18,822 research outputs found
Structure formation from non-Gaussian initial conditions: multivariate biasing, statistics, and comparison with N-body simulations
We study structure formation in the presence of primordial non-Gaussianity of
the local type with parameters f_NL and g_NL. We show that the distribution of
dark-matter halos is naturally described by a multivariate bias scheme where
the halo overdensity depends not only on the underlying matter density
fluctuation delta, but also on the Gaussian part of the primordial
gravitational potential phi. This corresponds to a non-local bias scheme in
terms of delta only. We derive the coefficients of the bias expansion as a
function of the halo mass by applying the peak-background split to common
parametrizations for the halo mass function in the non-Gaussian scenario. We
then compute the halo power spectrum and halo-matter cross spectrum in the
framework of Eulerian perturbation theory up to third order. Comparing our
results against N-body simulations, we find that our model accurately describes
the numerical data for wavenumbers k < 0.1-0.3 h/Mpc depending on redshift and
halo mass. In our multivariate approach, perturbations in the halo counts trace
phi on large scales and this explains why the halo and matter power spectra
show different asymptotic trends for k -> 0. This strongly scale-dependent bias
originates from terms at leading order in our expansion. This is different from
what happens using the standard univariate local bias where the scale-dependent
terms come from badly behaved higher-order corrections. On the other hand, our
biasing scheme reduces to the usual local bias on smaller scales where |phi| is
typically much smaller than the density perturbations. We finally discuss the
halo bispectrum in the context of multivariate biasing and show that, due to
its strong scale and shape dependence, it is a powerful tool for the detection
of primordial non-Gaussianity from future galaxy surveys.Comment: 26 pages, 16 figures. Minor modifications, version accepted by Phys.
Rev.
Bias and Hierarchical Clustering
It is now well established that galaxies are biased tracers of the
distribution of matter, although it is still not known what form this bias
takes. In local bias models the propensity for a galaxy to form at a point
depends only on the overall density of matter at that point. Hierarchical
scaling arguments allow one to build a fully-specified model of the underlying
distribution of matter and to explore the effects of local bias in the regime
of strong clustering. Using a generating-function method developed by
Bernardeau & Schaeffer (1992), we show that hierarchical models lead one
directly to the conclusion that a local bias does not alter the shape of the
galaxy correlation function relative to the matter correlation function on
large scales. This provides an elegant extension of a result first obtained by
Coles (1993) for Gaussian underlying fields and confirms the conclusions of
Scherrer & Weinberg (1998) obtained using a different approach. We also argue
that particularly dense regions in a hierarchical density field display a form
of bias that is different from that obtained by selecting such peaks in
Gaussian fields: they are themselves hierarchically distributed with scaling
parameters . This kind of bias is also factorizable, thus in
principle furnishing a simple test of this class of models.Comment: Latex, accepted for publication in ApJL; moderate revision
Inhibitory Effect of Berberine on Zeste Homolog 2 (Ezh2) Enhancement in Human Esophageal Cell Lines
Purpose: To investigate the inhibitory effect of berberine treatment on enhancement of zeste of homolog 2 (Ezh2) expressions in KYSE450 human esophageal cancer cells.Methods: Transwell motility chambers were used to analyze cell migration and invasion. Bio-Rad protein assay was used for the determination of protein concentration. Chemiluminescence with ECL system was employed for the detection of protein bands as per the manufacturer’s protocol. Staining was carried out with Alexa-Fluor 647 mouse anti-BrdU antibody. Flow cytometry was performed after adding DAPI. Annexin-V/DAPI staining and flow cytometry were used for the quantification of apoptotic cell death. Total RNA was isolated from KYSE450 cells using an RNA isolation kit.Results: Berberine-induced inhibition of Ezh2 expression led to inhibition of cell proliferation by G1 phase cell cycle arrest and induced anti-invasive properties of KYSE450 cells in Boyden chamber assays. There was 92 % reduction in invasive tendency of KYSE450 cells following treatment with berberine. Histone methylation inhibitor, 3-deazaneoplanocin A (DZNep), also led to a similar effect on cell proliferation of KYSE450 cells. Berberine treatment also resulted in strong transcriptional reduction of the AXL receptor kinase. The results of qRT-PCR and FACS analyses showed significant inhibition of AXL mRNA and protein expression in KYSE450 carcinoma cells after treatment with berberine.Conclusion: Berberine may be an effective therapeutic agent in the treatment of esophageal carcinoma.Keywords: Berberine, Histone methylation inhibitor, Anti-invasive, Cell proliferation, Human Esophageal cance
Properties of Galaxy Groups in the SDSS: II.- AGN Feedback and Star Formation Truncation
Successfully reproducing the galaxy luminosity function and the bimodality in
the galaxy distribution requires a mechanism that can truncate star formation
in massive haloes. Current models of galaxy formation consider two such
truncation mechanisms: strangulation, which acts on satellite galaxies, and AGN
feedback, which predominantly affects central galaxies. The efficiencies of
these processes set the blue fraction of galaxies as function of galaxy
luminosity and halo mass. In this paper we use a galaxy group catalogue
extracted from the Sloan Digital Sky Survey (SDSS) to determine these
fractions. To demonstrate the potential power of this data as a benchmark for
galaxy formation models, we compare the results to the semi-analytical model
for galaxy formation of Croton et al. (2006). Although this model accurately
fits the global statistics of the galaxy population, as well as the shape of
the conditional luminosity function, there are significant discrepancies when
the blue fraction of galaxies as a function of mass and luminosity is compared
between the observations and the model. In particular, the model predicts (i)
too many faint satellite galaxies in massive haloes, (ii) a blue fraction of
satellites that is much too low, and (iii) a blue fraction of centrals that is
too high and with an inverted luminosity dependence. In the same order, we
argue that these discrepancies owe to (i) the neglect of tidal stripping in the
semi-analytical model, (ii) the oversimplified treatment of strangulation, and
(iii) improper modeling of dust extinction and/or AGN feedback. The data
presented here will prove useful to test and calibrate future models of galaxy
formation and in particular to discriminate between various models for AGN
feedback and other star formation truncation mechanisms.Comment: 16 pages, 5 figures, submitted to MNRA
The Bispectrum of IRAS Galaxies
We compute the bispectrum for the galaxy distribution in the IRAS QDOT, 2Jy,
and 1.2Jy redshift catalogs for wavenumbers 0.05<k<0.2 h/Mpc and compare the
results with predictions from gravitational instability in perturbation theory.
Taking into account redshift space distortions, nonlinear evolution, the survey
selection function, and discreteness and finite volume effects, all three
catalogs show evidence for the dependence of the bispectrum on configuration
shape predicted by gravitational instability. Assuming Gaussian initial
conditions and local biasing parametrized by linear and non-linear bias
parameters b_1 and b_2, a likelihood analysis yields 1/b_1 =
1.32^{+0.36}_{-0.58}, 1.15^{+0.39}_{-0.39} and b_2/b_1^2=-0.57^{+0.45}_{-0.30},
-0.50^{+0.31}_{-0.51}, for the for the 2Jy and 1.2Jy samples, respectively.
This implies that IRAS galaxies trace dark matter increasingly weakly as the
density contrast increases, consistent with their being under-represented in
clusters. In a model with chi^2 non-Gaussian initial conditions, the bispectrum
displays an amplitude and scale dependence different than that found in the
Gaussian case; if IRAS galaxies do not have bias b_1> 1 at large scales, \chi^2
non-Gaussian initial conditions are ruled out at the 95% confidence level. The
IRAS data do not distinguish between Lagrangian or Eulerian local bias.Comment: 30 pages, 11 figure
Where can we really find the First Stars' Remnants today?
A number of recent numerical investigations concluded that the remnants of
rare structures formed at very high redshift, such as the very first stars and
bright redshift z~6 QSOs, are preferentially located at the center of the most
massive galaxy clusters at redshift z=0. In this paper we readdress this
question using a combination of cosmological simulations of structure formation
and extended Press-Schechter formalism and we show that the typical remnants of
Population III stars are instead more likely to be found in a group
environment, that is in dark matter halos of mass ~2x10^{13} h^{-1}M_sun.
Similarly, the descendants of the brightest z~6 QSOs are expected to be in
medium-sized clusters (mass of a few 10^{14} h^{-1}M_sun), rather than in the
most massive superclusters (M>10^{15} h^{-1}M_sun) found within the typical 1
Gpc^3 cosmic volume where a bright z~6 QSO lives. The origin of past claims
that the most massive clusters preferentially host these remnants is rooted in
the numerical method used to initialize their numerical simulations: Only a
small region of the cosmological volume of interest was simulated with
sufficient resolution to identify low-mass halos at early times, and this
region was chosen to host the most massive halo in the cosmological volume at
late times. The conclusion that the earliest structures formed in the entire
cosmological volume evolve into the most massive halo at late times was thus
arrived at by construction. We demonstrate that, to the contrary, the first
structures to form in a cosmological region evolve into relatively typical
objects at later times. We propose alternative numerical methods for simulating
the earliest structures in cosmological volumes.Comment: 18 pages, 5 figures, ApJ accepted, high resolution version of the
paper available at http://www.stsci.edu/~trenti/papers/halo_evolution.pd
Constraints on primordial non-Gaussianity from WMAP7 and Luminous Red Galaxies power spectrum and forecast for future surveys
We place new constraints on the primordial local non-Gaussianity parameter
f_NL using recent Cosmic Microwave Background anisotropy and galaxy clustering
data. We model the galaxy power spectrum according to the halo model,
accounting for a scale dependent bias correction proportional to f_NL/k^2. We
first constrain f_NL in a full 13 parameters analysis that includes 5
parameters of the halo model and 7 cosmological parameters. Using the WMAP7 CMB
data and the SDSS DR4 galaxy power spectrum, we find f_NL=171\pm+140 at 68%
C.L. and -69<f_NL<+492 at 95% C.L.. We discuss the degeneracies between f_NL
and other cosmological parameters. Including SN-Ia data and priors on H_0 from
Hubble Space Telescope observations we find a stronger bound: -35<f_NL<+479 at
95% C.L.. We also fit the more recent SDSS DR7 halo power spectrum data
finding, for a \Lambda-CDM+f_NL model, f_NL=-93\pm128 at 68% C.L. and
-327<f_{NL}<+177 at 95% C.L.. We finally forecast the constraints on f_NL from
future surveys as EUCLID and from CMB missions as Planck showing that their
combined analysis could detect f_NL\sim 5.Comment: 10 pages, 5 figures, 3 table
Quasiparticle dynamics and spin-orbital texture of the SrTiO3 two-dimensional electron gas
Two-dimensional electron gases (2DEGs) in SrTiO have become model systems
for engineering emergent behaviour in complex transition metal oxides.
Understanding the collective interactions that enable this, however, has thus
far proved elusive. Here we demonstrate that angle-resolved photoemission can
directly image the quasiparticle dynamics of the -electron subband ladder of
this complex-oxide 2DEG. Combined with realistic tight-binding supercell
calculations, we uncover how quantum confinement and inversion symmetry
breaking collectively tune the delicate interplay of charge, spin, orbital, and
lattice degrees of freedom in this system. We reveal how they lead to
pronounced orbital ordering, mediate an orbitally-enhanced Rashba splitting
with complex subband-dependent spin-orbital textures and markedly change the
character of electron-phonon coupling, co-operatively shaping the low-energy
electronic structure of the 2DEG. Our results allow for a unified understanding
of spectroscopic and transport measurements across different classes of
SrTiO-based 2DEGs, and yield new microscopic insights on their functional
properties.Comment: 10 pages including supplementary information, 4+4 figure
Gravity and Large-Scale Non-local Bias
The relationship between galaxy and matter overdensities, bias, is most often
assumed to be local. This is however unstable under time evolution, we provide
proofs under several sets of assumptions. In the simplest model galaxies are
created locally and linearly biased at a single time, and subsequently move
with the matter (no velocity bias) conserving their comoving number density (no
merging). We show that, after this formation time, the bias becomes unavoidably
non-local and non-linear at large scales. We identify the non-local
gravitationally induced fields in which the galaxy overdensity can be expanded,
showing that they can be constructed out of the invariants of the deformation
tensor (Galileons). In addition, we show that this result persists if we
include an arbitrary evolution of the comoving number density of tracers. We
then include velocity bias, and show that new contributions appear, a dipole
field being the signature at second order. We test these predictions by
studying the dependence of halo overdensities in cells of fixed matter density:
measurements in simulations show that departures from the mean bias relation
are strongly correlated with the non-local gravitationally induced fields
identified by our formalism. The effects on non-local bias seen in the
simulations are most important for the most biased halos, as expected from our
predictions. The non-locality seen in the simulations is not fully captured by
assuming local bias in Lagrangian space. Accounting for these effects when
modeling galaxy bias is essential for correctly describing the dependence on
triangle shape of the galaxy bispectrum, and hence constraining cosmological
parameters and primordial non-Gaussianity. We show that using our formalism we
remove an important systematic in the determination of bias parameters from the
galaxy bispectrum, particularly for luminous galaxies. (abridged)Comment: 26 pages, 9 figures. v2: improved appendix
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