Ursa Major II - reproducing the observed properties through tidal disruption

• Published in: Monthly notices of the Royal Astronomical Society Vol. 433; no. 3; pp. 2529 - 2544
• Main Authors: Smith, R., Fellhauer, M., Candlish, G. N., Wojtak, R., Farias, J. P., Blaña, M.
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 11.08.2013
• Subjects: Constellations; Dark matter; Dispersion; Simulation; Spectrum analysis; Star & galaxy formation; galaxies: dwarf; galaxies: individual: Ursa Major II; galaxies: kinematics and dynamics; methods: numerical; galaxies: evolution
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stt925

Recent deep photometry of the dwarf spheroidal Ursa Major II's morphology, and spectroscopy of individual stars, have provided a number of new constraints on its properties. With a velocity dispersion ∼6 km s−1, and under the assumption that the galaxy is virialized, the mass-to-light ratio is found to be approaching ∼2000 - apparently heavily dark matter dominated. Using N-body simulations, we demonstrate that the observed luminosity, ellipticity, irregular morphology, velocity gradient and the velocity dispersion can be well reproduced through processes associated with tidal mass-loss, and in the absence of dark matter. These results highlight the considerable uncertainty that exists in measurements of the dark matter content of Ursa Major II (UMaII). The dynamics of the inner tidal tails, and tidal stream, causes the observed velocity dispersion of stars to be boosted to values of >5 km s−1. These dispersion boosts occur at each apocentre, and last throughout the time the galaxy is close to apocentre. The model need not be close to destruction to have a boosted velocity dispersion. We additionally note that the velocity dispersion at apocentre is periodically enhanced substantially (e.g >20 km s−1). This occurs most strongly when the model's trajectory is close to perpendicular with the Galaxy's disc at pericentre. This effect is responsible for raising the velocity dispersion of our model to (and beyond) the observed values in UMaII. We test an iterative rejection technique for removing unbound stars from samples of UMaII stars whose positions on the sky, and line-of-sight velocities, are provided. We find that this technique is very effective at providing an accurate bound mass from this information, and only fails when the galaxy has a bound mass less than 10 per cent of its initial mass. However, when <2 per cent mass remains bound, mass overestimation by >3 orders of magnitude are seen. Additionally we find that the technique's mass measurements are sensitive to measurement uncertainty in line-of-sight velocities. Measurement uncertainties of 1-4 km s−1 result in mass overestimates by a factor of ∼1.3-5.7.