Formation et Evolution des Galaxies - Groupe de Recherche IAP

Date	Sujet	Présenté par
10 septembre 2009	Modelling mass indepedent of anisotropy: connecting observations to simulations Mass profile determinations for dispersion supported galaxies from line-of-sight velocities are subject to large uncertainties associated with the unknown stellar velocity anisotropy. We demonstrate both analytically and with available kinematic data (for systems spanning over seven decades in mass) that the mass-anisotropy degeneracy is effectively eliminated at a characteristic radius that is close to the 3D deprojected half-light radius of the stars. This allows a simple, yet accurate formula to describe the total half-light masses of all hot systems, including dwarf spheroidal galaxies (dSphs), based on directly observable parameters: M_1/2 = 4 sigma_LOS^2 R_half G, where R_half is the 2D projected half-light radius and sigma_LOS is the line-of-sight velocity dispersion. The fact that masses are well-constrained within a characteristic stellar radius has allowed our group to perform systematic, accurate mass determinations for Milky Way dSphs and to conclude that they all have a common mass scale of approximately 10^7 M_\odot within 300 pc of their centers. We extend this work to the satellite population of Andromeda using Keck/DEIMOS spectroscopy of individual stars. We find that the Andromeda dSphs are also consistent with sharing a common mass, but that it is offset from the scale of the Milky Way dSphs by a factor of nearly 2. With this, we will discuss implications for galaxy formation scenarios. Lastly, we will discuss what advances can be made by utilizing proper motion measurements of individual stars in these dSphs.	Joe Wolf Center for Cosmology, Univ. California, Irvine

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