The stellar populations trapped at the L4/L5 Lagrange (corotation) points of a bar

Abstract

Bars are prevalent features in disc galaxies and play an important role in driving secular evolution. One important effect of bars is resonances. Stars at corotation resonance with the bar are known to librate around the stable Lagrange points (L4, L5). Here, we investigate the orbits and properties of corotating stars as well as the effects of bar weakening on the corotation resonance. By performing an orbital frequency analysis on a subsample of stars in an N-body+SPH simulation of an isolated, barred galaxy, at two distinct times at which the bar has different strengths, we obtain corotating and non-corotating subsamples. The results at both times are compared numerically to evaluate the consequences of bar weakening. A selection of stellar orbits are plotted over a short timespan to investigate the different trajectories followed by corotating and non-corotating stars. Trends in rform, age, and metallicity are systematically investigated by sectioning the galactic plane into radial and azimuthal bins, and comparing the properties of both categories of stars within each bin, using a K-S test to check statistical significance. We confirm the existence of important systematic differences between the distributions of the radii of formation, ages, and metallicities of the corotating stars with respect to the surrounding non-corotating stars. Trends in rform and age are found to vary radially and suggest that corotating stars are dragged away from the corotation radius in both directions, whereas the metallicities of stars in corotation are found to be distinctly higher. We find that bar weakening largely disrupts these trends, by diminishing the bar’s ability to retain captured stars or trap new stars at corotation. We also show that stars at corotation may be trapped only temporarily and escape before following a well-defined orbit around the Lagrange points, which implies that tracking their trajectories is not a suitable way of identifying corotating stars.