40 research outputs found
The Pairwise Peculiar Velocity Dispersion of Galaxies: Effects of the Infall
We study the reliability of the reconstruction method which uses a modelling
of the redshift distortions of the two-point correlation function to estimate
the pairwise peculiar velocity dispersion of galaxies. In particular, the
dependence of this quantity on different models for the infall velocity is
examined for the Las Campanas Redshift Survey. We make extensive use of
numerical simulations and of mock catalogs derived from them to discuss the
effect of a self-similar infall model, of zero infall, and of the real infall
taken from the simulation. The implications for two recent discrepant
determinations of the pairwise velocity dispersion for this survey are
discussed.Comment: minor changes in the discussion; accepted for publication in ApJ; 8
pages with 2 figures include
The cross-correlation between galaxies of different luminosities and Colors
We study the cross-correlation between galaxies of different luminosities and
colors, using a sample selected from the SDSS Dr 4. Galaxies are divided into 6
samples according to luminosity, and each of these samples is divided into red
and blue subsamples. Projected auto-correlation and cross-correlation is
estimated for these subsample. At projected separations r_p > 1\mpch, all
correlation functions are roughly parallel, although the correlation amplitude
depends systematically on luminosity and color. On r_p < 1\mpch, the auto- and
cross-correlation functions of red galaxies are significantly enhanced relative
to the corresponding power laws obtained on larger scales. Such enhancement is
absent for blue galaxies and in the cross-correlation between red and blue
galaxies. We esimate the relative bias factor on scales r > 1\mpch for each
subsample using its auto-correlation function and cross-correlation functions.
The relative bias factors obtained from different methods are similar. For blue
galaxies the luminosity-dependence of the relative bias is strong over the
luminosity range probed (-23.0<M_r < -18.0),but for red galaxies the dependence
is weaker and becomes insignificant for luminosities below L^*. To examine
whether a significant stochastic/nonlinear component exists in the bias
relation, we study the ratio R_ij= W_{ii}W_{jj}/W_{ij}^2, where W_{ij} is the
projected correlation between subsample i and j. We find that the values of
R_ij are all consistent with 1 for all-all, red-red and blue-blue samples,
however significantly larger than 1 for red-blue samples. For faint red - faint
blue samples the values of R_{ij} are as high as ~ 2 on small scales r_p < 1
\mpch and decrease with increasing r_p.Comment: 25 pages, 18 figures, Accepted for publication in Ap
Scaling properties of the redshift power spectrum: theoretical models
We report the results of an analysis of the redshift power spectrum
in three typical Cold Dark Matter (CDM) cosmological models, where
is the cosine of the angle between the wave vector and the line-of-sight.
Two distinct biased tracers derived from the primordial density peaks of
Bardeen et al. and the cluster-underweight model of Jing, Mo, & B\"orner are
considered in addition to the pure dark matter models. Based on a large set of
high resolution simulations, we have measured the redshift power spectrum for
the three tracers from the linear to the nonlinear regime. We investigate the
validity of the relation - guessed from linear theory - in the nonlinear regime
where
is the real space power spectrum, and equals . The
damping function which should generally depend on , , and
, is found to be a function of only one variable
. This scaling behavior extends into the nonlinear regime,
while can be accurately expressed as a Lorentz function - well known from
linear theory - for values . The difference between
and the pairwise velocity dispersion defined by the 3-D peculiar velocity of
the simulations (taking ) is about 15%. Therefore is a
good indicator of the pairwise velocity dispersion. The exact functional form
of depends on the cosmological model and on the bias scheme. We have given
an accurate fitting formula for the functional form of for the models
studied.Comment: accepted for publication in ApJ;24 pages with 7 figures include
Spatial correlation functions and the pairwise peculiar velocity dispersion of galaxies in the PSCz survey: implications for the galaxy biasing in cold dark matter models
We report on the measurement of the two-point correlation function, and the
pairwise peculiar velocity of galaxies in the IRAS PSCz survey. We compute
these statistics first in redshift space, and then obtain the projected
functions which have simple relations to the real-space correlation functions
on the basis of the method developed earlier in analyzing the Las Campanas
Redshift Survey (LCRS) by Jing, Mo, & B\"orner (1998). We find that the real
space two-point correlation function can be fitted to a power law with and r_0=3.70 \mpc. The pairwise peculiar
velocity dispersion is close to 400 \kms at r_p=3\mpc
and decreases to about 150 \kms at r_p \approx 0.2 \mpc. These values are
significantly lower than those obtained from the LCRS. In order to understand
the implications of those measurements on the galaxy biasing, we construct mock
samples for a low density spatially-flat cold dark matter model (, , , ) using a set of
high-resolution N-body simulations. Applying a stronger cluster-underweight
biasing () than for the LCRS (), we are
able to reproduce these observational data, except for the strong decrease of
the pairwise peculiar velocity at small separations. This is qualitatively
ascribed to the different morphological mixture of galaxies in the two
catalogues. Disk-dominated galaxy samples drawn from the theoretically
constructed GIF catalog yield results rather similar to our mock samples with
the simple cluster-underweight biasing.Comment: accepted for publication in ApJ; 24 pages with 9 figure
The Three-point Correlation Function of Galaxies Determined from the 2dF Galaxy Redshift Survey
In a detailed analysis of the three point correlation function (3PCF) for the
2dF Galaxy Redshift Survey we have accurately measured the 3PCF for galaxies of
different luminosity. The 3PCF amplitudes [\Qsu or \Qrpu] of the galaxies
generally decrease with increasing triangle size and increase with the shape
parameter , in qualitative agreement with the predictions for the clustering
of dark matter in popular hierarchical CDM models. The 2dFGRS results agree
well with the results of Jing & B\"orner for the Las Camapanas Redshift Survey
(LCRS), though the measurement accuracy is greatly improved in the present
study. The dependence of the 3PCF on luminosity is not significant, but there
seems to be a trend for the brightest galaxy sample to have a lower amplitude
than the fainter ones. Comparing the measured 3PCF amplitudes [\Qsu or
\Qrpu] to the prediction of a WMAP concordance model, we find that the
measured values are consistently lower than the predicted ones for dark matter.
This is most pronounced for the brightest galaxies, for which about one-half of
the predicted value provides a good description of \Qrpu for the 2dFGRS
data. For a less luminous sample, the values are also smaller than in the
dark matter model on small scales, but on scales larger than s=8 \mpc and
r_p=3.25 \mpc they reach the model values. We discuss implications for
current theories of galaxy formation.Comment: 39 pages, 18 figures, ApJ, published version; minor changes to the
previous versio
Semi-analytical model of galaxy formation with high-resolution N-body simulations
We model the galaxy formation in a series of high-resolution N-body
simulations using the semi-analytical approach. Unlike many earlier
investigations based on semi-analytical models, we make use of the subhalos
resolved in the -body simulations to follow the mergers of galaxies in dark
halos, and we show that this is pivotal in modeling correctly the galaxy
luminosity function at the bright end and the bimodal nature of galaxy color
distribution. Mergers of galaxies based on subhalos also result in many more
bright red galaxies at high . The semi-analytical model we adopt is similar
to those used in earlier semi-analytical studies, except that we consider the
effect of a prolonged cooling in small halos and that we explicitly follow the
chemical enrichment in the interstellar medium. We use our model to make
predictions for the properties of the galaxy population at low redshift and
compare them with various current observations. We find that our model
predictions can match the luminosity functions of galaxies in various wavebands
redder than the u-band. The shape of the luminosity function at bright end is
well reproduced if galaxy mergers are modeled with the merger trees of subhalos
and the steep faint-end slope can be moderated if the gas cooling time in
low-mass halos is comparable to the age of the universe. The model with
subhalos resolved can reproduce the main features in the observed color bimodal
distribution, though it still predicts too many bright blue galaxies. The same
model can also match the color-magnitude relation for elliptical galaxies in
clusters, the metallicity-luminosity relation.Comment: 57 pages, 1 table, 18 figures, a new figure added, more discussion on
color bimodality, to appear in Ap
The decline of the Serengeti Thomson's gazelle population
The population of Thomson's gazelles in the Serengeti National Park, Tanzania has declined by almost two thirds over a 13 year period. In the early 1970s, numbers stood at 0.66 million animals but had decreased to less than 0.25 million animals in 1985 as estimated by 5 different censuses using two different counting techniques. Predation, interspecific competition and disease are all factors that could have contributed to this decline, and at least one of these factors, predation, could now prevent the Thomson's gazelle population from increasing.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47770/1/442_2004_Article_BF00376974.pd
TEOSINTE BRANCHED1 regulates height and stem internode length in bread wheat
Regulation of plant height and stem elongation has contributed significantly to improvement of cereal productivity by reducing lodging and improving distribution of assimilates to the inflorescence and grain. In wheat, genetic control of height has been largely contributed by the Reduced height-1 alleles that confer gibberellin insensitivity; the beneficial effects of these alleles are associated with less favourable effects involving seedling emergence, grain quality, and inflorescence architecture that have driven new research investigating genetic variation of stem growth. Here, we show that TEOSINTE BRANCHED1 (TB1) regulates height of wheat, with TB1 being expressed at low levels in nodes of the main culm prior to elongation, and increased dosage of TB1 restricting elongation of stem internodes. The effect of TB1 on stem growth is not accompanied by poor seedling emergence, as transgenic lines with increased activity of TB1 form longer coleoptiles than null transgenic controls. Analysis of height in a multiparent mapping population also showed that allelic variation for TB1 on the B genome influences height, with plants containing the variant TB-B1b allele being taller than those with the wild-type TB-B1a allele. Our results show that TB1 restricts height and stem elongation in wheat, suggesting that variant alleles that alter the expression or function of TB1 could be used as a new source of genetic diversity for optimizing architecture of wheat in breeding programmes
Retinoids cause apoptosis in pancreatic cancer cells via activation of RAR-γ and altered expression of Bcl-2/Bax
All-trans-retinoic acid and 9-cis-retinoic acid have been reported to have inhibitory effects on pancreatic adenocarcinoma cells and we have shown that this is partly due to induction of apoptosis. In this study, the mechanisms whereby 9-cis-retinoic acid induces apoptosis in these cells were investigated. An involvement of the Bcl-2 family of proteins was shown, such that 9-cis-retinoic acid causes a decrease in the Bcl-2/Bax ratio. Overexpression of Bcl-2 also resulted in inhibition of apoptosis induced by 9-cis-retinoic acid. Furthermore, two broad-range caspase inhibitors blocked DNA fragmentation induced by 9-cis-retinoic acid, but had no effect on viability defined by mitochondrial activity. Using synthetic retinoids, which bind selectively to specific retinoic acid receptor subtypes, we further established that activation of retinoic acid receptor-γ is essential for induction of apoptosis. Only pan-retinoic acid receptor and retinoic acid receptor-γ selective agonists reduced viability and a cell line expressing very low levels of retinoic acid receptor-γ is resistant to the effects of 9-cis-retinoic acid. A retinoic acid receptor-β/γ selective antagonist also suppressed the cytotoxic effects of 9-cis-retinoic acid in a dose-dependent manner. This study provides important insight into the mechanisms involved in suppression of pancreatic tumour cell growth by retinoids. Our results encourage further work evaluating the clinical use of receptor subtype selective retinoids in pancreatic carcinoma
Naturally Occurring Triggers that Induce Apoptosis-Like Programmed Cell Death in Plasmodium berghei Ookinetes
Several protozoan parasites have been shown to undergo a form of programmed cell death that exhibits morphological features associated with metazoan apoptosis. These include the rodent malaria parasite, Plasmodium berghei. Malaria zygotes develop in the mosquito midgut lumen, forming motile ookinetes. Up to 50% of these exhibit phenotypic markers of apoptosis; as do those grown in culture. We hypothesised that naturally occurring signals induce many ookinetes to undergo apoptosis before midgut traversal. To determine whether nitric oxide and reactive oxygen species act as such triggers, ookinetes were cultured with donors of these molecules. Exposure to the nitric oxide donor SNP induced a significant increase in ookinetes with condensed nuclear chromatin, activated caspase-like molecules and translocation of phosphatidylserine that was dose and time related. Results from an assay that detects the potential-dependent accumulation of aggregates of JC-1 in mitochondria suggested that nitric oxide does not operate via loss of mitochondrial membrane potential. L-DOPA (reactive oxygen species donor) also caused apoptosis in a dose and time dependent manner. Removal of white blood cells significantly decreased ookinetes exhibiting a marker of apoptosis in vitro. Inhibition of the activity of nitric oxide synthase in the mosquito midgut epithelium using L-NAME significantly decreased the proportion of apoptotic ookinetes and increased the number of oocysts that developed. Introduction of a nitric oxide donor into the blood meal had no effect on mosquito longevity but did reduce prevalence and intensity of infection. Thus, nitric oxide and reactive oxygen species are triggers of apoptosis in Plasmodium ookinetes. They occur naturally in the mosquito midgut lumen, sourced from infected blood and mosquito tissue. Up regulation of mosquito nitric oxide synthase activity has potential as a transmission blocking strategy