Kinematic Fractionation of the Stellar Populations in Barred Galaxies

Abstract

The formation and evolution of the central regions of barred galaxies are the subject of significant ongoing study. Research has indicated that the morphology and kinematics of stellar populations are dependent on their age. Simulations suggest that this dependence occurs because populations are separated by the bar. Populations with varying radial velocity dispersions evolve differently in the presence of a growing bar, a process termed kinematic fractionation.

Using star-forming simulations of barred galaxies which undergo kinematic fractionation, we have reproduced the observed trends of metal-rich and metal-poor Milky Way bulge main-sequence stars observed with the Hubble Space Telescope. The old and young stellar populations in the models were born before and after the bar, respectively. As each population traces varying bulge structures with different bar strengths, their kinematics project onto distinct observed motions as seen from the heliocentric perspective. The predictions from these models allow us to propose follow-up observations to test the kinematics of differently-aged populations in fields away from the bulge minor axis.

To trace structures in the Milky Way using stellar ages, we explore new data from the Gaia satellite. We define and characterise a clean sample of Mira variables using data-motivated cuts on their relative frequency error and variability amplitude, maximising their separation from SRVs. Mira candidates were found to follow the expected trends of age in their spatial and kinematic distributions when separated by variability period. However, we determined that a larger sample of Miras is needed to better constrain the Milky Way bar. Nevertheless, we were also able to characterise the effects of the large variability amplitude of Miras on astrometric and photometric solutions within Gaia, allowing us to make recommendations for future study and observation of these stars.

We have identified barred galaxies in the TNG50 cosmological simulation and defined a method based on kinematics to find those which form a box/peanut bulge. By studying the evolutionary history of bar and box/peanut strength, we can determine their formation epoch. This method is also able to distinguish between box/-peanut bulges formed through strong buckling or those which form through slower resonance heating or weak buckling. Our results show that TNG50 reproduces a similar dependence of box/peanut fraction with stellar mass but does not have as high a fraction at high mass. However, in TNG50, we find the same characteristic upturn to higher fractions of box/peanuts from low mass to high mass.

Having a large self-consistent sample of barred galaxies from TNG50, we study kinematic fractionation in a fully cosmological context. We find that the spatial distribution of kinematic quantities of populations in the bar, such as the in-plane anisotropy, can trace the bar strength as measured by the density of that population. We also find that populations within the bar of the galaxy retain the dynamical memory of the bar formation epoch. Studying box/peanut bulges in TNG50 galaxies, we find the separation of the density bimodality increases as a function of decreasing age.