Building up the stellar halo of the Galaxy
Abstract
We study numerical simulations of satellite galaxy disruption in a potential resembling that of the Milky Way. Our goal is to assess whether a merger origin for the stellar halo would leave observable fossil structure in the phase-space distribution of nearby stars. We show how mixing of disrupted satellites can be quantified using a coarse-grained entropy. Although after 10Gyr few obvious asymmetries remain in the distribution of particles in configuration space, strong correlations are still present in velocity space. We give a simple analytic description of these effects, based on a linearized treatment in action-angle variables, which shows how the kinematic and density structure of the debris stream changes with time. By applying this description we find that a single dwarf elliptical-like satellite of current luminosity 10^8L_solar disrupted 10Gyr ago from an orbit circulating in the inner halo (mean apocentre ~12kpc) would contribute about ~30 kinematically cold streams with internal velocity dispersions below 5kms^-1 to the local stellar halo. If the whole stellar halo were built by such disrupted satellites, it should consist locally of 300-500 such streams. Clear detection of all these structures would require a sample of a few thousand stars with 3D velocities accurate to better than 5kms^-1. Even with velocity errors several times worse than this, the expected clumpiness should be quite evident. We apply our formalism to a group of stars detected near the North Galactic Pole, and derive an order-of-magnitude estimate for the initial properties of the progenitor system.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- August 1999
- DOI:
- 10.1046/j.1365-8711.1999.02616.x
- arXiv:
- arXiv:astro-ph/9901102
- Bibcode:
- 1999MNRAS.307..495H
- Keywords:
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- Astrophysics
- E-Print:
- 28 pages, 10 figures, minor changes, matches the version to appear in MNRAS, Vol. 307, p.495-517 (August 1999)