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Register a new userConstraining the Galaxys dark halo with RAVE stars
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We use the kinematics of $sim200,000$ giant stars that lie within $sim 1.5$ kpc of the plane to measure the vertical profile of mass density near the Sun. We find that the dark mass contained within the isodensity surface of the dark halo that passes through the Sun ($(6pm0.9)times10^{10},mathrm{M_odot}$), and the surface density within $0.9$ kpc of the plane ($(69pm10),mathrm{M_odot,pc^{-2}}$) are almost independent of the (oblate) halos axis ratio $q$. If the halo is spherical, 46 per cent of the radial force on the Sun is provided by baryons, and only 4.3 per cent of the Galaxys mass is baryonic. If the halo is flattened, the baryons contribute even less strongly to the local radial force and to the Galaxys mass. The dark-matter density at the location of the Sun is $0.0126,q^{-0.89},mathrm{M_odot,pc^{-3}}=0.48,q^{-0.89},mathrm{GeV,cm^{-3}}$. When combined with other literature results we find hints for a mildly oblate dark halo with $q simeq 0.8$. Our value for the dark mass within the solar radius is larger than that predicted by cosmological dark-matter-only simulations but in good agreement with simulations once the effects of baryonic infall are taken into account. Our mass models consist of three double-exponential discs, an oblate bulge and a Navarro-Frenk-White dark-matter halo, and we model the dynamics of the RAVE stars in the corresponding gravitational fields by finding distribution functions $f(mathbf{J})$ that depend on three action integrals. Statistical errors are completely swamped by systematic uncertainties, the most important of which are the distance to the stars in the photometric and spectroscopic samples and the solar distance to the Galactic centre. Systematics other than the flattening of the dark halo yield overall uncertainties $sim 15$ per cent.
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The local velocity distribution of dark matter plays an integral role in interpreting the results from direct detection experiments. We previously showed that metal-poor halo stars serve as excellent tracers of the virialized dark matter velocity distribution using a high-resolution hydrodynamic simulation of a Milky Way--like halo. In this paper, we take advantage of the first textit{Gaia} data release, coupled with spectroscopic measurements from the RAdial Velocity Experiment (RAVE), to study the kinematics of stars belonging to the metal-poor halo within an average distance of $sim 5$ kpc of the Sun. We study stars with iron abundances [Fe/H]$ < -1.5$ and $-1.8$ that are located more than $1.5$ kpc from the Galactic plane. Using a Gaussian mixture model analysis, we identify the stars that belong to the halo population, as well as some kinematic outliers. We find that both metallicity samples have similar velocity distributions for the halo component, within uncertainties. Assuming that the stellar halo velocities adequately trace the virialized dark matter, we study the implications for direct detection experiments. The Standard Halo Model, which is typically assumed for dark matter, is discrepant with the empirical distribution by $sim6sigma$ and predicts fewer high-speed particles. As a result, the Standard Halo Model overpredicts the nuclear scattering rate for dark matter masses below $sim 10$ GeV. The kinematic outliers that we identify may potentially be correlated with dark matter substructure, though further study is needed to establish this correspondence.
We estimate the 3D density profile of the Galactic dark matter (DM) halo within $r lesssim 30$ kpc from the Galactic centre by using the astrometric data for halo RR Lyrae stars from Gaia DR2. We model both the stellar halo distribution function and the Galactic potential, fully taking into account the survey selection function, the observational errors, and the missing line-of-sight velocity data for RR Lyrae stars. With a Bayesian MCMC method, we infer the model parameters, including the density flattening of the DM halo $q$, which is assumed to be constant as a function of radius. We find that 99% of the posterior distribution of $q$ is located at $q>0.963$, which strongly disfavours a flattened DM halo. We cannot draw any conclusions as to whether the Galactic DM halo at $r lesssim 30$ kpc is prolate, because we restrict ourselves to axisymmetric oblate halo models with $qleq1$. Our result is inconsistent with predictions from cosmological hydrodynamical simulations that advocate more oblate ($langle{q}rangle sim0.8 pm 0.15$) DM halos within $sim 15%$ of the virial radius for Milky-Way-sized galaxies. An alternative possibility, based on our validation tests with a cosmological simulation, is that the true value $q$ of the Galactic halo could be consistent with cosmological simulations but that disequilibrium in the Milky Way potential is inflating our measurement of $q$ by 0.1-0.2. As a by-product of our analysis, our model constrains the DM density in the Solar neighbourhood to be $rho_{mathrm{DM},odot} = (9.01^{+0.18}_{-0.20})times10^{-3}M_odot mathrm{pc}^{-3} = 0.342^{+0.007}_{-0.007}$ $;mathrm{GeV} mathrm{cm}^{-3}$.
The last two years have seen widespread acceptance of the idea that the Milky Way halo was largely created in an early (8-10 Gyr ago) and massive ($> 10^{10} M_odot$) merger. The roots of this idea pre-date the Gaia mission, but the exquisite proper motions available from Gaia have made the hypothesis irresistible. We trace the history of this idea, reviewing the series of papers that led to our current understanding.
We present measurements of the radial gravitational acceleration around isolated galaxies, comparing the expected gravitational acceleration given the baryonic matter with the observed gravitational acceleration, using weak lensing measurements from the fourth data release of the Kilo-Degree Survey. These measurements extend the radial acceleration relation (RAR) by 2 decades into the low-acceleration regime beyond the outskirts of the observable galaxy. We compare our RAR measurements to the predictions of two modified gravity (MG) theories: MOND and Verlindes emergent gravity. We find that the measured RAR agrees well with the MG predictions. In addition, we find a difference of at least $6sigma$ between the RARs of early- and late-type galaxies (split by S{e}rsic index and $u-r$ colour) with the same stellar mass. Current MG theories involve a gravity modification that is independent of other galaxy properties, which would be unable to explain this behaviour. The difference might be explained if only the early-type galaxies have significant ($M_{gas} approx M_*$) circumgalactic gaseous haloes. The observed behaviour is also expected in $Lambda$CDM models where the galaxy-to-halo mass relation depends on the galaxy formation history. We find that MICE, a $Lambda$CDM simulation with hybrid halo occupation distribution modelling and abundance matching, reproduces the observed RAR but significantly differs from BAHAMAS, a hydrodynamical cosmological galaxy formation simulation. Our results are sensitive to the amount of circumgalactic gas; current observational constraints indicate that the resulting corrections are likely moderate. Measurements of the lensing RAR with future cosmological surveys will be able to further distinguish between MG and $Lambda$CDM models if systematic uncertainties in the baryonic mass distribution around galaxies are reduced.
It has recently been proposed that if the Galactic dark matter halo were triaxial it would induce lumpiness in the velocity distribution of halo stars in the Solar Neighbourhood through orbital resonances. These substructures could therefore provide a way of measuring its shape. We explore the robustness of this proposal by integrating numerically orbits starting from a realistic set of initial conditions in dark halo potentials of different shape. We have analysed the resulting velocity distributions in Solar neighbourhood-like volumes, and have performed statistical tests for the presence of kinematic substructures. Furthermore, we have characterized the particles orbits using a Fourier analysis. The local velocity distributions obtained are relatively smooth, statistically consistent with being devoid of substructures even for a dark halo potential with significant but plausible triaxiality. Although resonances are indeed present and associated with specific regions of velocity space, the fraction of stars associated to these is relatively minor. The most significant imprint of the triaxiality of the dark halo is in fact, a variation in the shape of the velocity ellipsoid with spatial location.
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