arXiv subjects

J. I. Read

Publications and source records attributed to J. I. Read.

At least 37 records · Page 2Linked to original sources

arXiv2011

SPHS: Smoothed Particle Hydrodynamics with a higher order dissipation switch

We present a novel implementation of Smoothed Particle Hydrodynamics (SPHS) that uses the spatial derivative of the velocity divergence as a higher order dissipation switch. Our switch -- which is second order accurate -- detects flow convergence before it occurs. If particle trajectories are going to cross, we switch on the usual SPH artificial viscosity, as well as conservative dissipation in all advected fluid quantities (for example, the entropy). The viscosity and dissipation terms (that are numerical errors) are designed to ensure that all fluid quantities remain single-valued as particles approach one another, to respect conservation laws, and to vanish on a given physical scale as the resolution is increased. SPHS alleviates a number of known problems with `classic' SPH, successfully resolving mixing, and recovering numerical convergence with increasing resolution. An additional key advantage is that -- treating the particle mass similarly to the entropy -- we are able to use multimass particles, giving significantly improved control over the refinement strategy. We present a wide range of code tests including the Sod shock tube, Sedov-Taylor blast wave, Kelvin-Helmholtz Instability, the `blob test', and some convergence tests. Our method performs well on all tests, giving good agreement with analytic expectations.

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arXiv2011

Dark matter profiles and annihilation in dwarf spheroidal galaxies: prospectives for present and future gamma-ray observatories - I. The classical dSphs

Due to their large dynamical mass-to-light ratios, dwarf spheroidal galaxies (dSphs) are promising targets for the indirect detection of dark matter (DM) in gamma-rays. We examine their detectability by present and future gamma-ray observatories. The key innovative features of our analysis are: (i) We take into account the angular size of the dSphs; while nearby objects have higher gamma ray flux, their larger angular extent can make them less attractive targets for background-dominated instruments. (ii) We derive DM profiles and the astrophysical J-factor (which parameterises the expected gamma-ray flux, independently of the choice of DM particle model) for the classical dSphs directly from photometric and kinematic data. We assume very little about the DM profile, modelling this as a smooth split-power law distribution, with and without sub-clumps. (iii) We use a Markov Chain Monte Carlo (MCMC) technique to marginalise over unknown parameters and determine the sensitivity of our derived J-factors to both model and measurement uncertainties. (iv) We use simulated DM profiles to demonstrate that our J-factor determinations recover the correct solution within our quoted uncertainties. Our key findings are: (i) Sub-clumps in the dSphs do not usefully boost the signal; (ii) The sensitivity of atmospheric Cherenkov telescopes to dSphs within 20 kpc with cored halos can be up to ~50 times worse than when estimated assuming them to be point-like. Even for the satellite-borne Fermi-LAT the sensitivity is significantly degraded on the relevant angular scales for long exposures, hence it is vital to consider the angular extent of the dSphs when selecting targets; (iii) No DM profile has been ruled out by current data, but using a prior on the inner dark matter cusp slope 0<=gamma<=1 provides J-factor estimates accurate to a factor of a few if an appropriate angular scale [abridged]

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arXiv2010

Core creation in galaxies and haloes via sinking massive objects

We perform a detailed investigation into the disruption of central cusps via the transfer of energy from sinking massive objects. Constant density inner regions form at the radius where the enclosed mass approximately matches the mass of the infalling body. We explore parameter space using numerical simulations and give an empirical relation for the size of the resulting core within structures that have different initial cusp slopes. We find that infalling bodies always stall at the edge of these newly formed cores, experiencing no dynamical friction over many dynamical times. As applications, we consider the resulting decrease in the dark matter annihilation flux due to centrally destroyed cusps, and we present a new theory for the formation of close binary nuclei -- the `stalled binary' model. We focus on one particularly interesting binary nucleus system, the dwarf spheroidal galaxy VCC 128 which is dark matter dominated at all radii. We show that its nuclei would rapidly coalesce within a few million years if it has a central dark matter cusp slope steeper than r^{-1}. However, if its initial dark matter cusp is slightly shallower than a log slope of -0.75 at ~0.1% of the virial radius, then the sinking nuclei naturally create a core equal to their observed separation and stall. This is close to the log slope measured a recent billion particle CDM halo simulation.

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arXiv2010

A dark matter disc in three cosmological simulations of Milky Way mass galaxies

Making robust predictions for the phase space distribution of dark matter at the solar neighbourhood is vital for dark matter direct detection experiments. To date, almost all such predictions have been based on simulations that model the dark matter alone. Here, we use three cosmological hydrodynamics simulations of bright, disc dominated galaxies to include the effects of baryonic matter self-consistently for the first time. We find that the addition of baryonic physics drastically alters the dark matter profile in the vicinity of the Solar neighbourhood. A stellar/gas disc, already in place at high redshift, causes merging satellites to be dragged preferentially towards the disc plane where they are torn apart by tides. This results in an accreted dark matter disc that contributes ~0.25 - 1.5 times the non-rotating halo density at the solar position. The dark disc, unlike dark matter streams, is an equilibrium structure that must exist in disc galaxies that form in a hierarchical cosmology. Its low rotation lag with respect to the Earth significantly boosts WIMP capture in the Earth and Sun, boosts the annual modulation signal, and leads to distinct variations in the flux as a function of recoil energy that allow the WIMP mass to be determined.

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arXiv2010

Resolving mixing in Smoothed Particle Hydrodynamics

Standard formulations of smoothed particle hydrodynamics (SPH) are unable to resolve mixing at fluid boundaries. We use an error and stability analysis of the generalised SPH equations of motion to prove that this is due to two distinct problems. The first is a leading order error in the momentum equation. This should decrease with increasing neighbour number, but does not because numerical instabilities cause the kernel to be irregularly sampled. We identify two important instabilities: the clumping instability and the banding instability, and we show that both are cured by a suitable choice of kernel. The second problem is the local mixing instability (LMI). This occurs as particles attempt to mix on the kernel scale, but are unable to due to entropy conservation. The result is a pressure discontinuity at boundaries that pushes fluids of different entropy apart. We cure the LMI by using a weighted density estimate that ensures that pressures are single valued throughout the flow. This also gives a better volume estimate for the particles, reducing errors in the continuity and momentum equations. We demonstrate mixing in our new Optimised Smoothed Particle Hydrodynamics (OSPH) scheme using a Kelvin Helmholtz instability (KHI) test with density contrast 1:2, and the 'blob test' - a 1:10 density ratio gas sphere in a wind tunnel - finding excellent agreement between OSPH and Eulerian codes.

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arXiv2010

Determining orbits for the Milky Way's dwarfs

We calculate orbits for the Milky Way dwarf galaxies with proper motions, and compare these to subhalo orbits in a high resolution cosmological simulation. We use the simulation data to assess how well orbits may be recovered in the face of measurement errors, a time varying triaxial gravitational potential, and satellite-satellite interactions. For present measurement uncertainties, we recover the apocentre r_a and pericentre r_p to ~40%. With improved data from the Gaia satellite we should be able to recover r_a and r_p to ~14%, respectively. However, recovering the 3D positions and orbital phase of satellites over several orbits is more challenging. This owes primarily to the non-sphericity of the potential and satellite interactions during group infall. Dynamical friction, satellite mass loss and the mass evolution of the main halo play a more minor role in the uncertainties. We apply our technique to nine Milky Way dwarfs with observed proper motions. We show that their mean apocentre is lower than the mean of the most massive subhalos in our cosmological simulation, but consistent with the most massive subhalos that form before z=10. This lends further support to the idea that the Milky Way's dwarfs formed before reionisation.

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arXiv2009

Determining orbits for the Milky Way's dwarfs

We calculate orbits for the Milky Way dwarf galaxies with proper motions, and compare these to subhalo orbits in a high resolution cosmological simulation. We use this same simulation to assess how well are able to recover orbits in the face of measurement errors, a time varying triaxial gravitational potential, and satellite-satellite interactions. We find that, for present measurement uncertainties, we are able to recover the apocentre r_a and pericentre r_p to ~ 40%. However, even with better data the non-sphericity of the potential and satellite interactions during group infall make the orbital recovery more challenging. Dynamical friction, satellite mass loss and the mass evolution of the main halo play a more minor role. We apply our technique to nine Milky Way dwarfs with observed proper motions. We show that their mean apocentre is consistent with the most massive subhalos that form before z=10, lending support to the idea that the Milky Way dwarfs formed before reionisation.

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arXiv2009

A new low mass for the Hercules dSph: the end of a common mass scale for the dwarfs?

We present a new mass estimate for the Hercules dwarf spheroidal galaxy (dSph), based on the revised velocity dispersion obtained by Aden et al. (2009, arXiv:0908.3489). The removal of a significant foreground contamination using newly acquired Stromgren photometry has resulted in a reduced velocity dispersion. Using this new velocity dispersion of 3.72 +/- 0.91 km/s, we find a mass of M_300=1.9^{+1.1}_{-0.8} 10^6 M_sun within the central 300 pc, which is also the half-light radius, and a mass of M_433=3.7_{-1.6}^{+2.2} 10^6 M_sun within the reach of our data to 433 pc, significantly lower than previous estimates. We derive an overall mass-to-light ratio of M_433/L=103^{+83}_{-48} M_sun/L_sun. Our mass estimate calls into question recent claims of a common mass scale for dSph galaxies. Additionally, we find tentative evidence for a velocity gradient in our kinematic data of 16 +/- 3 km/s/kpc, and evidence of an asymmetric extension in the light distribution at about 0.5 kpc. We explore the possibility that these features are due to tidal interactions with the Milky Way. We show that there is a self-consistent model in which Hercules has an assumed tidal radius of r_t = 485 pc, an orbital pericentre of r_p = 18.5 +/- 5 kpc, and a mass within r_t of M_{tid,r_t}=5.2 +/- 2.7 10^6 M_sun. Proper motions are required to test this model. Although we cannot exclude models in which Hercules contains no dark matter, we argue that Hercules is more likely to be a dark matter dominated system which is currently experiencing some tidal disturbance of its outer parts.

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arXiv2009

Measuring Dark Matter Substructure with Galaxy-Galaxy Flexion Statistics

It is of great interest to measure the properties of substructures in dark matter halos at galactic and cluster scales. Here we suggest a method to constrain substructure properties using the variance of weak gravitational flexion in a galaxy-galaxy lensing context. We show the effectiveness of flexion variance in measuring substructures in N-body simulations of dark matter halos, and present the expected galaxy-galaxy lensing signals. We show the insensitivity of the method to the overall galaxy halo mass, and predict the method's signal-to-noise for a space-based all-sky survey, showing that the presence of substructure down to 10^9 M_\odot halos can be reliably detected.

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arXiv2009

A dark matter disc in the Milky Way

Predicting the local flux of dark matter particles is vital for dark matter direct detection experiments. To date, such predictions have been based on simulations that model the dark matter alone. Here we include the influence of the baryonic matter for the first time. We use two different approaches. Firstly, we use dark matter only simulations to estimate the expected merger history for a Milky Way mass galaxy, and then add a thin stellar disc to measure its effect. Secondly, we use three cosmological hydrodynamic simulations of Milky Way mass galaxies. In both cases, we find that a stellar/gas disc at high redshift (z~1) causes merging satellites to be preferentially dragged towards the disc plane. This results in an accreted dark matter disc that contributes ~0.25 - 1 times the non-rotating halo density at the solar position. An associated thick stellar disc forms with the dark disc and shares a similar velocity distribution. If these accreted stars can be separated from those that formed in situ, future astronomical surveys will be able to infer the properties of the dark disc from these stars. The dark disc, unlike dark matter streams, is an equilibrium structure that must exist in disc galaxies that form in a hierarchical cosmology. Its low rotation lag with respect to the Earth significantly boosts WIMP capture in the Earth and Sun, increases the likelihood of direct detection at low recoil energy, boosts the annual modulation signal, and leads to distinct variations in the flux as a function of recoil energy that allow the WIMP mass to be determined (see contribution from T. Bruch this volume).

astro-ph.GA↗

arXiv2008

Thin, thick and dark discs in LCDM

In a LCDM cosmology, the Milky Way accretes satellites into the stellar disc. We use cosmological simulations to assess the frequency of near disc plane and higher inclination accretion events, and collisionless simulations of satellite mergers to quantify the final state of the accreted material and the effect on the thin disc. On average, a Milky Way-sized galaxy has 3 subhalos with vmax>80km/s; 7 with vmax>60km/s; and 15 with vmax>40km/s merge at redshift z>1. Assuming isotropic accretion, a third of these merge at an impact angle <20 degrees and are dragged into the disc plane by dynamical friction. Their accreted stars and dark matter settle into a thick disc. The stellar thick disc qualitatively reproduces the observed thick disc at the solar neighbourhood, but is less massive by a factor ~2-10. The dark matter disc contributes 0.25-1 times the halo density at the solar position. Although not likely to be dynamically interesting, the dark disc has important implications for the direct detection of dark matter because of its low velocity with respect to the Earth. Higher inclination encounters (>20 degrees) are twice as likely as low inclination ones. These lead to structures that closely resemble the recently discovered inner/outer stellar halos. They also do more damage to the Milky Way stellar disc creating a more pronounced flare, and warp; both long-lived and consistent with current observations. The most massive mergers (vmax>80km/s) heat the thin disc enough to produce a thick disc. These heated thin disc stars are essential for obtaining a thick disc as massive as that seen in the Milky Way; they likely comprise some ~50-90% of the thick disc stars. The Milky Way thin disc must reform from fresh gas after z=1 [abridged].

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arXiv2007

Radial density profiles of time-delay lensing galaxies

We present non-parametric radial mass profiles for ten QSO strong lensing galaxies. Five of the galaxies have profiles close to $ρ(r)\propto r^{-2}$, while the rest are closer to r^{-1}, consistent with an NFW profile. The former are all relatively isolated early-types and dominated by their stellar light. The latter --though the modeling code did not know this-- are either in clusters, or have very high mass-to-light, suggesting dark-matter dominant lenses (one is a actually pair of merging galaxies). The same models give $H_0^{-1} = 15.2_{-1.7}^{+2.5}\Gyr$ ($H_0 = 64_{-9}^{+8} \legacy$), consistent with a previous determination. When tested on simulated lenses taken from a cosmological hydrodynamical simulation, our modeling pipeline recovers both H_0 and $ρ(r)$ within estimated uncertainties. Our result is contrary to some recent claims that lensing time delays imply either a low H_0 or galaxy profiles much steeper than r^{-2}. We diagnose these claims as resulting from an invalid modeling approximation: that small deviations from a power-law profile have a small effect on lensing time-delays. In fact, as we show using using both perturbation theory and numerical computation from a galaxy-formation simulation, a first-order perturbation of an isothermal lens can produce a zeroth-order change in the time delays.

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arXiv2006

Dynamical friction in constant density cores: a failure of the Chandrasekhar formula

Using analytic calculations and N-body simulations we show that in constant density (harmonic) cores, sinking satellites undergo an initial phase of very rapid (super-Chandrasekhar) dynamical friction, after which they experience no dynamical friction at all. For density profiles with a central power law profile of log-slope, $-α$, the infalling satellite heats the background and causes $α$ to decrease. For $α< 0.5$ initially, the satellite generates a small central constant density core and stalls as in the $α= 0$ case. We discuss some astrophysical applications of our results to decaying satellite orbits, galactic bars and mergers of supermassive black hole binaries. In a companion paper we show that a central constant density core can provide a natural solution to the timing problem for Fornax's globular clusters.

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arXiv2006

Two strong-lensing clusters confront universal dark-matter profiles

In the galaxy clusters SSDS J1004+411 and ACO 1689, a large number of multiply-imaged background objects have recently been observed. We use these data to map the projected mass distribution in the inner regions of these clusters. The source redshifts span a large range, which eliminates the degeneracies that plague nearly all lensing work. As a result the mass maps are very well-constrained, despite very weak prior assumptions. ACO 1689 lenses so many objects that we can afford to map it twice using disjoint sets of images, thus verifying our internal error estimates. We then deproject the mass maps (pretending for this purpose that they are spherical) and obtain inner profiles consistent with rho propto r^{-1} and indistinguishable from recent cold dark matter simulations. Assuming that baryons make only a small difference to the profile outside sim 10 kpc, these results support the prediction of a universal inner profile for dark matter structures, independent of any parametrization of that profile.

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arXiv2006

On the formation of dwarf galaxies and stellar halos

Using analytic arguments and a suite of very high resolution (10^3 Msun per particle) cosmological hydro-dynamical simulations, we argue that high redshift, z ~ 10, M ~ 10^8 Msun halos, form the smallest `baryonic building block' (BBB) for galaxy formation. These halos are just massive enough to efficiently form stars through atomic line cooling and to hold onto their gas in the presence of supernovae winds and reionisation. These combined effects, in particular that of the supernovae feedback, create a sharp transition: over the mass range 3-10x10^7 Msun, the BBBs drop two orders ofmagnitude in stellar mass. Below ~2x10^7 Msun, galaxies will be dark with almost no stars and no gas. Above this scale is the smallest unit of galaxy formation: the BBB. A small fraction (~100) of these gas rich BBBs fall in to a galaxy the size of the Milky Way. Ten percent of these survive to become the observed LG dwarf galaxies at the present epoch. Those in-falling halos on benign orbits which keep them far away from the Milky Way or Andromeda manage to retain their gas and slowly form stars - these become the smallest dwarf irregular galax ies; those on more severe orbits lose their gas faster than they can form stars and become the dwarf spheroidals. The remaining 90% of the BBBs will be accreted. We show that this gives a metallicity and total stellar mass consistent with the Milky Way old stellar halo (abridged).

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arXiv2006

Does the Fornax dwarf spheroidal have a central cusp or core?

The dark matter dominated Fornax dwarf spheroidal has five globular clusters orbiting at ~1 kpc from its centre. In a cuspy CDM halo the globulars would sink to the centre from their current positions within a few Gyrs, presenting a puzzle as to why they survive undigested at the present epoch. We show that a solution to this timing problem is to adopt a cored dark matter halo. We use numerical simulations and analytic calculations to show that, under these conditions, the sinking time becomes many Hubble times; the globulars effectively stall at the dark matter core radius. We conclude that the Fornax dwarf spheroidal has a shallow inner density profile with a core radius constrained by the observed positions of its globular clusters. If the phase space density of the core is primordial then it implies a warm dark matter particle and gives an upper limit to its mass of ~0.5 keV, consistent with that required to significantly alleviate the substructure problem.

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arXiv2006

The internal kinematics of dwarf spheroidal galaxies

The status of kinematic observations in Local Group dwarf spheroidal galaxies (dSphs) is reviewed. Various approaches to the dynamical modelling of these data are discussed and some general features of dSph dark matter haloes based on simple mass models are presented.

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arXiv2006

The importance of tides for the Local Group dwarf spheroidals

There are two main tidal effects which can act on the Local Group dwarf spheroidals (dSphs): tidal stripping and tidal shocking. Using N-body simulations, we show that tidal stripping always leads to flat or rising projected velocity dispersions beyond a critical radius; it is $\sim 5$ times more likely, when averaging over all possible projection angles, that the cylindrically averaged projected dispersion will rise, rather than be flat. In contrast, the Local Group dSphs, as a class, show flat or falling projected velocity dispersions interior to $\sim 1$ kpc. This argues for tidal stripping being unimportant interior to $\sim 1$ kpc for most of the Local Group dSphs observed so far. These results have important implications for the formation of the dSphs and for cosmology. A tidal origin for the formation of these Local Group dSphs (in which they contain no dark matter) is strongly disfavoured. In the cosmological context, a naive solution to the missing satellites problem is to allow only the most massive substructure dark matter halos around the Milky Way to form stars. It is possible for dSphs to reside within these halos ($\sim 10^{10}$M$_\odot$) and have their velocity dispersions lowered through the action of tidal shocks, but only if they have a central density core in their dark matter, rather than a cusp. dSphs can reside within cuspy dark matter halos if their halos are less massive ($\sim 10^{9}$M$_\odot$) and therefore have smaller central velocity dispersions initially (abridged).

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