72 research outputs found
The Growth and Structure of Dark Matter Haloes
In this paper, we analyse in detail the mass-accretion histories and
structural properties of dark haloes in high-resolution N-body simulations.
Modeling the density distribution in individual haloes with the NFW profile, we
find, for all main progenitors of a given halo, there is a tight correlation
between its inner scale radius and the mass within it, , which is
the basic reason why halo structural properties are closely related to their
mass-accretion histories. This correlation can be used to predict accurately
the structural properties of a dark halo at any time from its mass-accretion
history. We also test our model with a large sample of GIF haloes. The build-up
of dark haloes in CDM models generally consists of an early phase of fast
accretion and a late phase of slow accretion [where increases with time
approximately as the expansion rate]. These two phases are separated at a time
when the halo concentration parameter . Haloes in the two accretion
phases show systematically different properties, for example, the circular
velocity increases rapidly with time in the fast accretion phase but
remain almost constant in the slow accretion phase,the inner properties of a
halo, such as and increase rapidly with time in the fast accretion
phase but change only slowly in the slow accretion phase. The potential well
associated with a halo is built up mainly in the fast accretion phase, even
though a large amount of mass (over 10 times) can be accreted in the slow
accretion phase. We discuss our results in connection to the formation of dark
haloes and galaxies in hierarchical models.Comment: 26 pages, including 10 figures. v2: some conceptual changes. Accepted
for publication in MNRA
The statistical nature of the brightest group galaxies
We examine the statistical properties of the brightest group galaxies (BGGs)
using a complete spectroscopic sample of groups/clusters of galaxies selected
from the Data Release 7 of the Sloan Digital Sky Survey. We test whether BGGs
and other bright members of groups are consistent with an ordered population
among the total population of group galaxies. We find that the luminosity
distributions of BGGs do not follow the predictions from the order statistics
(OS). The average luminosities of BGGs are systematically brighter than OS
predictions. On the other hand, by properly taking into account the brightening
effect of the BGGs, the luminosity distributions of the second brightest
galaxies are in excellent agreement with the expectations of OS. The
brightening of BGGs relative to the OS expectation is consistent with a
scenario that the BGGs on average have over-grown about 20 percent masses
relative to the other member galaxies. The growth () is not
stochastic but correlated with the magnitude gap () between the
brightest and the second brightest galaxy. The growth () is larger
for the groups having more prominent BGGs (larger ) and averagely
contributes about 30 percent of the final of the groups of galaxies.Comment: ApJ accepted, replaced with the accepted versio
A two-phase model of galaxy formation: II. The size-mass relation of dynamically hot galaxies
In Paper-I we developed a two-phase model to connect dynamically hot galaxies
(such as ellipticals and bulges) with the formation of self-gravitating,
turbulent gas clouds (SGC) associated with the fast assembly of dark matter
halos. Here we explore the implications of the model for the size-stellar mass
relation of dynamically hot galaxies. Star-forming sub-clouds produced by the
fragmentation of the SGC inherit its spatial structure and dynamical hotness,
which produces a tight and 'homologous' relation, , between the size of a dynamically hot galaxy ()
and that of its host halo assembled in the fast assembly regime (),
independent of redshift and halo mass. This relation is preserved by the 'dry'
expansion driven by dynamical heating when a galaxy becomes gas-poor due to
inefficient cooling, and is frozen during the slow assembly regime when the
bulge stops growing in mass and dynamical heating is no longer effective. The
size-stellar mass relation is thus a simple combination of the galaxy-halo
homology and the non-linear relation between stellar mass and halo mass. Using
a set of halo assembly histories we demonstrate that this model can reproduce
all properties in the observed size-mass relation of dynamically hot galaxies,
including the flattening of the relation in the low-mass end and the upturn in
the very massive end. The predicted evolution of this relation matches
observational data currently available to redshift , and can be
tested in the future at higher . Our results indicate that the sizes of
dynamically hot galaxies are produced by the dissipation and collapse of gas in
dark matter halos to establish self-gravitating systems of sub-clouds in which
stars form.Comment: 9 pages, 4 figures, 1 table; submitted to MNRA
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