The Location of Stellar Clusters in the Big Dent: an Age Gradient along the Z-Axis

The ages and locations of stellar clusters and individual Cepheid stars in the depression called the "Big Dent" are analyzed. The Z-locations of the cluster sample display a well-defined Z-age stratification. The Cepheid sample has ages between 3 and 8 × 10⁷ yr and shows a departure from the midplane similar to the one observed for clusters of the same age range. The existence of such an age gradient seems to corroborate previous hypotheses suggesting that the star formation activity was probably triggered by the same strong perturbation which originated the depression. A model in which the Big Dent was created by a collision of a high-velocity cloud with the Galactic disk is able to reproduce the observed gradient.