We use numerical simulations to investigate the origin and structure of the luminous halos that surround isolated galaxies .
These stellar structures extend out to several hundred kpc away from a galaxy , and consist of stars shed by merging subunits during the many accretion events that characterize the hierarchical assembly of galaxies .
Such origin suggests that outer luminous halos are ubiquitous and that they should appear as an excess of light over extrapolations of the galaxy ’ s inner profile beyond its traditional luminous radius .
The mass profile of the accreted stellar component is well approximated by a model where the logarithmic slope steepens monotonically with radius ; from \rho \propto r ^ { -3 } at the luminous edge of the galaxy to r ^ { -4 } or steeper near the virial radius of the system .
Such spatial distribution is consistent with that of Galactic and M31 globular clusters , suggesting that many of the globulars were brought in by accretion events , in a manner akin to the classic Searle-Zinn scenario .
Luminous halos are similar in shape to their dark matter counterparts , which are only mildly triaxial and much rounder than dark halos formed in simulations that do not include a dissipative luminous component .
The outer stellar spheroid is supported by a velocity dispersion tensor with a substantial and radially increasing radial anisotropy ; from \sigma _ { r } ^ { 2 } / \sigma _ { t } ^ { 2 } \sim 2 at the edge of the central galaxy to \sim 5 at the virial radius .
These properties distinguish the stellar halo from the dark matter component , which is more isotropic in velocity space , as well as from some tracers of the outer spheroid such as satellite galaxies .
Most stars in the outer halo formed in progenitors that have since merged with the central galaxy ; very few stars in the halo are contributed by satellites that survive as self-bound entities at the present .
These features are in reasonable agreement with recent observations of the outer halo of the Milky Way , of M31 , and of other isolated spirals , and suggest that all of these systems underwent an early period of active merging , as envisioned in hierarchical models of galaxy formation .