Astrometric mock observations for determining the local dark matter density^⋆

¹ National Astronomical Observatory of Japan, Mitaka, 181-8588 Tokyo, Japan
e-mail: shigeki.inoue@nao.ac.jp
² Korea Astronomy and Space Science Institute 776, Daedeokdae-ro, Yuseong-gu, 305-348 Daejeon, Republic of Korea

Received: 24 February 2013
Accepted: 3 May 2013

Abstract

Context. To determine the local dark matter density of the solar system is a classical problem in astronomy. Recently, a novel method of determining the local dark matter density from stellar distribution and vertical velocity dispersion profiles perpendicular to the Galactic plane was devised. This method has the advantage of abolishing conventional approximations and using only a few assumptions.

Aims. Our aims are to carefully scrutinize this method and to examine influences by uncertainties of astrometric observations. We discuss how the determinations of the local dark matter density vary with observational precisions on parallax, proper motion, and line-of-sight velocity measurements.

Methods. To examine the influences by the observational imprecision, we created mock observation data for stars that are dynamical tracers based on an analytical galaxy model and applied parametrized observational errors to the mock data. We evaluated the accuracy of determining the dark matter density by applying the method to the mock data. In addition, we estimated a sample size and observational precision required to determine the dark matter density with accuracy.

Results. We find that the method is capable of determining the local dark matter density with accuracy if the sample size and observational precisions are satisfactory. The required sample size is approximately 6000 stars. The random errors of parallaxes and proper motions can cause systematic overestimation of the dark matter density. We estimate the required precisions of the parallax measurements to be approximately 0.1 − 0.3 milliarcsec at 1  kpc away from the Sun; the proper motion precisions do not seem to be as important as the parallaxes. Moreover, we find that the line-of-sight velocity errors can cause either underestimation or overestimation of the dark matter density, which is contingent on distance-dependence of the errors.

Conclusions. From these results, we expect that using the Hipparcos catalog would overestimate the local dark matter density because of the imprecise parallax measurements if this method is applied; however, we emphasize the capability of the method. We expect that Gaia will provide data precise enough to determine the local dark matter density.