Physics behind bars and bulges
The central region of the Milky Way holds its most massive and complex stellar component — the bulge. Primarily composed of disc stars, the bulge's structure is intricately (re-)shaped by the evolution of the Galactic bar.
​​
Over the years, alongside my colleagues, I have contributed to advancing our understanding of the Milky Way's bar and bulge's origin, kinematics, and chemical abundance patterns. Through this collaborative effort, we have uncovered key insights into the dynamic processes governing these main Galactic components, shedding light on their role in the overall evolution of the Milky Way.
Recently, using the MW/M31 analogues from the TNG50 simulations (see above the face-on images of 189 galaxies sorted by the bar strength), we have explored what initial conditions are suitable for the formation of bars whose properties resemble the one in the Milky Way. We showed that galaxies with Milky Way-like bars form rotationally-supported discs faster compared to weakly or non-barred galaxies, with the largest angular momentum of IGM at z>3-5. We therefore conclude that the Milky Way appeared to be a disc-dominated galaxy already at redshift ~2, implying even earlier spin-up of its stellar populations.
Using N-body simulations of the Milky Way-like galaxy, we have introduced a novel mechanism of radial migration, where stars in the inner Galaxy, trapped by the bar resonances, are carried outward as the resonances themselves shift due to the deceleration of the bar. Recently, using the data from APOGEE DR 17, we have confirmed that this process indeed took place in the Milky Way. Moreover, using the radial variations of the age distribution of the very metal-rich stars across the disc, in agreement with my chemo-dynamic simulations, we have demonstrated that they have departed from the inner Galaxy at the time of the bar formation, implying that the bar of the Milky Way formed 9 Gyr ago.
The process of the bar-induced radial migration of stars from the inner galaxy is illustrated in the animation above. On the left, it shows the orbits of stars moving outwards together with the bar resonances (magenta lines). The right panels show the time evolution of the resonance location and parameters of the bar (strength, pattern speed).
Over the last years, I have been deeply engaged in investigating the kinematics, chemical composition, and origin of secular bulges in the Milky Way bulge and other galaxies. Together with my colleagues, we have used the N-body models I developed to study the complex dynamical processes shaping this crucial Galactic component. Our research has focused on understanding the redistribution of thin and thick disc stars following the formation of the Galactic bar, a process that gives rise to X-shaped, boxy/peanut-shaped bulges — components strikingly similar to what we observe in the Milky Way today.
The animation above illustrates the X-shaped bulge formation (perhaps one of the most extreme cases) via vertical bar instability shown for stellar populations with different pre-installed kinematics: total (left), thin disc (middle) and thick disc (right). The animation shows that the pre-existing parameters of stellar populations affect how the disc populations are mapped into the density structure, resulting in a complex chemical and kinematical composition of the Milky Way bulge.