Constraints on galaxy structure and evolution from the light of nearby systems (original) (raw)

The structural and photometric properties of early-type galaxies in hierarchical models

Monthly Notices of The Royal Astronomical Society, 2006

We present predictions for the structural and photometric properties of early-type galaxies in the cold dark matter cosmology from the published semi-analytical galaxy formation models of Baugh et al. and Bower et al. These calculations were made with the GALFORM code, which tracks the evolution of the disc and bulge components of a galaxy, using a self-consistent model to compute the scalelengths. The sizes of galactic discs are determined by the conservation of the angular momentum of cooling gas. The sizes of merger remnants are computed by applying the virial theorem and conserving the binding energy of the progenitors and their orbital energy. We compare the model predictions with observational results derived from the SDSS. The model enjoys a number of notable successes, such as giving reasonable reproductions of the local Faber-Jackson relation, the velocity dispersion-age relation, and the fundamental plane relating the luminosity, velocity dispersion and effective radius of spheroids. These achievements are all the more remarkable when one bears in mind that none of the parameters have been adjusted to refine the model predictions. We study how the residuals around the fundamental plane relation depend on galaxy properties. We examine in detail the physical ingredients of the calculation of galaxy sizes. We also study the evolution of the scaling relations with redshift. However, there are some important disagreements between the predictions of the model and observations: the brightest model spheroids have effective radii smaller than observed and the zero-point of the fundamental plane shows little or no evolution with redshift in the model.

Stars beyond Galaxies: The Origin of Extended Luminous Halos around Galaxies

Mon Notic Roy Astron Soc, 2005

(Abridged) 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 -3 at the luminous edge of the galaxy to -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. The outer stellar spheroid is supported by a velocity dispersion tensor with a substantial and radially increasing radial anisotropy. 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 MW, 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.