Weighing the local dark matter with RAVE red clump stars

Abstract

We determine the Galactic potential in the solar neigbourhood from RAVE observations. We select red clump stars for which accurate distances, radial velocities, and metallicities have been measured. Combined with data from the 2MASS and UCAC catalogues, we build a sample of ∼4600 red clump stars within a cylinder of 500 pc radius oriented in the direction of the South Galactic Pole, in the range of 200 pc to 2000 pc distances. We deduce the vertical force and the total mass density distribution up to 2 kpc away from the Galactic plane by fitting a distribution function depending explicitly on three isolating integrals of the motion in a separable potential locally representing the Galactic one with four free parameters. Because of the deep extension of our sample, we can determine nearly independently the dark matter mass density and the baryonic disc surface mass density. We find (i) at 1 kpc K z /(2πG) = 68.5 ± 1.0 M pc −2 ; and (ii) at 2 kpc K z /(2πG) = 96.9 ± 2.2 M pc −2 . Assuming the solar Galactic radius at R 0 = 8.5 kpc, we deduce the local dark matter density ρ DM (z = 0) = 0.0143 ± 0.0011 M pc −3 = 0.542 ± 0.042 Gev cm −3 and the baryonic surface mass density Σ bar = 44.4 ± 4.1 M pc −2 . Our results are in agreement with previously published K z determinations up to 1 kpc, while the extension to 2 kpc shows some evidence for an unexpectedly large amount of dark matter. A flattening of the dark halo of order 0.8 can produce such a high local density in combination with a circular velocity of 240 km s −1 . It could also be consistent with a spherical cored dark matter profile whose density does not drop sharply with radius. Another explanation, allowing for a lower circular velocity, could be the presence of a secondary dark component, a very thick disc resulting either from the deposit of dark matter from the accretion of multiple small dwarf galaxies, or from the presence of an effective "phantom" thick disc in the context of effective galactic-scale modifications of gravity.

Spiral arm kinematics for Milky Way stellar populations

“…(41) Our MW potential model with the values of Table 2 presents a value of |Fz (1.1kpc)| 2πG = 70.0 for the total vertical force on the plane that match exactly the standard literature values of Kuijken & Gilmore (1989a) (see also Kuijken & Gilmore 1989c,b) and |Fz (2.0kpc)| 2πG = 87.9 at the solar potion R⊙ = 8.0 and φ⊙ = 0.0 (e.g., see Bienaymé et al 2014, for compatible values at R⊙ = 8.5). We consider these as the major contributors to the shape of the underlying MW potential.…”

Bringing the Galaxy's dark halo to life

“…Using a very different approach to that of P14, Bienaymé et al (2014) determined the local dm density from a subset of the data used by P14. The local dm densities of these two studies agree for halo axis ratio q in the range 0.79 to 0.94.…”

“…Since the potentials of the discs cause a dark halo that is spherical when isolated to flatten in its inner region, we have focused on the model in P14 that has a mildly flattened dark halo (minor-major axis ratio q = 0.8). Moreover, P14 remark that comparison of their results with those of Bienaymé et al (2014) favours a flattened model with axis ratio q ≃ 0.8. We shall refer to this model as our 'reference model'.…”

Dark Matter in the Milky Way

“…In principle, one can iterate the fits with different Galactic potentials until the best-fitting potential is found, giving access to the underlying mass distribution. This is the philosophy we followed in [23], where a parametrized separable potential was used together with a fixed distribution depending on three isolating integrals of the motion, and was fit to the kinematics of 4600 red clump giants from the RAVE survey in a cylinder of 500 pc radius around the Sun. This allowed us to demonstrate that the local dark matter density is of the order of ρ DM = 0.5GeV cm −3 , which should be used as a benchmark for local direct dark matter detection searches instead of the usually lower values used (different estimates from other studies vary by about a factor of two), nevertheless keeping in mind the caveat of the possible effects of non-axisymmetries mentioned above.…”