Systematic uncertainties in the determination of the local dark matter density

A precise determination of the local dark matter density and an accurate control over the corresponding uncertainties are of paramount importance for dark matter (DM) searches. Using very recent high-resolution numerical simulations of a Milky Way like object, we study the systematic uncertainties that affect the determination of the local dark matter density based on dynamical measurements in the Galaxy. In particular, extracting from the simulation with baryons the orientation of the Galactic stellar disk with respect to the DM distribution, we study the DM density for an observer located at $\sim 8\mathrm{kpc}$ from the Galactic center on the stellar disk, ${\rho }_0$. This quantity is found to be always larger than the average density in a spherical shell of the same radius ${\overline{\rho }}_0$, which is the quantity inferred from dynamical measurements in the Galaxy, and to vary in the range ${\rho }_0/{\overline{\rho }}_0=1.01–1.41$. This suggests that the actual dark matter density in the solar neighborhood is on average 21% larger than the value inferred from most dynamical measurements, and that the associated systematic errors are larger than the statistical errors recently discussed in the literature.

Figure 1

The shape parameters $b/a$, $c/a$, and $T$ for SR6-n01e1ML (solid black lines) and for the corresponding pure dark matter realization (dashed red lines), both at $z=0$. Upper (lower) thin lines show $b/a$ ($c/a$), whereas thick curves represent the triaxiality parameter $T$. Also shown are the Sun galactocentric distance $R_0\simeq 8\mathrm{kpc}$ and the virial radius $R_{\mathrm{vir}}\equiv R_{200c}$.

Figure 2

The angle between the normal to the stellar disk ${\overrightarrow{n}}_{\mathrm{sd}}$ and the minor axis ${\overrightarrow{j}}_c$. The solid black (dot-dashed blue) line corresponds to SR6-n01e1ML (SR6-n01e5ML) at $z=0$. The dashed red line shows the angle between the minor axis in the pure dark matter simulation and the normal to the stellar disk in SR6-n01e1ML. Also shown are the Sun galactocentric distance $R_0\simeq 8\mathrm{kpc}$ and the virial radius $R_{\mathrm{vir}}\equiv R_{200c}$.

Figure 3

The dark matter density in the spherical shell $7.5<R/\mathrm{kpc}<8.5$ along the stellar disk plane and two perpendicular planes for SR6-n01e1ML (top), and along the planes perpendicular to the principal axes for the pure dark matter simulation (bottom). The solid horizontal line represents the mean of the points and the dashed line shows the value of the mean density in the whole shell, dubbed ${\overline{\rho }}_0$. The sinusoidal curve shown in each plot is the best fit to the points in the form $c_1+c_2\sin (2(\phi +c_3))$.