Life and death of a hero – lessons learned from modelling the dwarf spheroidal Hercules: an incorrect orbit?

Abstract

Hercules is a dwarf spheroidal satellite of the Milky Way, found at a distance of ≈ 138 kpc, and showing evidence of tidal disruption. It is very elongated and exhibits a velocity gradient of 16 ± 3 km s −1 kpc −1 . Using this data a possible orbit of Hercules has previously been deduced in the literature. In this study we make use of a novel approach to find a best fit model that follows the published orbit. Instead of using trial and error, we use a systematic approach in order to find a model that fits multiple observables simultaneously. As such, we investigate a much wider parameter range of initial conditions and ensure we have found the best match possible. Using a dark matter free progenitor that undergoes tidal disruption, our best-fit model can simultaneously match the observed luminosity, central surface brightness, effective radius, velocity dispersion, and velocity gradient of Hercules. However, we find it is impossible to reproduce the observed elongation and the position angle of Hercules at the same time in our models. This failure persists even when we vary the duration of the simulation significantly, and consider a more cuspy density distribution for the progenitor. We discuss how this suggests that the published orbit of Hercules is very likely to be incorrect.

Could Segue 1 be a destroyed star cluster? – a dynamical perspective

“…We see that in the right half of the panel the results in the upper part are deviating from the power-law. This behaviour, described already in Blaña et al (2015), stems from results which belong to the first regime of resulting objects, i.e., here we have bound objects which lose only part of their initial mass. Furthermore, it is clear that the power-laws on this side of the panel can not extend to final masses, which are higher than the initial ones.…”

Exploding Satellites—the Tidal Debris of the Ultra-Faint Dwarf Galaxy Hercules

“…The satellite remnant is surrounded by lots of debris in this stage, 3. a "stream regime" in which the satellite is completely disrupted and the debris is spread along the progenitor's orbit. Blaña et al (2015) observe the "flip" in orientation of the debris when the satellite has been completely destroyed in the last pericenter passage, that is, when it is in between the "tidal" and the "stream" regime. Interestingly, their best-fit models for the other observational constraints are also in this intermediate regime, where Herc is right on the edge of destruction.…”

“…Interestingly, their best-fit models for the other observational constraints are also in this intermediate regime, where Herc is right on the edge of destruction. Since none of their models reproduces all of Hercules' observational constraints, Blaña et al (2015) argue that the orbit they chose is most likely wrong. This conclusion appears reasonable, given that Martin & Jin (2010) assumed that the orientation of the elongation of the debris must follow the orbit of the progenitor.…”

“…Model 5 is a transition case between 4 and 6. It is capturing the moment when the orientation of the debris "flips", as described by Blaña et al (2015).…”

Modelling the Canes Venatici I dwarf spheroidal galaxy

“…Following the analysis made by Blaña et al (2015) we fit the solutions where the object is almost destroyed, that is objects with low initial masses and high Plummer radii, trying to match the velocity dispersion mentioned above. On this part of the parameter space, the central part of the remnant, still bounded, is surrounded by a large number of unbounded particles (the ones that form the tidal tails of the object and can be found at the background and foreground of the centre of the remnant).…”