Tidal tails around 20 Galactic globular clusters - 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. Fiel d and cluster stars are sorted with the help of color-magnitude diagrams, a nd 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 th e overdensities of these stars using the wavelet transform of the star coun ts in order to filter the background density noise and to detect the weak s tructures. at large scale, formed by the numerous stars previously members of the clusters. We associate these stellar overdensities with the stars ev aporated from the clusters because of dynamical relaxation and/or tidal str ipping from the clusters by the galactic gravitational field. We take into account the strong observational biases induced by the clustering of galact ic field stars and of background galaxies, along with the fluctuations of t he background due to dust extinction. Most of the globular clusters in our sample display strong evidence of tida l interactions with the galactic plane in the form of large and extended de formations. These tidal tails exhibit projected directions preferentially t owards the galactic center. All the clusters observed, which do not suffer from strong observational biases, present such tidal tails, tracing their d ynamical evolution (evaporation, tidal shocking, tidal torquing, and bulge shocking) in the Galaxy. The clusters exhibit different regimes of mass los s rate, detected using the radial density slope in the outer parts of the c lusters. For NGC 5139 (omega Centauri), we estimate, taking into account th e possible presence of mass segregation in its outer parts, that about 0.6 to 1% of its mass has been lost during the current disk shocking event. In the case of NGC 6254, we tentatively estimate, in the cluster reference fra me, 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 orie ntations of these observed tidal tails are perfectly reproduced by N-body s imulations of globular clusters in the galactic potential well. We present these results in a companion paper (Combes et al. 1999). As a by-product of this study, we detect several new galaxy clusters toward s the different fields studied at high galactic latitude. The estimation of the tidal radius of some of the globular clusters could have been overesti mated because of these galaxy clusters.