Quantifying torque from the Milky Way bar using Gaia DR2

Abstract

We determine the mass of the Milky Way bar and the torque it causes, using Gaia DR2, by applying the orbital arc method. Based on this, we have found that the gravitational acceleration is not directed towards the centre of our Galaxy but a few degrees away from it. We propose that the tangential acceleration component is caused by the bar of the Galaxy. Calculations based on our model suggest that the torque experienced by the region around the Sun is $\approx 2400\, {\rm km^2\, s^{-2}}$ per solar mass. The mass estimate for the bar is $\sim 1.6\pm 0.3\times 10^{10}\, \mathrm{M_\odot }$. Using greatly improved data from Gaia DR2, we have computed the acceleration field to great accuracy by adapting the orbital Probability Density Function (oPDF) method (Han et al. 2016) locally and used the phase space coordinates of ∼4 × 105 stars within a distance of 0.5 kpc from the Sun. In the orbital arc method, the first step is to guess an acceleration field and then reconstruct the stellar orbits using this acceleration for all the stars within a specified region. Next, the stars are redistributed along orbits to check if the overall phase space distribution has changed. We repeat this process until we find an acceleration field that results in a new phase space distribution that is the same as the one that we started with; we have then recovered the true underlying acceleration.

Non-equilibrium in the Solar Neighbourhood using dynamical modelling with Gaia DR2

“…The improvements of the method presented by Kipper et al (2020) are related to both dynamics and statistics. We describe them in subsequent sections.…”

“…The orbital arc method developed in Kipper et al (2020) relies on seeking of consistency between assumptions and data. The data in our case is the distribution of stars in a specific region.…”

“…It is also necessary to estimate how reasonable the assumption of a stationary potential is or within what timescale one can handle integrals of motions to be constant for a particular stellar orbit. For example, by modelling the Milky Way bar with Gaia DR2 data Kipper et al (2020) estimated that due to torque from the central bar the angular momentum of the Sun Lz might change within one orbital period about 30 per cent. It indicates that in barred galaxies Lz is not as good integral of motion as one would like it to be.…”

“…In our earlier paper (Kipper et al 2019) we presented a method to calculate gravitational potential derivatives (accelerations) in sufficiently small regions of a galaxy if one knows phase-space coordinates of a large number of stars in these regions. In a later paper (Kipper et al 2020) the method was applied for the Milky Way (MW) galaxy in the Solar neighbourhood (SN) using Gaia DR2 data. The model included a non-axisymmetric central bar but assumed stationarity.…”

“…In Section 2 we characterise the advancements we did in modelling compared to Kipper et al (2020) in more detailed, Section 3 checks the validity of the improvements with mock data. Thereafter, in Section 4 we present what we learned about the SN using this approach.…”