784 research outputs found
Morphology of Mock SDSS Catalogues
We measure the geometry, topology and morphology of the superclusters in mock
SDSS catalogues prepared by Cole et al.(1998). The mock catalogues refer to
CDM and \LCDM {\em flat} cosmological models and are populated by
galaxies so that these act as biased tracers of mass, conforming with the
correlation function measured using APM catalogue. We compute the Minkowski
Functionals (MFs) for the cosmic density fields using SURFGEN (Sheth et
al.2003) and use the available 10 realizations of CDM to study the effect
of cosmic variance in estimation of MFs and Shapefinders, which we find to be
extremely well constrained statistics. Although all the mock catalogues of
galaxies have the same two-point correlation function and similar clustering
amplitude, the global MFs due to CDM show systematically lower amplitude
compared to those due to \LCDM; an indirect, but detectable effect due to
nonzero, higher order correlation functions. The characteristic thickness (T),
breadth (B) and length (L) of the superclusters are measured using the
available 10 realizations of CDM. While TB and T, B[1,17]
hMpc, we find the top 10 superclusters to be as long as 90 hMpc,
with the longest superclusters identified at percolation to be rare objects
with their length as large as 150 hMpc. The CDM superclusters are
found to be significantly longer than those in \LCDM. Thickness (T), breadth
(B), planarity (P) and mass/volumeweighted planarity and filamentarity of
the superclusters are found to be useful to compare the two models (abridged).Comment: 23 Pages, 12 Figures, MNRAS Style. Minor modifications to the text.
New references adde
Youth Involvement in Community Development: Implications and Possibilities for Extension
There is a need for Extension program/policy developers to better understand the role of youth in the community development process. While often seen as suited only for 4-H programs, youth can significantly contribute to a variety of Extension activities. Through active engagement, youth can take on ownership and become lifelong contributors to local well-being. This mixed-methods research reflects data from a survey of 418 Florida youth and 12 in-depth key informant interviews. The findings provide insights into the factors most directly shaping youth attitudes and involvement in their communities. From these, implications for applied use in Extension programs are presented
Can Baryonic Features Produce the Observed 100 Mpc Clustering?
We assess the possibility that baryonic acoustic oscillations in adiabatic
models may explain the observations of excess power in large-scale structure on
100h^-1 Mpc scales. The observed location restricts models to two extreme areas
of parameter space. In either case, the baryon fraction must be large
(Omega_b/Omega_0 > 0.3) to yield significant features. The first region
requires Omega_0 < 0.2h to match the location, implying large blue tilts
(n>1.4) to satisfy cluster abundance constraints. The power spectrum also
continues to rise toward larger scales in these models. The second region
requires Omega_0 near 1, implying Omega_b well out of the range of big bang
nucleosynthesis constraints; moreover, the peak is noticeably wider than the
observations suggest. Testable features of both solutions are that they require
moderate reionization and thereby generate potentially observable (about 1 uK)
large-angle polarization, as well as sub-arc-minute temperature fluctuations.
In short, baryonic features in adiabatic models may explain the observed excess
only if currently favored determinations of cosmological parameters are in
substantial error or if present surveys do not represent a fair sample of
100h^-1 Mpc structures.Comment: LaTeX, 7 pages, 5 Postscript figures, submitted to ApJ Letter
Measuring Feedback Using the Intergalactic Medium State and Evolution Inferred from the Soft X-ray Background
We explore the intergalactic medium (IGM) as a potential source of the
unresolved soft X-ray background (XRB) and the feasibility to extract the IGM
state and evolution from XRB observations. We build two analytical models, the
continuum field model and the halo model, to calculate the IGM XRB mean flux,
angular auto correlation and cross correlation with galaxies. Our results
suggest that the IGM may contribute a significant fraction to the unresolved
soft XRB flux and correlations. We calibrated non-Gaussian errors estimated
against our moving mesh hydro simulation and estimate that the ROSAT
all sky survey plus Sloan galaxy photometric redshift survey would allow a
accuracy in the IGM XRB-galaxy cross correlation power spectrum
measurement for and a accuracy in the redshift resolved
X-ray emissivity-galaxy cross correlation power spectrum measurement for
. At small scales, non-gravitational heating, e.g. feedback,
dominates over gravity and leaves unique signatures in the IGM XRB, which
allows a comparable accuracy in the measurement of the amount of
non-gravitational heating and the length scales where non-gravitational energy
balances gravity.Comment: 17 pages, 5 figures. Will appear on ApJ May issu
Star formation history in the solar neighborhood: the link between stars and cosmology
Using a cosmological galactic evolutionary approach to model the Milky Way,
we calculate the star formation history (SFH) of the solar neighborhood. The
good agreement we obtain with the observational inferences suggests that our
physical model describes accurately the long term/large spatial trends of the
local and global Milky Way SFH. In this model, star formation is triggered by
disk gravitational instabilities and self-regulated by an energy balance in the
ISM. The drivers of the SFH are the cosmological gas infall rate and the gas
surface density determined by the primordial spin parameter. A LambdaCDM
cosmology was used throughout.Comment: 8 pages, uses kluwer.cls. Invited talk, to appear in "New Quests in
Stellar Astrophysics: The link between Stars and Cosmology", eds. M. Chavez,
A. Bressan, A. Buzzoni & D. Mayya, Kluwer Academic Publisher
Numerical Analyses of Weakly Nonlinear Velocity-Density Coupling
We study evolution of various statistical quantities of smoothed cosmic
density and velocity fields using N-body simulations. The parameter
characterizes nonlinear coupling of
these two fields and determines behavior of bulk velocity dispersion as a
function of local density contrast.
It is found that this parameter depends strongly on the smoothing scale even
in quasi-linear regimes where the skewness parameter
is nearly constant and close to the predicted value by the second-order
perturbation theory. We also analyze weakly nonlinear effects caused by an
adaptive smoothing known as the gather approach.Comment: 22 pages, 4 figures, to appear in ApJ (558, Sep 10
The environment of active objects in the nearby universe
We study the galaxy environment of active galaxies, radio-loud and
radio-quiet quasars in the redshift range . We use APM
galaxies in order to explore the local galaxy overdensity and the
colour distribution of neighbouring galaxies of these target samples. For
comparison, we perform similar analysis on samples of Abell clusters with X-ray
emission, and samples of Abell clusters with richness R=1 and R=0. The
projected cross-correlations show that the samples of quasars and active
galaxies reside in regions of galaxy density enhancements lower than those
typical of R=0 clusters. We also find that in the nearby universe the local
galaxy overdensity of radio-loud and radio-quiet quasars are comparable. The
analysis of the distribution of galaxy colour indexes suggests that the
environment of quasars is not strongly dominated by a population of red
galaxies, characteristic of rich Abell cluster, an effect that is more clearly
appreciated for our sample of radio-loud quasars.Comment: 5 pages, 3 figures, to appear in MNRA
Origin and evolution of halo bias in linear and non-linear regimes
We present results from a study of bias and its evolution for galaxy-size
halos in a large, high-resolution simulation of a LCDM model. We consider the
evolution of bias estimated using two-point correlation function (b_xi), power
spectrum (b_P), and a direct correlation of smoothed halo and matter
overdensity fields (b_d). We present accurate estimates of the evolution of the
matter power spectrum probed deep into the stable clustering regime
(k~[0.1-200]h/Mpc at z=0). The halo power spectrum evolves much slower than the
power spectrum of matter and has a different shape which indicates that the
bias is time- and scale-dependent. At z=0, the halo power spectrum is
anti-biased with respect to the matter power spectrum at wavenumbers
k~[0.15-30]h/Mpc, and provides an excellent match to the power spectrum of the
APM galaxies at all probed k. In particular, it nicely matches the inflection
observed in the APM power spectrum at k~0.15h/Mpc. We complement the power
spectrum analysis with a direct estimate of bias using smoothed halo and matter
overdensity fields and show that the evolution observed in the simulation in
linear and mildly non-linear regimes can be well described by the analytical
model of Mo & White (1996), if the distinction between formation redshift of
halos and observation epoch is introduced into the model. We present arguments
and evidence that at higher overdensities, the evolution of bias is
significantly affected by dynamical friction and tidal stripping operating on
the satellite halos in high-density regions of clusters and groups; we
attribute the strong anti-bias observed in the halo correlation function and
power spectrum to these effects. (Abridged)Comment: submitted to the Astrophys.Journal; 19 pages, 9 figures LaTeX (uses
emulateapj.sty
The Hierarchical Build-Up of Massive Galaxies and the Intracluster Light since z=1
We use a set of simulation-based models for the dissipationless evolution of
galaxies since z=1 to constrain the fate of accreted satellites embedded in
dark matter subhalos. These models assign stellar mass to dark matter halos at
z=1 by relating the observed galaxy stellar mass function (GSMF) to the
halo+subhalo mass function monotonically. The evolution of the stellar mass
content is then followed using halo merger trees extracted from N-body
simulations. Our models are differentiated only in the fate assigned to
satellite galaxies once subhalos, within which satellites are embedded,
disrupt. These models are confronted with the observed evolution in the massive
end of the GSMF, the z~0 brightest cluster galaxy (BCG)-cluster mass relation,
and the combined BCG and intracluster light (ICL) luminosity distribution --
all observables expected to evolve approximately dissipationlessly since z=1.
The combined observational constraints favor a model in which the vast majority
(>80%) of satellite stars from disrupted subhalos go into the ICL. Conversely,
models that leave behind a significant population of satellite galaxies once
the subhalo has disrupted are strongly disfavored, as are models that put a
significant fraction of satellite stars into the BCG. Our results show that
observations of the ICL provide useful and unique constraints on models of
galaxy merging and the dissipationless evolution of galaxies in groups and
clusters.Comment: 12 pages, 6 figures. Submitted to Ap
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