2,305 research outputs found
The Ages of Elliptical Galaxies in a Merger Model
The tightness of the observed colour-magnitude and Mg- velocity
dispersion relations for elliptical galaxies has often been cited as an
argument against a picture in which ellipticals form by the merging of spiral
disks. A common view is that merging would mix together stars of disparate ages
and produce a large scatter in these relations. Here I use semi-analytic models
of galaxy formation to derive the distribution of the mean ages, colours and
metallicities of the stars in elliptical galaxies formed by mergers in a flat
CDM universe. It is seen that most of the stars in ellipticals form at
relatively high redshift (z > 1.9) and that the predicted scatter in the
colour-magnitude and Mg_2 - sigma relations falls within observational bounds.
I conclude that the apparent homogeneity in the properties of the stellar
populations of ellipticals is not inconsistent with a merger scenario for the
origin of these systems.Comment: latex file, figures available upon reques
Clustering of Galaxies in a Hierarchical Universe: I. Methods and Results at z=0
We introduce a new technique for following the formation and evolution of
galaxies in cosmological N-body simulations. Dissipationless simulations are
used to track the formation and merging of dark matter halos as a function of
redshift. Simple prescriptions, taken directly from semi-analytic models of
galaxy formation, are adopted for cooling, star formation, supernova feedback
and the merging of galaxies within the halos. This scheme enables us to study
the clustering properties of galaxies and to investigate how selection by type,
colour or luminosity influences the results. In this paper, we study properties
of the galaxy distribution at z=0. These include luminosity functions, colours,
correlation functions, pairwise peculiar velocities, cluster M/L ratios and
star formation rates. We focus on two variants of a CDM cosmology: a high-
density model with Gamma=0.21 (TCDM) and a low-density model with Omega=0.3 and
Lambda=0.7 (LCDM). Both are normalized to reproduce the I-band Tully-Fisher
relation near a circular velocity of 220 km/s. Our results depend strongly both
on this normalization and on the prescriptions for star formation and feedback.
Very different assumptions are required to obtain an acceptable model in the
two cases. For TCDM, efficient feedback is required to suppress the growth of
galaxies low-mass field halos. Without it, there are too many galaxies and the
correlation function turns over below 1 Mpc. For LCDM, feedback must be weak,
otherwise too few L* galaxies are produced and the correlation function is too
steep. Given the uncertainties in modelling some of the key physical processes,
we conclude that it is not yet possible to draw conclusions about the values of
cosmological parameters from studies of this kind. Further work on global star
formation and feedback effects is required to narrow the range of possibilitiesComment: 43 pages, Latex, 16 figures included, 2 additional GIF format
figures, submitted to MNRA
A possible observational bias in the estimation of the virial parameter in virialized clumps
The dynamics of massive clumps, the environment where massive stars
originate, is still unclear. Many theories predict that these regions are in a
state of near-virial equilibrium, or near energy equi-partition, while others
predict that clumps are in a sub-virial state. Observationally, the majority of
the massive clumps are in a sub-virial state with a clear anti-correlation
between the virial parameter and the mass of the clumps ,
which suggests that the more massive objects are also the more gravitationally
bound. Although this trend is observed at all scales, from massive clouds down
to star-forming cores, theories do not predict it. In this work we show how,
starting from virialized clumps, an observational bias is introduced in the
specific case where the kinetic and the gravitational energies are estimated in
different volumes within clumps and how it can contribute to the spurious
anti-correlation in these data. As a result, the observed
effective virial parameter , and in some
circumstances it might not be representative of the virial state of the
observed clumps.Comment: A&A letter, accepte
Twinning phenomena along and beyond the bain path
Twinning is a phenomenon that occurs, e.g., during deformation, martensitic transformation and film growth. The present study shows that the crystallography of twinning can be described by two twinning modes along the complete Bain transformation path and beyond connecting body-centered and face-centered cubic structures. To probe this concept, we used strained epitaxial films of the Fe-Pd magnetic shape memory system. As the substrate acts as an absolute reference frame, we could show by pole figure measurements that all observed twinning can be a body-centered and face-centered cubic twinning mode. This continuously transforms towards identity when approaching the complementary structure
Non-linear Stochastic Galaxy Biasing in Cosmological Simulations
We study the biasing relation between dark-matter halos or galaxies and the
underlying mass distribution, using cosmological -body simulations in which
galaxies are modelled via semi-analytic recipes. The nonlinear, stochastic
biasing is quantified in terms of the mean biasing function and the scatter
about it as a function of time, scale and object properties. The biasing of
galaxies and halos shows a general similarity and a characteristic shape, with
no galaxies in deep voids and a steep slope in moderately underdense regions.
At \sim 8\hmpc, the nonlinearity is typically \lsim 10 percent and the
stochasticity is a few tens of percent, corresponding to percent
variations in the cosmological parameter . Biasing
depends weakly on halo mass, galaxy luminosity, and scale. The time evolution
is rapid, with the mean biasing larger by a factor of a few at
compared to , and with a minimum for the nonlinearity and stochasticity at
an intermediate redshift. Biasing today is a weak function of the cosmological
model, reflecting the weak dependence on the power-spectrum shape, but the time
evolution is more cosmology-dependent, relecting the effect of the growth rate.
We provide predictions for the relative biasing of galaxies of different type
and color, to be compared with upcoming large redshift surveys. Analytic models
in which the number of objects is conserved underestimate the evolution of
biasing, while models that explicitly account for merging provide a good
description of the biasing of halos and its evolution, suggesting that merging
is a crucial element in the evolution of biasing.Comment: 27 pages, 21 figures, submitted to MNRA
Spectral and morphological properties of quasar hosts in SPH simulations of AGN feeding by mergers
We present a method for generating virtual observations from
smoothed-particle-hydrodynamics (SPH) simulations. This method includes stellar
population synthesis models and the reprocessing of starlight by dust to
produce realistic galaxy images. We apply this method and simulate the merging
of two identical giant Sa galaxies. The merger remnant is an elliptical galaxy.
The merger concentrates the gas content of the two galaxies into the nuclear
region. The gas that flows into the nuclear region refuels the central black
holes of the merging galaxies. We follow the refuelling of the black holes
during the merger semi-analytically.
In the simulation presented in this article, the black holes grow from 3 x
10^7 to 1.8X 10^8 Solar masses, with a peak AGN luminosity of M_B ~ -23.7. We
study how the morphological and spectral properties of the system evolve during
the merger and work out the predictions of this scenario for the properties of
host galaxies during the active phase. The peak of AGN activity coincides with
the merging of the two galactic nuclei and occurs at a stage when the remnant
looks like a lenticular galaxy. The simulation predicts the formation of a
circumnuclear starburst ring/dusty torus with an opening angle of 30-40 degrees
and made of clouds with n_H=10^24 cm^-2. The average optical depth of the torus
is quite high, but the obscuring medium is patchy, so that there still exist
lines of sight where the AGN is visible in a nearly edge-on view. For the same
reason, there are lines of sight where the AGN is completely obscured in the
face-on view.Comment: 14 pages, 11 figure
The Evolution of Early-Type Galaxies in Distant Clusters
We present results from an optical-IR photometric study of early-type
galaxies in 19 galaxy clusters out to z=0.9. The galaxy sample is selected on
the basis of morphologies determined from HST WFPC2 images, and is
photometrically defined in the K-band to minimize redshift-dependent selection
biases. The optical-IR colors of the early-type cluster galaxies become bluer
with increasing redshift in a manner consistent with the passive evolution of
an old stellar population formed at an early cosmic epoch. The degree of color
evolution is similar for clusters at similar redshift, and does not depend
strongly on the optical richness or X-ray luminosity of the cluster, suggesting
that the history of early-type galaxies is relatively insensitive to
environment. The slope of the color-magnitude relationship shows no significant
change out to z=0.9, providing evidence that it arises from a correlation
between galaxy mass and metallicity, not age. Finally, the intrinsic scatter in
the optical-IR colors is small and nearly constant with redshift, indicating
that the majority of giant, early-type galaxies in clusters share a common star
formation history, with little perturbation due to uncorrelated episodes of
later star formation. Taken together, our results are consistent with models in
which most early-type galaxies in rich clusters are old, formed the majority of
their stars at high redshift in a well-synchronized fashion, and evolved
quiescently thereafter.Comment: 55 pages, 24 figures, uses AASTeX. Accepted for publication in The
Astrophysical Journa
Cosmic Voids and Galaxy Bias in the Halo Occupation Framework
(Abridged) We investigate the power of void statistics to constrain galaxy
bias and the amplitude of dark matter fluctuations. We use the halo occupation
distribution (HOD) framework to describe the relation between galaxies and dark
matter. After choosing HOD parameters that reproduce the mean space density
n_gal and projected correlation function w_p measured for galaxy samples with
M_r<-19 and M_r<-21 from the Sloan Digital Sky Survey (SDSS), we predict the
void probability function (VPF) and underdensity probability function (UPF) of
these samples by populating the halos of a large, high-resolution N-body
simulation. If we make the conventional assumption that the HOD is independent
of large scale environment at fixed halo mass, then models constrained to match
n_gal and w_p predict nearly identical void statistics, independent of the
scatter between halo mass and central galaxy luminosity or uncertainties in HOD
parameters. Models with sigma_8=0.7 and sigma_8=0.9 also predict very similar
void statistics. However, the VPF and UPF are sensitive to environmental
variations of the HOD in a regime where these variations have little impact on
w_p. For example, doubling the minimum host halo mass in regions with large
scale (5 Mpc/h) density contrast delta<-0.65 has a readily detectable impact on
void probabilities of M_r<-19 galaxies, and a similar change for delta<-0.2
alters the void probabilities of M_r<-21 galaxies at a detectable level. The
VPF and UPF provide complementary information about the onset and magnitude of
density- dependence in the HOD. By detecting or ruling out HOD changes in low
density regions, void statistics can reduce systematic uncertainties in the
cosmological constraints derived from HOD modeling, and, more importantly,
reveal connections between halo formation history and galaxy properties.Comment: emulateapj, 16 pages, 13 figure
Dark-matter sterile neutrinos in models with a gauge singlet in the Higgs sector
Sterile neutrino with mass of several keV can be the cosmological dark
matter, can explain the observed velocities of pulsars, and can play an
important role in the formation of the first stars. We describe the production
of sterile neutrinos in a model with an extended Higgs sector, in which the
Majorana mass term is generated by the vacuum expectation value of a
gauge-singlet Higgs boson. In this model the relic abundance of sterile
neutrinos does not necessarily depend on their mixing angles, the
free-streaming length can be much smaller than in the case of warm dark matter
produced by neutrino oscillations, and, therefore, some of the previously
quoted bounds do not apply. The presence of the gauge singlet in the Higgs
sector has important implications for the electroweak phase transition,
baryogenesis, and the upcoming experiments at the Large Hadron Collider and a
Linear Collider.Comment: 12 pages, 7 figure
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