Tidal Tails of Star Clusters as Tools for Studying the Milky-Way Potential
Final Report Abstract
The gravitational potential of the Milky Way is a complex but yet not well understood com- position of three major parts: i) the Milky Way disk, ii) the central bulge, and iii) a large dark halo whose nature is still under debate. The goal of this project was to investigate how tidal tails of star clusters can be used to constrain the shape and the granularity of the gravitational potential of the Milky Way. Tidal tails of star clusters are cold dynamical structures of low surface brightness. They are being detected in imaging and radial velocity surveys, and span up to several degrees on the sky. Moreover, the most prominent tails have been shown to contain significant substructure of yet unknown origin. We therefore aimed at investigating the formation of substructure in tidal tails via two distinct mechanisms: a) via epicyclic motion of stars within the tails, and b) via encounters of tail stars with dark matter sub haloes. These dark matter sub haloes are expected from _CDM cosmology to orbit within the gravitational potential of the Milky Way. By disentangling the effects from epicyclic motion and dark matter sub haloes on the appearance of tidal tails of star clusters and by applying our findings to observed tidal tails in the Galactic halo, we aimed at constraining the sub halo content of the Milky Way and test _CDM cosmology. Although we originally planned to study these effects by means of direct N-body simulations, we instead developed a method to generate realistic tidal streams at a much lower computa- tional cost. We found that N-body simulations are better suited for detailed studies of specific clusters and their tidal tails, like we demonstrated for some of the Palomar clusters, but are less useful for systematic studies for which thousands of models are needed. With the so-called streakline method, however, this is now possible. We are, for example, able to insert tidal tails of star clusters into simulations of large-scale structure formation and follow the evolution of substructure in tidal tails throughout the build-up of the host galaxy’s potential. In an ongoing effort we included this method into the Via Lactea II simulation of a Milky-Way sized dark matter halo, and added thousands of artificial streams to systematically test the effects of dark matter sub haloes on star cluster streams. Furthermore, the streakline method can be used to predict shapes and extents of globular cluster tidal tails. It can be used to guide observational campaigns looking for valuable targets like 47 Tucanae, whose tidal tails are perfectly suited for constraining the gravitational potential of the Milky Way disk. By now, we are involved in several projects which are aimed at finding the tidal tails of promising candidate clusters in the MilkyWay halo.We demonstrate the power of our method by constraining the mass of the Milky Way to an unprecedented precision using the available observations on the tidal tail of Pal 5. This project greatly enhanced our understanding of tidal tails of star clusters and how they can be used to study the Milky Way potential. The theoretical explanation of the epicyclic over-densities spaced regularly along tidal tails is one of the major results achieved. Moreover, it enabled and initiated a variety of new observational and theoretical studies aimed at this problem which were not anticipated at the beginning of this project.
Publications
- 2011. Direct N-body simulations of globular clusters - I. Palomar 14. Monthly Notices of the Royal Astronomical Society 411, 1989-2001
Zonoozi, A. H., Küpper, A. H. W., Baumgardt, H., Haghi, H., Kroupa, P., Hilker, M.
- 2011. Mass segregation and fractal substructure in young massive clusters - I . The McLuster code and method calibration. Monthly Notices of the Royal Astronomical Society 417, 2300-2317
Küpper, A. H. W., Maschberger, T., Kroupa, P., Baumgardt, H.
- 2011. The curious case of Palomar 13: the influence of the orbital phase on the appearance of galactic satellites. Monthly Notices of the Royal Astronomical Society 413, 863-877
Küpper, A. H. W., Mieske, S., Kroupa, P.
- 2012. More on the structure of tidal tails. Monthly Notices of the Royal Astronomical Society 420, 2700-2714
Küpper, A. H. W., Lane, R. R., Heggie, D. C.
- 2012. The tidal tails of 47Tucanae. Monthly Notices of the Royal Astronomical Society 423, 2845-2853
Lane, R. R., Küpper, A. H. W., Heggie, D. C.
- 2015. Globular Cluster Streams as Galactic High-Precision Scales—the Poster Child Palomar 5. The Astrophysical Journal 803, 80-106
Küpper, A. H. W., Balbinot, E., Bonaca, A., Johnston, K. V., Hogg, D.W., Kroupa, P., Santiago, B. X.
(See online at https://doi.org/10.1088/0004-637X/803/2/80)
