Paper detail

The eye of Gaia on globular clusters structure: tidal tails

I analyse the projected density distribution of member stars over a wide area surrounding 18 Galactic globular clusters using the photometric and astrometric information provided by the second data release of the Gaia mission. A 5D mixture modelling technique has been employed to optimally isolate the signal of the cluster stellar population from the contamination of the Galactic field, taking advantage of its different distribution in the space formed by colours, magnitudes, parallaxes and proper motions. In 7 clusters I detect collimated overdensities at a >3 sigma level above the background density extending well beyond the cluster tidal radius, consistent with the distortion expected as a result of the tidal interaction with the Milky Way potential. In five of these clusters (NGC288, NGC2298, NGC5139, NGC6341 and NGC7099) spectacular tidal tails extend up to the border of the analysed field of view at 5 degrees from the centre. At large distances from the cluster centre, the orientation of the detected overdensities appears to be systematically aligned with the cluster orbital path, in agreement with the predictions of N-body simulations. The fraction of stars contained in the tidal tails of these clusters is also used to determine the first observational estimate of their present-day destruction rates.