Tidal Tails Around 20 Galactic Globular Cluster: Observational Evidence for Gravitational Disk/Bulge Shocking

Abstract

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. 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 crossing (Abridged)