The vertical stellar kinematics in face-on barred galaxies: Estimating the ages of bars

In order to perform a detailed study of the stellar kinematics in the vertical axis of bars, we obtained high signal-to-noise spectra along the major and minor axes of the bars in a sample of 14 face-on galaxies and used them to determine the line-of-sight stellar velocity distribution, parameterized as a Gauss-Hermite series. With these data, we developed a diagnostic tool that allows one to distinguish between recently formed and evolved bars, as well as to estimate their ages, assuming that bars form in vertically thin disks that are recognizable by low values for the vertical velocity dispersion sigma(z). Through N-body realizations of bar unstable disk galaxies we were also able to check the timescales involved in the processes that give bars an important vertical structure. We show that sigma(z) in evolved bars is roughly 100 km s(-1), which translates to a height scale of about 1.4 kpc, giving support to scenarios in which bulges form through disk material. Furthermore, the bars in our numerical simulations have values for sigma(z) generally smaller than 50 km s(-1), even after evolving for 2 Gyr, suggesting that a slow process is responsible for making bars as vertically thick as we observe. We verify theoretically that the Spitzer-Schwarzschild mechanism is quantitatively able to explain these observations if we assume that giant molecular clouds are twice as concentrated along the bar as in the rest of the disk.