Signatures of the resonances of a large Galactic bar in local velocity space

Abstract

The second data release of the Gaia mission has revealed a very rich structure in local velocity space. In terms of in-plane motions, this rich structure is also seen as multiple ridges in the actions of the axisymmetric background potential of the Galaxy. These ridges are probably related to a combination of effects from ongoing phase-mixing and resonances from the spiral arms and the bar. We have recently developed a method to capture the behaviour of the stellar phase-space distribution function at a resonance, by reexpressing it in terms of a new set of canonical actions and angles variables valid in the resonant region. Here, by properly treating the distribution function at resonances, and by using a realistic model for a slowly rotating large Galactic bar with pattern speed Ω b = 39 km s −1 kpc −1 , we show that no less than six ridges in local action space can be related to resonances with the bar. Two of these at low angular momentum correspond to the corotation resonance, and can be associated to the Hercules moving group in local velocity space. Another one at high angular momentum corresponds to the outer Lindblad resonance, and can tentatively be associated to the velocity structure seen as an arch at high azimuthal velocities in Gaia data. The other ridges are associated to the 3:1, 4:1 and 6:1 resonances. The latter can be associated to the so-called 'horn' of the local velocity distribution. While it is clear that effects from spiral arms and incomplete phase-mixing related to external perturbations also play a role in shaping the complex kinematics revealed by Gaia data, the present work demonstrates that, contrary to common misconceptions, the bar alone can create multiple prominent ridges in velocity and action space.

Fluctuations in galactic bar parameters due to bar–spiral interaction

“…the uv plane). Due to the large number of particles required, typically such studies are done with test-particle integrations (Dehnen 2000;Fragkoudi et al 2019) or analytically (Monari et al 2017a(Monari et al , 2019, in which case the bar is in a steady state. What creates the structure in the uv plane are the locations of resonances in the disc, which are set by the pattern speed.…”

“…Starting with Wegg, Gerhard & Portail (2015), Sormani, Binney & Magorrian (2015), Li et al (2016), and Portail et al (2017), more recent works using different data sets and methods have suggested a significantly longer bar than previously thought (∼5 kpc) and a pattern speed much lower than previously accepted (35-45 km s −1 kpc −1 ; e.g. Hunt & Bovy 2018;Bovy et al 2019;Clarke et al 2019;Monari et al 2019;Sanders et al 2019). In contrast, Anders et al (2019) found a bar-shaped feature inclined by ∼40 • with respect to the solar azimuth and a length of ∼3.5 kpc in the stellar density distribution of Gaia DR2 data (Gaia Collaboration et al 2018a) for stars brighter than G = 18, using distances derived with the STARHORSE code (Santiago et al 2016;Queiroz et al 2018).…”

Constraining the Milky Way non-axisymmetries with Gaia

“…In Monari et al (2017), we devised a method to treat the response of such a DF to a rotating perturber potential, such as the bar or a spiral mode, at resonances where treatments based on linearizing the collisionless Blotzmann equation diverge. In Monari et al (2019a), we used this method to study the response of the Galactic disc DF to the large bar Galactic potential model developed by Portail et al (2017), with pattern speed 39 km s −1 kpc −1 , demonstrating that the bar alone can create multiple prominent ridges in local velocity and action space, including Hercules (corotation resonance), the 'horn' (6:1 resonance) and the 'hat' (2:1 outer Lindblad resonance). However, many of these can be explained by different combinations of non-axisymmetric perturbations, making their modelling degenerate.…”

“…In Monari et al (2019a), we used the method developed in Monari et al (2017) and described in Sect. 1 to study the response of the Galactic disc DF to the large bar Galactic potential model developed by Portail et al (2017), with pattern speed 39 km s −1 kpc −1 .…”

Galactic dynamics in the era of Gaia

“…Bovy et al (2016) argued for a near spherical inner halo by modelling the nearby Pal-5 and GD-1 streams. Also Wegg et al (2019) argued for a spherical halo by studying the kinematics of RR Lyrae stars. On the other hand, oblate models were favoured by studying SDSS halo stars with new data from Gaia, LAMOST and SEQUE (Loebman et al 2014), and by studying the GD-1 stream solely (Malhan & Ibata 2019).…”