On the Galactic rotation curve inferred from the Jeans equations Assessing its robustness using Gaia DR3 and cosmological simulations

Abstract

Several works have recently applied Jeans modelling to Gaia-based datasets to infer the circular velocity curve for the Milky Way. Such works have consistently found evidence for a continuous decline in the rotation curve beyond $\sim$15kpc possibly indicative of a light dark matter halo. We used Gaia DR3 RVS data, supplemented with Bayesian distances to determine the radial variation of the second moments of the velocity distribution for stars close to the Galactic plane. We have used these profiles to determine the rotation curve using the Jeans equations under the assumption of axisymmetry and explored how they vary with azimuth and above and below the Galactic disk plane. We have applied the same methodology to an N-body simulation of a Milky Way-like galaxy impacted by a satellite akin the Sagittarius dwarf and to the Auriga suite of cosmological simulations. We reveal evidence of disequilibrium and deviations from axisymmetry closer in. We find that the second moment of $V_R$ flattens out at $R \gtrsim 12.5$kpc, and that the second moment of $V_{\phi}$ is different above and below the plane for $R \gtrsim 11$kpc. The simulations indicate that these features are typical of galaxies that have been perturbed by external satellites. They also suggest that the difference between the true circular velocity curve and that inferred from Jeans equations can be as high as 15$\%$, but is likely of order 10$\%$ for the Milky Way. This is of larger amplitude than the systematics inherent to Jeans equations. However, if the density of the tracer population were truncated at large radii, the erroneous conclusion of a steeply declining rotation curve can be reached. We find that steady-state axisymmetric Jeans modelling becomes less robust at large radii, indicating that particular caution is needed when interpreting the rotation curve inferred in those regions.