Weighing the Galactic disk using the Jeans equation: lessons from simulations

TL;DR

This study evaluates the effectiveness of Jeans equation analysis in determining the total surface density of galactic disks, including dark matter, using simulated galaxy data, highlighting key sensitivities and methodological considerations.

Contribution

It demonstrates the accuracy and limitations of Jeans analysis in simulated galaxies, emphasizing the importance of precise disk parameters and the inclusion of radial forces at higher altitudes.

Findings

Vertical velocity dispersion is the dominant kinematic quantity.

Using only the vertical force is effective at low heights.

Including radial force improves accuracy at higher altitudes.

Abstract

Using three-dimensional stellar kinematic data from simulated galaxies, we examine the efficacy of a Jeans equation analysis in reconstructing the total disk surface density, including the dark matter, at the "Solar" radius. Our simulation dataset includes galaxies formed in a cosmological context using state-of-the-art high resolution cosmological zoom simulations, and other idealised models. The cosmologically formed galaxies have been demonstrated to lie on many of the observed scaling relations for late-type spirals, and thus offer an interesting surrogate for real galaxies with the obvious advantage that all the kinematical data are known perfectly. We show that the vertical velocity dispersion is typically the dominant kinematic quantity in the analysis, and that the traditional method of using only the vertical force is reasonably effective at low heights above the disk plane. At higher heights the inclusion of the radial force becomes increasingly important. We also show that the method is sensitive to uncertainties in the measured disk parameters, particularly the scale lengths of the assumed double exponential density distribution, and the scale length of the radial velocity dispersion. In addition, we show that disk structure and low number statistics can lead to significant errors in the calculated surface densities. Finally we examine the implications of our results for previous studies of this sort, suggesting that more accurate measurements of the scale lengths may help reconcile conflicting estimates of the local dark matter density in the literature.