The Milky Way's dwarf satellite galaxies in [L]CDM: orbital ellipticities and internal structure




Barber, Christopher

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Current models of cosmology and galaxy formation are possibly at odds with observations of small-scale galaxies. Such is the case for the dwarf spheroidal (dSph) galaxies of the Milky Way (MW), where tension exists in explaining their observed abundance, mass, and internal structure. Here we present an analysis of the substructure surrounding MW-sized haloes in a Lambda Cold Dark Matter (LCDM) simulation suite. Combined with a semi-analytic model of galaxy formation and evolution, we identify substructures that are expected to host dSph galaxies similar to the satellites of the MW. We subsequently use these simulations to investigate the orbital properties of dSph satellite galaxies to make contact with those orbiting the MW. After accretion into the main halo, the higher mass ``luminous'' substructure remains on highly radial orbits while the orbits of lower mass substructure, which are not expected to host stars, tend to scatter off of the luminous substructure, and thus circularize over time. The orbital ellipticity distribution of the luminous substructure shows little dependence on the mass or formation history of the main halo, making this distribution a robust prediction of LCDM. Through comparison with the ellipticity distribution computed from the positions and velocities of the nine MW dSph galaxies that currently have proper motion estimates as a function of the assumed MW mass, we present a novel means of estimating the virial mass of the Milky Way. The best match is obtained assuming a mass of 1.1 x 10^12 M_sun with 95 per cent confidence limits of (0.6 - 3.1) x 10^12 M_sun. The uncertainty in this estimate is dominated by the large uncertainties in the proper motions and small number of MW satellites used, and will improve significantly with better proper motion measurements from Gaia. We also measure the shape of the gravitational potential of subhaloes likely to host dSphs, down to radii comparable to the half-light radii of MW dSphs. Field haloes are triaxial in general, while satellite haloes become more spherical over time due to tidal interactions with the host. Thus through the determination of the shape of a MW dSph's gravitational potential via line of sight velocity measurements, one could in principle deduce the impact of past tidal interactions with the MW, and thus determine its dynamical history. Additionally, luminous subhaloes experience a radial alignment of their major axes with the direction to the host halo over time, caused by tidal torquing with the host's gravitational potential during close pericentric passages. This effect is seen at all radii, even down to the half-light radii of the satellites. Radial alignment must be taken into account when calibrating weak-lensing surveys which often assume isotropic orientations of satellite galaxies surrounding host galaxies and clusters.



dark matter, methods numerical, galaxy halo, galaxies, dwarf, evolution, formation