Smoothed Particle Hydrodynamics in cosmology: a comparative study of implementations

We analyse the performance of twelve different implementations of Smoothed Particle Hydrodynamics (SPH) using seven tests designed to isolate key hydrodynamic elements of cosmological simulations which are known to cause the SPH algorithm problems. In order, we consider a shock tube, spherical adiabatic collapse, cooling flow model, drag, a cosmological simulation, rotating cloud-collapse and disc stability. In the implementations special attention is given to the way in which force symmetry is enforced in the equations of motion. We study in detail how the hydrodynamics are affected by different implementations of the artificial viscosity including those with a shear-correction modification. We present an improved first-order smoothing-length update algorithm that is designed to remove instabilities that are present in the Hernquist and Katz (1989) algorithm. For all tests we find that the artificial viscosity is the most important factor distinguishing the results from the various implementations. The second most important factor is the way force symmetry is achieved in the equation of motion. Most results favour a kernel symmetrization approach. The exact method by which SPH pressure forces are included has comparatively little effect on the results. Combining the equation of motion presented in Thomas and Couchman (1992) with a modification of the Monaghan and Gingold (1983) artificial viscosity leads to an SPH scheme that is both fast and reliable.Comment: 30 pages, 26 figures and 9 tables included. Submitted to MNRAS. Postscript version available at ftp://phobos.astro.uwo.ca/pub/etittley/papers/sphtest.ps.g

The nongravitational interactions of dark matter in colliding galaxy clusters

Collisions between galaxy clusters provide a test of the nongravitational forces acting on dark matter. Dark matter’s lack of deceleration in the “bullet cluster” collision constrained its self-interaction cross section σDM/m < 1.25 square centimeters per gram (cm2/g) [68% confidence limit (CL)] (σDM, self-interaction cross section; m, unit mass of dark matter) for long-ranged forces. Using the Chandra and Hubble Space Telescopes, we have now observed 72 collisions, including both major and minor mergers. Combining these measurements statistically, we detect the existence of dark mass at 7.6σ significance. The position of the dark mass has remained closely aligned within 5.8 ± 8.2 kiloparsecs of associated stars, implying a self-interaction cross section σDM/m < 0.47 cm2/g (95% CL) and disfavoring some proposed extensions to the standard model

Dark matter astrometry: accuracy of subhalo positions for the measurement of self-interaction cross-sections

Direct evidence for the existence of dark matter and measurements of its interaction cross-section have been provided by the physical offset between dark matter and intracluster gas in merging systems like the Bullet Cluster. Although a smaller signal, this effect is more abundant in minor mergers where infalling substructure dark matter and gas are segregated. In such low-mass systems the gravitational lensing signal comes primarily from weak lensing. A fundamental step in determining such an offset in substructure is the ability to accurately measure the positions of dark matter subpeaks. Using simulated Hubble Space Telescope observations, we make a first assessment of the precision and accuracy with which we can measure infalling groups using weak gravitational lensing. We demonstrate that using an existing and well-used mass reconstruction algorithm can measure the positions of 1.5 × 1013 M⊙ substructures that have parent haloes 10 times more massive with a bias of less than 0.3 arcsec. In this regime, our analysis suggests the precision is sufficient to detect (at 3σ statistical significance) the expected mean offset between dark matter and baryonic gas in infalling groups from a sample of ∼50 massive clusters

The Mysterious Merger of NGC6868 and NGC6861 in the Telescopium Group

We use Chandra X-ray observations of the hot gas in and around NGC6868 and NGC6861 in the Telescopium galaxy group (AS0851) to probe the interaction history between these galaxies. Mean surface brightness profiles for NGC6868 and NGC6861 are each well described by double beta-models, suggesting that they are each the dominant galaxy in a galaxy subgroup about to merge. Surface brightness and temperature maps of the brightest group galaxy NGC6868 show a cold front edge ~23 kpc to the north, and a cool 0.62 keV spiral-shaped tail to the south. Analysis of the temperature and density across the cold front constrains the relative motion between NGC6868 and the ambient group gas to be at most transonic; while the spiral morphology of the tail strongly suggests that the cold front edge and tail are the result of gas sloshing due to the subgroup merger. The cooler central region of NGC6861 is surrounded by a sheath of hot gas to the east and hot, bifurcated tails of X-ray emission to the west and northwest. We discuss supersonic infall of the NGC6861 subroup, sloshing from the NGC6868 and NGC6861 subgroup merger, and AGN heating as possible explanations for these features, and discuss possible scenarios that may contribute to the order of magnitude discrepancy between the Margorrian and black hole mass - sigma predictions for its central black hole.Comment: 17 pages, 23 figures, submitted to Ap

Metal transport by gas sloshing in M87

We present the results of an XMM-Newton mosaic covering the central ~200 kpc of the nearby Virgo cluster. We focus on a strong surface brightness discontinuity in the outskirts of the brightest cluster galaxy, M87. Using both XMM-Newton and Suzaku, we derive accurate temperature and metallicity profiles across this feature and show that it is a cold front probably due to sloshing of the Virgo ICM. It is also associated with a discontinuity in the chemical composition. The gas in the inner, bright region of the front is ~40% more abundant in Fe than the gas outside the front, suggesting the important role of sloshing in transporting metals through the ICM. For the first time, we provide a quantitative estimate of the mass of Fe transported by a cold front. This amounts to ~6% of the total Fe mass within the radial range affected by sloshing, significantly more than the amount of metals transported by the AGN in the same cluster core. The very low Fe abundance of only ~0.2 solar immediately outside the cold front at a radius of 90 kpc suggests we are witnessing first-hand the transport of higher metallicity gas into a pristine region, whose abundance is typical of the cluster outskirts. The Mg/Fe and O/Fe abundance ratios remain approximately constant over the entire radial range between the centre of M87 and the faint side of the cold front, which requires the presence of a centrally peaked distribution not only for Fe but also for core-collapse type supernova products. This peak may stem from the star formation triggered as the BCG assembled during the protocluster phase.Comment: accepted for publication in MNRA

Dynamical friction of bodies orbiting in a gaseous sphere

The dynamical friction experienced by a body moving in a gaseous medium is different from the friction in the case of a collisionless stellar system. Here we consider the orbital evolution of a gravitational perturber inside a gaseous sphere using three-dimensional simulations, ignoring however self-gravity. The results are analysed in terms of a `local' formula with the associated Coulomb logarithm taken as a free parameter. For forced circular orbits, the asymptotic value of the component of the drag force in the direction of the velocity is a slowly varying function of the Mach number in the range 1.0-1.6. The dynamical friction timescale for free decay orbits is typically only half as long as in the case of a collisionless background, which is in agreement with E.C. Ostriker's recent analytic result. The orbital decay rate is rather insensitive to the past history of the perturber. It is shown that, similar to the case of stellar systems, orbits are not subject to any significant circularization. However, the dynamical friction timescales are found to increase with increasing orbital eccentricity for the Plummer model, whilst no strong dependence on the initial eccentricity is found for the isothermal sphere.Comment: 13 pages, 13 figures, MNRAS accepte

Gas sloshing, cold fronts, Kelvin-Helmholtz instabilities and the merger history of the cluster of galaxies Abell 496

We investigate the origin and nature of the multiple sloshing cold fronts in the core of Abell 496 by direct comparison between observations and dedicated hydrodynamical simulations. Our simulations model a minor merger with a 4{\times}10^13M{\circ} subcluster crossing A496 from the south-west to the north-north-east, passing the cluster core in the south-east at a pericentre distance 100 to a few 100 kpc about 0.6 to 0.8 Gyr ago. The gas sloshing triggered by the merger can reproduce almost all observed features, e.g. the characteristic spiral-like brightness residual distribution in the cluster centre and its asymmetry out to 500 kpc, also the positions of and contrasts across the cold fronts. If the subcluster passes close (100 kpc) to the cluster core, the resulting shear flows are strong enough to trigger Kelvin-Helmholtz instabilities that in projection resemble the peculiar kinks in the cold fronts of Abell 496. Finally, we show that sloshing does not lead to a significant modification of the global ICM profiles but a mild oscillation around the initial profiles.Comment: MNRAS, accepted, 19 page

Sloshing Gas in the Core of the Most Luminous Galaxy Cluster RXJ1347.5-1145

We present new constraints on the merger history of the most X-ray luminous cluster of galaxies, RXJ1347.5-1145, based its unique multiwavelength morphology. Our X-ray analysis confirms the core gas is undergoing "sloshing" resulting from a prior, large scale, gravitational perturbation. In combination with extensive multiwavelength observations, the sloshing gas points to the primary and secondary clusters having had at least two prior strong gravitational interactions. The evidence supports a model in which the secondary subcluster with mass M=4.8$\pm2.4 \times$ 10$^{14}$ M$_{\odot}$ has previously ($\gtrsim$0.6 Gyr ago) passed by the primary cluster, and has now returned for a subsequent crossing where the subcluster's gas has been completely stripped from its dark matter halo. RXJ1347 is a prime example of how core gas sloshing may be used to constrain the merger histories of galaxy clusters through multiwavelength analyses.Comment: 17 pages, 5 figures; higher resolution figures available in online ApJ versio

What is a Cool-Core Cluster? A Detailed Analysis of the Cores of the X-ray Flux-Limited HIFLUGCS Cluster Sample

We use the largest complete sample of 64 galaxy clusters (HIghest X-ray FLUx Galaxy Cluster Sample) with available high-quality X-ray data from Chandra, and apply 16 cool-core diagnostics to them, some of them new. We also correlate optical properties of brightest cluster galaxies (BCGs) with X-ray properties. To segregate cool core and non-cool-core clusters, we find that central cooling time, t_cool, is the best parameter for low redshift clusters with high quality data, and that cuspiness is the best parameter for high redshift clusters. 72% of clusters in our sample have a cool core (t_cool < 7.7 h_{71}^{-1/2} Gyr) and 44% have strong cool cores (t_cool <1.0 h_{71}^{-1/2} Gyr). For the first time we show quantitatively that the discrepancy in classical and spectroscopic mass deposition rates can not be explained with a recent formation of the cool cores, demonstrating the need for a heating mechanism to explain the cooling flow problem. [Abridged]Comment: 45 pages, 19 figures, 7 tables. Accepted for publication in A&A. Contact Person: Rupal Mittal ([email protected]

A Chandra X-ray Analysis of Abell 1664: Cooling, Feedback and Star Formation in the Central Cluster Galaxy

The brightest cluster galaxy (BCG) in the Abell 1664 cluster is unusually blue and is forming stars at a rate of ~ 23 M_{\sun} yr^{-1}. The BCG is located within 5 kpc of the X-ray peak, where the cooling time of 3.5x10^8 yr and entropy of 10.4 keV cm^2 are consistent with other star-forming BCGs in cooling flow clusters. The center of A1664 has an elongated, "bar-like" X-ray structure whose mass is comparable to the mass of molecular hydrogen, ~ 10^{10} M_{\sun} in the BCG. We show that this gas is unlikely to have been stripped from interloping galaxies. The cooling rate in this region is roughly consistent with the star formation rate, suggesting that the hot gas is condensing onto the BCG. We use the scaling relations of Birzan et al. 2008 to show that the AGN is underpowered compared to the central X-ray cooling luminosity by roughly a factor of three. We suggest that A1664 is experiencing rapid cooling and star formation during a low-state of an AGN feedback cycle that regulates the rates of cooling and star formation. Modeling the emission as a single temperature plasma, we find that the metallicity peaks 100 kpc from the X-ray center, resulting in a central metallicity dip. However, a multi-temperature cooling flow model improves the fit to the X-ray emission and is able to recover the expected, centrally-peaked metallicity profile.Comment: 15 pages, 13 figure