Observational Evidence for Gravitational Disk/Bulge Shocking

Large-field multi-color images of 20 galactic globular clusters are used to investigate the presence of tidal tails around these stellar systems. Field and cluster stars are sorted with the help of color-magnitude diagrams, and star-count analysis is performed on the selected cluster stars in order to increase the signal-to-noise ratio of their surface density. We study the overdensities of these stars using the wavelet transform of the star counts in order to filter the background density noise and to detect the weak structures, at large scale, formed by the numerous stars previously members of the clusters. We associate these stellar overdensities with the stars evaporated from the clusters because of dynamical relaxation and/or tidal stripping from the clusters by the galactic gravitational field. We take into account the strong observational biases induced by the clustering of galactic field stars and of background galaxies, along with the fluctuations of the background due to dust extinction. Most of the globular clusters in our sample display strong evidence of tidal interactions with the galactic plane in the form of large and extended deformations. These tidal tails exhibit projected directions preferentially towards the galactic center. All the clusters observed, which do not suffer from strong observational biases, present such tidal tails, tracing their dynamical evolution (evaporation, tidal shocking, tidal torquing, and bulge shocking) in the Galaxy. The clusters exhibit different regimes of mass loss rate, detected using the radial density slope in the outer parts of the clusters. For NGC 5139 (ω Centauri), we estimate, taking into account the possible presence of mass segregation in its outer parts, that about 0.6 to 1 % of its mass has been lost during the Send offprint requests to: S. Leon ([email protected]) ⋆ Based on observations made at the European Southern Observatory, La Silla, Chile ⋆⋆ Plate scanning done with the MAMA (Machine Automatique à Mesurer pour l’Astronomie), a facility developed and operated by the INSU (Institut National des Sciences de l’Univers) at the Observatoire de Paris, France current disk shocking event. In the case of NGC 6254, we tentatively estimate, in the cluster reference frame, for the radial diffusion velocity of the stars stripped, a value of the order of the velocity dispersion in the cluster itself. The sizes and orientations of these observed tidal tails are perfectly reproduced by N-body simulations of globular clusters in the galactic potential well. We present these results in a companion paper (Combes, Leon & Meylan 1999). As a by-product of this study, we detect several new galaxy clusters towards the different fields studied at high galactic latitude. The estimation of the tidal radius of some of the globular clusters could have been overestimated because of these galaxy clusters.