On the local dark matter density

¹ Instituto de Astronomía, Universidad Católica del NorteAv. Angamos 0610 Antofagasta Chile
e-mail: cmoni@ucn.cl
² Departamento de Astronomía, Universidad de Concepción, Casilla 160- C Concepción, Chile
³ European Southern Observatory, 3107 Alonso de Cordova, Vitacura, Santiago, Chile
⁴ Dipartimento di Fisica e Astronomia, Universitá di Padova, Vicolo Osservatorio 3, 35122 Padova, Italy
⁵ Universidad de Chile, Departamento de Astronomía, Casilla 36- D Santiago, Chile

Received: 24 July 2014
Accepted: 17 September 2014

Abstract

Context. In 2012, we applied a three-dimensional formulation to kinematic measurements of the Galactic thick disk and derived a surprisingly low dark matter density at the solar position. This result was challenged by Bovy & Tremaine (2012, ApJ, 756, 89), who claimed that the observational data are consistent with the expected local dark matter density if a one-dimensional approach is adopted.

Aims. We aim at clarifying whether their work definitively explains our result by analyzing the assumption at the base of their formulation and their claim that this returns a lower limit for the local dark matter density, which is accurate within 20%.

Methods. We find that the validity of their formulation depends on the underlying mass distribution. We therefore analyze the predictions that their hypothesis casts on the radial gradient of the azimuthal velocity ∂_Rv̅ and compare it with observational data as a testbed for the validity of their formulation.

Results. We find that their hypothesis requires too steep a profile of ∂_Rv̅(Z), which is inconsistent with the observational data both in the Milky Way and in external galaxies. As a consequence, their results are biased and largely overestimate the mass density. Dynamical simulations also show that, contrary to their claims, low values of ∂_Rv̅ are compatible with a Milky Way-like potential with radially constant circular velocity. We nevertheless confirm that, according to their criticism, our assumption ∂_Rv̅ = 0 is only an approximation. If this hypothesis is released, and the available information about ∂_Rv̅ in the thick disk is used, the resulting local dark matter density increases by a tiny amount, from 0 ± 1 to 2 ± 3 mM_⊙ pc^-3, with an upper limit of ~3.5 mM_⊙ pc^-3. Hence, this approximation has negligible influence on our results.

Conclusions. Our analysis shows that their criticism is not a viable explanation for the inferred lack of dark matter at the solar position detected by us. More studies are required to understand these unexpected results.