Showing papers by "Justin I. Read published in 2014"

The local dark matter density

Abstract: I review current efforts to measure the mean density of dark matter near the Sun. This encodes valuable dynamical information about our Galaxy and is also of great importance for ?direct detection? dark matter experiments. I discuss theoretical expectations in our current cosmology; the theory behind mass modelling of the Galaxy; and I show how combining local and global measures probes the shape of the Milky Way dark matter halo and the possible presence of a ?dark disc?. I stress the strengths and weaknesses of different methodologies and highlight the continuing need for detailed tests on mock data?particularly in the light of recently discovered evidence for disequilibria in the Milky Way disc. I collate the latest measurements of ?dm and show that, once the baryonic surface density contribution ?b is normalized across different groups, there is remarkably good agreement. Compiling data from the literature, I estimate ?b = 54.2 ? 4.9?M?pc?2, where the dominant source of uncertainty is in the H?i gas contribution. Assuming this contribution from the baryons, I highlight several recent measurements of ?dm in order of increasing data complexity and prior, and, correspondingly, decreasing formal error bars. Comparing these measurements with spherical extrapolations from the Milky Way?s rotation curve, I show that the Milky Way is consistent with having a spherical dark matter halo at R0 ? 8 kpc. The very latest measures of ?dm based on ?10?000 stars from the Sloan Digital Sky Survey appear to favour little halo flattening at R0, suggesting that the Galaxy has a rather weak dark matter disc, with a correspondingly quiescent merger history. I caution, however, that this result hinges on there being no large systematics that remain to be uncovered in the SDSS data, and on the local baryonic surface density being ?b ? 55?M?pc?2. I conclude by discussing how the new Gaia satellite will be transformative. We will obtain much tighter constraints on both ?b and ?dm by having accurate 6D phase space data for millions of stars near the Sun. These data will drive us towards fully three dimensional models of our Galactic potential, moving us into the realm of precision measurements of??dm.

The Local Dark Matter Density

Abstract: I review current efforts to measure the mean density of dark matter near the Sun. This encodes valuable dynamical information about our Galaxy and is also of great importance for 'direct detection' dark matter experiments. I discuss theoretical expectations in our current cosmology; the theory behind mass modelling of the Galaxy; and I show how combining local and global measures probes the shape of the Milky Way dark matter halo and the possible presence of a 'dark disc'. I stress the strengths and weaknesses of different methodologies and highlight the continuing need for detailed tests on mock data - particularly in the light of recently discovered evidence for disequilibria in the Milky Way disc. I highlight several recent measurements in order of increasing data complexity and prior, and, correspondingly, decreasing formal error bars. Comparing these measurements with spherical extrapolations from the Milky Way's rotation curve, I show that the Milky Way is consistent with having a spherical dark matter halo at the Solar position R0. The very latest measures based on ~10,000 stars from the Sloan Digital Sky Survey appear to favour little halo flattening at R0, suggesting that the Galaxy has a rather weak dark matter disc, with a correspondingly quiescent merger history [Abridged].

The hunt for the Milky Way's accreted disc

Abstract: The Milky Way is expected to host an accreted disc of stars and dark matter. This forms as massive greater than or similar to 1 : 10 mergers are preferentially dragged towards the disc plane by dynamical friction and then tidally shredded. The accreted disc likely contributes only a tiny fraction of the MilkyWay's thin and thick stellar disc. However, it is interesting because (i) its associated 'dark disc' has important implications for experiments hoping to detect a dark matter particle in the laboratory; and (ii) the presence or absence of such a disc constrains the merger history of our Galaxy. In this work, we develop a chemodynamical template to hunt for the accreted disc. We apply our template to the high-resolution spectroscopic sample from Ruchti et al., finding at present no evidence for accreted disc stars. Our results are consistent with a quiescent Milky Way with no greater than or similar to 1 : 10 mergers since the disc formed and a correspondingly light ` dark disc'. However, we caution that while our method can robustly identify accreted stars, our incomplete stellar sample makes it more challenging to definitively rule them out. Larger unbiased stellar samples will be required for this. (Less)

The formation of entropy cores in non-radiative galaxy cluster simulations: smoothed particle hydrodynamics versus adaptive mesh refinement

Abstract: We simulate the formation and evolution of a massive galaxy cluster in aCDM Universe us- ing three different approaches to solving the equations of h ydrodynamics in the absence of ra- diative cooling: one based on the 'classic' Smoothed Partic le Hydrodynamics (SPH) method; one based on a novel SPH algorithm with a higher order dissipation switch (SPHS); and one based on an adaptive mesh refinement (AMR) method. We find that SPHS and the AMR code are in excellent agreement with one another: in both, the sph erically averaged entropy profile forms a well-defined core that rapidly converges with increa sing mass and force resolution. By contrast, in agreement with previous work, SPH exhibits rather different behaviour. At low redshift, the entropy profile shows a systematic decreas e with decreasing cluster-centric radius, converging on ever lower central entropy with incre asing resolution. At higher redshift (z � 1), SPH is in better agreement with the other codes but shows much poorer numerical convergence. We trace the reason for these discrepancies to a known artificial surface tension in SPH that appears at phase boundaries. At early times, the passage of massive substructures close to the cluster centre during its violent assembly stir s and shocks the gas to build up an entropy core. At late times, the artificial surface tension c auses low entropy gas - that ought to mix with the higher entropy gas - to sink artificially to the centre of the cluster. We use SPHS - in which we can fully control the amount of numerical dissipation - to study the contribution of numerical versus physical dissip ation on the resultant entropy core. We argue that numerical dissipation is required to ensure si ngle-valued fluid quantities in converging flows. However, provided this dissipation occur s only at the resolution limit, and provided that it does not propagate errors to larger scales, its effect is benign. There is no re- quirement to build 'sub-grid' models of unresolved turbule nce for galaxy cluster simulations. We conclude that entropy cores in non-radiative simulations of galaxy clusters are physical, resulting from entropy generation in shocked gas during the cluster assembly process. This fi- nally puts to rest the long-standing puzzle of cluster entro py cores in AMR simulations versus their apparent absence in classic SPH simulations.

Gravitational lens recovery with glass: measuring the mass profile and shape of a lens

Abstract: We use a new non-parametric gravitational modelling tool { Glass { to determine what quality of data (strong lensing, stellar kinematics, and/or stellar masses) are required to measure the circularly averaged mass prole of a lens and its shape. Glass uses an under-constrained adaptive grid of mass pixels to model the lens, searching through thousands of models to marginalise over model uncertainties. Our key ndings

Growing galaxies via superbubble-driven accretion flows

Abstract: We use a suite a cooling halo simulations to study a new mechanism for rapid accretion of hot halo gas onto star-forming galaxies. Correlated supernovae events create converging 'superbubbles' in the halo gas. Where these collide, the density increases, driving cooling filaments of low metallicity gas that feed the disc. At our current numerical resolution (20 pc) we are only able to resolve the most dramatic events; these could be responsible for the build-up of galaxy discs after the most massive gas-rich mergers have completed (z < 1). As we increase the numerical resolution, we find that the filaments persist for longer, driving continued late-time star formation. This suggests that SNe-driven accretion could act as an efficient mechanism for extracting cold gas from the hot halo, driving late-time star formation in disc galaxies. We show that such filament feeding leads to a peak star formation rate (SFR) of $\sim 3$ M$_{\rm sun}$ yr$^{-1}$, consistent with estimates for the Milky Way. By contrast, direct cooling from the hot halo ('hot-mode' accretion, not present in the simulations that show filament feeding) falls short of the SNe-driven SFR by a factor of 3-4, and is sustained over a shorter time period. The filaments we resolve extend to $\sim$ 50 kpc, reaching column densities of $\sim 10^{18}$ cm$^{-2}$. We show that such structures can plausibly explain the broad dispersion in Mg II absorption seen along sight lines to quasars. Our results suggest a dual role for stellar feedback in galaxy formation, suppressing hot-mode accretion while promoting cold-mode accretion along filaments. This ultimately leads to more star formation, suggesting that the positive feedback effect outweighs the negative. Finally, since the filamentary gas has higher angular momentum than that coming from hot-mode accretion, we show that this leads to the formation of substantially larger gas discs.

Estimating the Galactic Coronal Density via Ram-Pressure Stripping from Dwarf Satellites

Abstract: Cosmological simulations and theories of galaxy formation predict that the Milky Way should be embedded in an extended hot gaseous halo or corona. To date, a definitive detection of such a corona in the Milky Way remains elusive. We have attempted to estimate the density of the Milky Way’s cosmological corona using the effect that it has on the surrounding population of dwarf galaxies. We have considered two dSphs close to the Galaxy: Sextans and Carina. Assuming that they have lost all their gas during the last pericentric passage via ram-pressure stripping, we were able to estimate the average density (n ∼ 2 ⋅ 10−4 cm−3) of the corona at a distance of ∼ 70 kpc from the Milky Way. If we consider an isothermal profile and extrapolate it at large radii, the corona could contain a significant fraction of the missing baryons associated to the Milky Way.