Life and death of a hero - Lessons learned from modeling the dwarf spheroidal Hercules: an incorrect orbit?

摘要

Hercules is a dwarf spheroidal satellite of the Milky Way, found at adistance of about 138 kpc, and showing evidence of tidal disruption. It is veryelongated and exhibits a velocity gradient of 16 +/- 3 km/s/kpc. Using thisdata a possible orbit of Hercules has previously been deduced in theliterature. In this study we make use of a novel approach to find a best fitmodel that follows the published orbit. Instead of using trial and error, weuse a systematic approach in order to find a model that fits multipleobservables simultaneously. As such, we investigate a much wider parameterrange of initial conditions and ensure we have found the best match possible.Using a dark matter free progenitor that undergoes tidal disruption, ourbest-fit model can simultaneously match the observed luminosity, centralsurface brightness, effective radius, velocity dispersion, and velocitygradient of Hercules. However, we find it is impossible to reproduce theobserved elongation and the position angle of Hercules at the same time in ourmodels. This failure persists even when we vary the duration of the simulationsignificantly, and consider a more cuspy density distribution for theprogenitor. We discuss how this suggests that the published orbit of Herculesis very likely to be incorrect.