Published online by Cambridge University Press: 10 November 2010
The age and location of stellar clusters and Wolf-Rayet stars in the third Galactic quadrant are analyzed. The cluster sample has been divided into three age groups: 1) younger than 107 yr, 2) between 107 and 3 × 107 yr and 3) between 3 × 107 and 108 yr. The mean z-locations of these samples in the central region of the Big Dent display a well defined z-age stratification. 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 originated by the collision of a high-velocity cloud with the Galactic disk is able to reproduce the observed gradient.
The analysis of the three-dimensional spatial distribution of a sample of young open clusters within 3 kpc around the Sun led Alfaro et al. (1991) to the discovery that the four nearest supercomplexes, previously detected by Efremov & Sitnik (1988), appear to be located below the formal Galactic plane. The one placed in the third Galactic quadrant (labelled III in ES88) shows the largest and deepest z-displacement and has been called the Big Dent.
The mechanisms able to explain the observed large z-departure as well as the different stages of star formation in the Big Dent need a source of mechanical energy able to inject in the Galactic disk a considerable amount of energy and momentum along the z-axis.