Colloquium on 6 August 2015
The local Dark Matter density


Title: The local Dark Matter density
Speaker: Frederic (Rick) Hessman (Institut für Astrophysik, Goettingen)
Date: Thursday, 6 August 2015
Time: 11:00 - 12:00
Venue: SAAO auditorium


The analysis of the vertical velocity dispersion of disc stars is the most direct astronomical means of estimating the local dark matter density, rho_DM. Current estimates based on the mid-plane dynamic density use a local baryonic correction that ignores the non-local effects of spiral structure and significantly underestimates the amount of dynamically relevant gas; the additional gas plus the remaining uncertainties make it practically impossible to measure rho_DM from mid-plane kinematics alone. The sampling of inhomogeneous tracer populations with different scale-heights and scale-lengths results in a systematic increase in the observed dispersion gradients and changes in the nominal density distributions that, if not properly considered, can be misinterpreted as a sign of more dark matter. If the disc gravity is modelled using an infinite disc, the local variation in the vertical gravity due to the globally exponential disc components results in an underestimation of the baryonic contribution by as much as ~40%. Given only the assumptions of stationarity, an axially and vertically symmetric disc, doubly exponential tracer and mass-component density profiles, a phenomenologically justified model for the R-z cross-dispersion and a realistic gravity model, it is possible to solve the full vertical Jeans equation analytically for the vertical dispersion and hence test the robustness of previous attempts at measuring rho_DM.  The amount of DM thus needed to model SDSS/SEGUE G dwarf star kinematics turns out to be significantly less than estimated before.

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