705 research outputs found
Evolution of galaxy groups in the Illustris simulation
We present the first study of evolution of galaxy groups in the Illustris
simulation. We focus on dynamically relaxed and unrelaxed galaxy groups
representing dynamically evolved and evolving galaxy systems, respectively. The
evolutionary state of a group is probed from its luminosity gap and separation
between the brightest group galaxy and the center of mass of the group members.
We find that the Illustris simulation, over-produces large luminosity gap
galaxy systems, known as fossil systems, in comparison to observations and the
probed semi-analytical predictions. However, this simulation is equally
successful in recovering the correlation between luminosity gap and luminosity
centroid offset, in comparison to the probed semi-analytic model. We find
evolutionary tracks based on luminosity gap which indicate that a large
luminosity gap group is rooted in a small luminosity gap group, regardless of
the position of the brightest group galaxy within the halo. This simulation
helps, for the first time, to explore the black hole mass and its accretion
rate in galaxy groups. For a given stellar mass of the brightest group
galaxies, the black hole mass is larger in dynamically relaxed groups with a
lower rate of mass accretion. We find this consistent with the latest
observational studies of the radio activities in the brightest group galaxies
in fossil groups. We also find that the IGM in dynamically evolved groups is
hotter for a given halo mass than that in evolving groups, again consistent
with earlier observational studies.Comment: 10 pages, 10 figures. Accepted for publication in Ap
Inhomogeneous metallic phase upon disordering a two dimensional Mott insulator
We find that isoelectronic disorder destroys the spectral gap in a
Mott-Hubbard insulator in 2D leading, most unexpectedly, to a new metallic
phase. This phase is spatially inhomogeneous with metallic behavior coexisting
with antiferromagnetic long range order. Even though the Mott gap in the pure
system is much larger than antiferromagnetic exchange, the spectral gap is
destroyed locally in regions where the disorder potential is high enough to
overcome the inter-electron repulsion thereby generating puddles where charge
fluctuations are enhanced. With increasing disorder, these puddles expand and
concomitantly the states at the Fermi energy get extended leading to a metallic
phase. We discuss the implications of our results for experiments.Comment: (4 pages, 5 figures
Evolution of the galaxy luminosity function in progenitors of fossil groups
Using the semi-analytic models based on the Millennium simulation, we trace
back the evolution of the luminosity function of galaxies residing in
progenitors of groups classified by the magnitude gap at redshift zero. We
determine the luminosity function of galaxies within , and for galaxy groups/clusters. The bright end of the galaxy
luminosity function of fossil groups shows a significant evolution with
redshift, with changes in by 1-2 mag between and
(for the central ), suggesting that the formation of the most
luminous galaxy in a fossil group has had a significant impact on the
galaxies e.g. it is formed as a result of multiple mergers of the
galaxies within the last Gyr. In contrast, the slope of the faint end,
, of the luminosity function shows no considerable redshift evolution
and the number of dwarf galaxies in the fossil groups exhibits no evolution,
unlike in non-fossil groups where it grows by towards low
redshifts. In agreement with previous studies, we also show that fossil groups
accumulate most of their halo mass earlier than non-fossil groups. Selecting
the fossils at a redshift of 1 and tracing them to a redshift 0, we show that
of the fossil groups () will lose their large magnitude gaps. However, about
of fossil clusters () will retain their large
gaps.Comment: Accepted for publication in A&A. 13 pages, 15 figure
Optically selected fossil groups; X-ray observations and galaxy properties
We report on the X-ray and optical observations of galaxy groups selected
from the 2dfGRS group catalog, to explore the possibility that galaxy groups
hosting a giant elliptical galaxy and a large optical luminosity gap present
between the two brightest group galaxies, can be associated with an extended
X-ray emission, similar to that observed in fossil galaxy groups. The X-ray
observations of 4 galaxy groups were carried out with Chandra telescope with
10-20 ksec exposure time. Combining the X-ray and the optical observations we
find evidences for the presence of a diffuse extended X-ray emission beyond the
optical size of the brightest group galaxy. Taking both the X-ray and the
optical criteria, one of the groups is identified as a fossil group and one is
ruled out because of the contamination in the earlier optical selection. For
the two remaining systems, the X-ay luminosity threshold is close to the
convention know for fossil groups. In all cases the X-ray luminosity is below
the expected value from the X-ray selected fossils for a given optical
luminosity of the group. A rough estimation for the comoving number density of
fossil groups is obtained and found to be in broad agreement with the
estimations from observations of X-ray selected fossils and predictions of
cosmological simulations.Comment: Accepted for publication in MNRA
A Study of Swelling Behaviour in a Tunnel Using Finite Element Methods
The aim of this research is to show swelling behaviour in a tunnel excavated through rocks by modeling them and using laboratory results. The engineering challenge is how to design a tunnel which contains swelling rocks such as marl. This aim is achieved through two methods. In the first method, the Field System Solution program (FISS) is calibrated using experimentally obtained laboratory graphs of the saturated rocks around the tunnel, and the parameters are applied to the geometry of the tunnel. Comparing a critical state model with stresses around the tunnel, stability of the rocks was examined. In the second method, stability of the rocks around the tunnel was investigated by using the Nisa-II program, adjusting the laboratory swelling graphs with the creep function to the program and, finally, drawing Von Mises stresses around the tunnel
The mass assembly of galaxy groups and the evolution of the magnitude gap
We investigate the assembly of groups and clusters of galaxies using the
Millennium dark matter simulation and the associated gas simulations and
semi-analytic catalogues of galaxies. In particular, in order to find an
observable quantity that could be used to identify early-formed groups, we
study the development of the difference in magnitude between their brightest
galaxies to assess the use of magnitude gaps as possible indicators. We select
galaxy groups and clusters at redshift z=1 with dark matter halo mass M(R200) >
1E13/h Msun, and trace their properties until the present time (z=0). We
consider only the systems with X-ray luminosity L_X> 0.25E42/h^2 erg/s at z=0.
While it is true that a large magnitude gap between the two brightest galaxies
of a particular group often indicates that a large fraction of its mass was
assembled at an early epoch, it is not a necessary condition. More than 90% of
fossil groups defined on the basis of their magnitude gaps (at any epoch
between 0<z<1) cease to be fossils within 4 Gyr, mostly because other massive
galaxies are assembled within their cores, even though most of the mass in
their haloes might have been assembled at early times. We show that, compared
to the conventional definition of fossil galaxy groups based on the magnitude
gap Delta m(12)> 2 (in the R-band, within 0.5R200 of the centre of the group),
an alternative criterion Delta m(14)>2.5 (within the same radius) finds 50%
more early-formed systems, and those that on average retain their fossil phase
longer. However, the conventional criterion performs marginally better at
finding early-formed groups at the high-mass end of groups. Nevertheless, both
criteria fail to identify a majority of the early-formed systems.Comment: 16 pages, 11 figures, 2 tables. Accepted for publication in MNRA
LoCuSS: Connecting the Dominance and Shape of Brightest Cluster Galaxies with the Assembly History of Massive Clusters
We study the luminosity gap, dm12, between the first and second ranked
galaxies in a sample of 59 massive galaxy clusters, using data from the Hale
Telescope, HST, Chandra, and Spitzer. We find that the dm12 distribution,
p(dm12), is a declining function of dm12, to which we fitted a straight line:
p(dm12) propto -(0.13+/-0.02)dm12. The fraction of clusters with "large"
luminosity gaps is p(dm12>=1)=0.37+/-0.08, which represents a 3sigma excess
over that obtained from Monte Carlo simulations of a Schechter function that
matches the mean cluster galaxy luminosity function. We also identify four
clusters with "extreme" luminosity gaps, dm12>=2, giving a fraction of
p(dm12>=2)=0.07+0.05-0.03. More generally, large luminosity gap clusters are
relatively homogeneous, with elliptical/disky brightest cluster galaxies
(BCGs), cuspy gas density profiles (i.e. strong cool cores), high
concentrations, and low substructure fractions. In contrast, small luminosity
gap clusters are heterogeneous, spanning the full range of
boxy/elliptical/disky BCG morphologies, the full range of cool core strengths
and dark matter concentrations, and have large substructure fractions. Taken
together, these results imply that the amplitude of the luminosity gap is a
function of both the formation epoch, and the recent infall history of the
cluster. "BCG dominance" is therefore a phase that a cluster may evolve
through, and is not an evolutionary "cul-de-sac". We also compare our results
with semi-analytic model predictions based on the Millennium Simulation. None
of the models are able to reproduce all of the observational results,
underlining the inability of current models to match the empirical properties
of BCGs. We identify the strength of AGN feedback and the efficiency with which
cluster galaxies are replenished after they merge with the BCG in each model as
possible causes of these discrepancies. [Abridged]Comment: 15 pages, 12 figures, accepted for publication in MNRA
- …