Stars beyond galaxies: the origin of extended luminous haloes around galaxies

We use numerical simulations to investigate the origin and structure of the luminous haloes 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 an origin suggests that outer luminous haloes 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 proportional to 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 haloes are similar in shape to their dark matter counterparts, which are only mildly triaxial and much rounder than dark haloes 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(2)(r)/sigma(2)(t)similar to 2 at the edge of the central galaxy to similar to 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 or have been substantially disrupted in its immediate surroundings; very few stars in the halo are contributed by satellites that survive as self-bound entities at the present. Although the stellar spheroid in the simulations is more prominent than in disc-dominated galaxies, many of 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.