 pages, 10 figures, 3 tables, accepted for publication in NewA new
version with references update
A global descriptor of spatial pattern interaction in the galaxy distribution
We present the function J as a morphological descriptor for point patterns
formed by the distribution of galaxies in the Universe. This function was
recently introduced in the field of spatial statistics, and is based on the
nearest neighbor distribution and the void probability function. The J
descriptor allows to distinguish clustered (i.e. correlated) from ``regular''
(i.e. anti-correlated) point distributions. We outline the theoretical
foundations of the method, perform tests with a Matern cluster process as an
idealised model of galaxy clustering, and apply the descriptor to galaxies and
loose groups in the Perseus-Pisces Survey. A comparison with mock-samples
extracted from a mixed dark matter simulation shows that the J descriptor can
be profitably used to constrain (in this case reject) viable models of cosmic
structure formation.Comment: Significantly enhanced version, 14 pages, LaTeX using epsf, aaspp4, 7
eps-figures, accepted for publication in the Astrophysical Journa
Overlapping inflow events as catalysts for supermassive black hole growth
One of the greatest issues in modelling black hole fuelling is our lack of understanding of the processes by which gas loses angular momentum and falls from galactic scales down to the nuclear region where an accretion disc forms, subsequently guiding the inflow of gas down to the black hole horizon. It is feared that gas at larger scales might still retain enough angular momentum and settle into a larger scale disc with very low or no inflow to form or replenish the inner accretion disc (on similar to 0.01 pc scales). In this paper we report on hydrodynamical simulations of rotating infalling gas shells impacting at different angles on to a pre-existing, primitive large-scale (similar to 10 pc) disc around a supermassive black hole. The aim is to explore how the interaction between the shell and the disc redistributes the angular momentum on scales close to the black hole's sphere of influence. Angular momentum redistribution via hydrodynamical shocks leads to inflows of gas across the inner boundary, enhancing the inflow rate by more than 2-3 orders of magnitude. In all cases, the gas inflow rate across the inner parsec is higher than in the absence of the interaction, and the orientation of the angular momentum of the flow in the region changes with time due to gas mixing. Warped discs or nested misaligned rings form depending on the angular momentum content of the infalling shell relative to the disc. In the cases in which the shell falls in near counter-rotation, part of the resulting flows settle into an inner dense disc which becomes more susceptible to mass transfer
LoCuSS: A comparison of cluster mass measurements from XMM-Newton and subaru - Testing deviation from hydrostatic equilibrium and non-thermal pressure support
We compare X-ray hydrostatic and weak-lensing mass estimates for a sample of 12 clusters that have been observed with both XMM-Newton and Subaru. At an over-density of \u394 = 500, we obtain 1 - M X/M WL = 0.01 \ub1 0.07 for the whole sample. We also divided the sample into undisturbed and disturbed sub-samples based on quantitative X-ray morphologies using asymmetry and fluctuation parameters, obtaining 1 - M X/M WL = 0.09 \ub1 0.06 and -0.06 \ub1 0.12 for the undisturbed and disturbed clusters, respectively. In addition to non-thermal pressure support, there may be a competing effect associated with adiabatic compression and/or shock heating which leads to overestimate of X-ray hydrostatic masses for disturbed clusters, for example, in the famous merging cluster A1914. Despite the modest statistical significance of the mass discrepancy, on average, in the undisturbed clusters, we detect a clear trend of improving agreement between M X and M WL as a function of increasing over-density, M^X/M^WL=(0.908 \ub1 0.004)+(0.187 \ub1 0.010) cdot log_{10} (\u394 /500). We also examine the gas mass fractions, f gas = M gas/M WL, finding that they are an increasing function of cluster radius, with no dependence on dynamical state, in agreement with predictions from numerical simulations. Overall, our results demonstrate that XMM-Newton and Subaru are a powerful combination for calibrating systematic uncertainties in cluster mass measurements