The local dark matter density Read, J I
Journal of physics. G, Nuclear and particle physics,
06/2014, Letnik:
41, Številka:
6
Journal Article
Recenzirano
Odprti dostop
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 ρdm and show that, once the baryonic surface density contribution Σb is normalized across different groups, there is remarkably good agreement. Compiling data from the literature, I estimate Σb = 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 ρdm 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 ρdm 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 Σb ∼ 55 M pc−2. I conclude by discussing how the new Gaia satellite will be transformative. We will obtain much tighter constraints on both Σb and ρdm 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 ρdm.
Abstract
We present a new non-parametric Jeans code, GravSphere, that recovers the density ρ(r) and velocity anisotropy β(r) of spherical stellar systems, assuming only that they are in a steady ...state. Using a large suite of mock data, we confirm that with only line-of-sight velocity data, GravSphere provides a good estimate of the density at the projected stellar half-mass radius, ρ(R
1/2), but is not able to measure ρ(r) or β(r), even with 10 000 tracer stars. We then test three popular methods for breaking this ρ − β degeneracy: using multiple populations with different R
1/2; using higher order ‘virial shape parameters’ (VSPs); and including proper motion data. We find that two populations provide an excellent recovery of ρ(r) in-between their respective R
1/2. However, even with a total of ∼7000 tracers, we are not able to well-constrain β(r) for either population. By contrast, using 1000 tracers with higher order VSPs we are able to measure ρ(r) over the range 0.5 < r/R
1/2 < 2 and broadly constrain β(r). Including proper motion data for all stars gives an even better performance, with ρ and β well-measured over the range 0.25 < r/R
1/2 < 4. Finally, we test GravSphere on a triaxial mock galaxy that has axis ratios typical of a merger remnant 1 : 0.8 : 0.6. In this case, GravSphere can become slightly biased. However, we find that when this occurs the data are poorly fit, allowing us to detect when such departures from spherical symmetry become problematic.
Dark matter heats up in dwarf galaxies Read, J I; Walker, M G; Steger, P
Monthly notices of the Royal Astronomical Society,
03/2019, Letnik:
484, Številka:
1
Journal Article
The case for a cold dark matter cusp in Draco Read, J I; Walker, M G; Steger, P
Monthly notices of the Royal Astronomical Society,
11/2018, Letnik:
481, Številka:
1
Journal Article
Dwarf spheroidal (dSph) galaxies are prime targets for present and future γ-ray telescopes hunting for indirect signals of particle dark matter. The interpretation of the data requires careful ...assessment of their dark matter content in order to derive robust constraints on candidate relic particles. Here, we use an optimized spherical Jeans analysis to reconstruct the ‘astrophysical factor’ for both annihilating and decaying dark matter in 21 known dSphs. Improvements with respect to previous works are: (i) the use of more flexible luminosity and anisotropy profiles to minimize biases, (ii) the use of weak priors tailored on extensive sets of contamination-free mock data to improve the confidence intervals, (iii) systematic cross-checks of binned and unbinned analyses on mock and real data, and (iv) the use of mock data including stellar contamination to test the impact on reconstructed signals. Our analysis provides updated values for the dark matter content of 8 ‘classical’ and 13 ‘ultrafaint’ dSphs, with the quoted uncertainties directly linked to the sample size; the more flexible parametrization we use results in changes compared to previous calculations. This translates into our ranking of potentially-brightest and most robust targets – namely Ursa Minor, Draco, Sculptor – and of the more promising, but uncertain targets – namely Ursa Major 2, Coma – for annihilating dark matter. Our analysis of Segue 1 is extremely sensitive to whether we include or exclude a few marginal member stars, making this target one of the most uncertain. Our analysis illustrates challenges that will need to be addressed when inferring the dark matter content of new ‘ultrafaint’ satellites that are beginning to be discovered in southern sky surveys.
Dark matter cores all the way down Read, J. I; Agertz, O; Collins, M. L. M
Monthly notices of the Royal Astronomical Society,
07/2016, Letnik:
459, Številka:
3
Journal Article
Recenzirano
Odprti dostop
We use high-resolution simulations of isolated dwarf galaxies to study the physics of dark matter cusp-core transformations at the edge of galaxy formation: M
200 = 107–109 M⊙. We work at a ...resolution (∼4 pc minimum cell size; ∼250 M⊙ per particle) at which the impact from individual supernovae explosions can be resolved, becoming insensitive to even large changes in our numerical ‘sub-grid’ parameters. We find that our dwarf galaxies give a remarkable match to the stellar light profile; star formation history; metallicity distribution function; and star/gas kinematics of isolated dwarf irregular galaxies. Our key result is that dark matter cores of size comparable to the stellar half-mass radius r
1/2 always form if star formation proceeds for long enough. Cores fully form in less than 4 Gyr for the M
200 = 108 M⊙ and ∼14 Gyr for the 109 M⊙ dwarf. We provide a convenient two parameter ‘coreNFW’ fitting function that captures this dark matter core growth as a function of star formation time and the projected stellar half-mass radius. Our results have several implications: (i) we make a strong prediction that if Λcold dark matter is correct, then ‘pristine’ dark matter cusps will be found either in systems that have truncated star formation and/or at radii r > r
1/2; (ii) complete core formation lowers the projected velocity dispersion at r
1/2 by a factor of ∼2, which is sufficient to fully explain the ‘too-big-to-fail problem’; and (iii) cored dwarfs will be much more susceptible to tides, leading to a dramatic scouring of the sub-halo mass function inside galaxies and groups.
Dwarf spheroidal galaxies have shallow central dark matter density profiles, low angular momentum and approximately exponential surface brightness distributions. Through N-body simulations and ...analytic calculations we investigate the extent to which these properties can be generated from ‘typical’ΛCDM galaxies, which differ in all of these properties, by the dynamical consequences of feedback. We find that, for a wide range of initial conditions, one impulsive mass-loss event will naturally produce a surface brightness profile in the remaining stellar component of a dwarf spheroidal galaxy (dSph) which is well-fitted over many scalelengths by an exponential, in good qualitative agreement with observations of Local Group dSphs. Furthermore, two impulsive mass-loss phases, punctuated by significant gas re-accretion, are found to be sufficient to transform a central density cusp in the dark matter profile into a near-constant density core. This may then provide the missing link between current cosmological simulations, which predict a central cusp in the dark matter density profile, and current observations, which find much shallower central density profiles. We also look at the angular momentum history of dSphs and demonstrate that if these galaxies have spent most of their lifetime in tidal isolation from massive galaxies then they cannot have formed from high angular momentum gas discs.
Abstract
We fit the rotation curves of isolated dwarf galaxies to directly measure the stellar mass–halo mass relation (M⋆–M200) over the mass range 5 × 105 ≲ M⋆/M⊙ ≲ 108. By accounting for cusp-core ...transformations due to stellar feedback, we find a monotonic relation with little scatter. Such monotonicity implies that abundance matching should yield a similar M⋆–M200 if the cosmological model is correct. Using the ‘field galaxy’ stellar mass function from the Sloan Digital Sky Survey (SDSS) and the halo mass function from the Λ cold dark matter Bolshoi simulation, we find remarkable agreement between the two. This holds down to M200 ∼ 5 × 109 M⊙, and to M200 ∼ 5 × 108 M⊙ if we assume a power-law extrapolation of the SDSS stellar mass function below M⋆ ∼ 107 M⊙. However, if instead of SDSS we use the stellar mass function of nearby galaxy groups, then the agreement is poor. This occurs because the group stellar mass function is shallower than that of the field below M⋆ ∼ 109 M⊙, recovering the familiar ‘missing satellites’ and ‘too big to fail’ problems. Our result demonstrates that both problems are confined to group environments and must, therefore, owe to ‘galaxy formation physics’ rather than exotic cosmology. Finally, we repeat our analysis for a Λ Warm Dark Matter cosmology, finding that it fails at 68 per cent confidence for a thermal relic mass of mWDM < 1.25 keV, and mWDM < 2 keV if we use the power-law extrapolation of SDSS. We conclude by making a number of predictions for future surveys based on these results.
We present a dynamical friction model based on Chandrasekhar's formula that reproduces the fast inspiral and stalling experienced by satellites orbiting galaxies with a large constant density core. ...We show that the fast inspiral phase does not owe to resonance. Rather, it owes to the background velocity distribution function for the constant density core being dissimilar from the usually assumed Maxwellian distribution. Using the correct background velocity distribution function and our semi-analytic model from previous work, we are able to correctly reproduce the infall rate in both cored and cusped potentials. However, in the case of large cores, our model is no longer able to correctly capture core-stalling. We show that this stalling owes to the tidal radius of the satellite approaching the size of the core. By switching off dynamical friction when r
t(r) = r (where r
t is the tidal radius at the satellite's position), we arrive at a model which reproduces the N-body results remarkably well. Since the tidal radius can be very large for constant density background distributions, our model recovers the result that stalling can occur for M
s/M
enc ≪ 1, where M
s and M
enc are the mass of the satellite and the enclosed galaxy mass, respectively. Finally, we include the contribution to dynamical friction that comes from stars moving faster than the satellite. This next-to-leading order effect becomes the dominant driver of inspiral near the core region, prior to stalling.
Standard formulations of smoothed particle hydrodynamics (SPH) are unable to resolve mixing at fluid boundaries. We use an error and stability analysis of the generalized 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 an 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 entropies 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 optimized smoothed particle hydrodynamics (OSPH) scheme using a Kelvin–Helmholtz instability (KHI) test with a density contrast of 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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