1,208 research outputs found
Inter-cluster filaments in a CDM Universe
The large--scale structure (LSS) in the Universe comprises a complicated
filamentary network of matter. We study this network using a high--resolution
simulation of structure formation of a Cold Dark Matter cosmology. We
investigate the distribution of matter between neighbouring large haloes whose
masses are comparable to massive clusters of galaxies. We identify a total of
228 filaments between neighbouring clusters. Roughly half of the filaments are
either warped or lie off the cluster--cluster axis. We find that straight
filaments on the average are shorter than warped ones. More massive clusters
are connected to more filaments than less massive ones on average. This finding
indicates that the most massive clusters form at the intersections of the
filamentary backbone of LSS. For straight filaments, we compute mass profiles.
Radial profiles show a fairly well--defined radius, , beyond which the
profiles follow an power law fairly closely. For the majority of
filaments, lies between 1.5 Mpc and 2.0 Mpc. The
enclosed overdensity inside varies between a few times up to 25 times
mean density, independent of the length of the filaments. Along the filaments'
axes, material is not distributed uniformly. Towards the clusters, the density
rises, indicating the presence of the cluster infall regions. In addition, we
also find some sheet--like connections between clusters. In roughly a fifth of
all cluster--cluster connections where we could not identify a filament or
sheet, projection effects lead to filamentary structures in the projected mass
distribution. (abridged)Comment: 10 pages, 18 figures; submitted to MNRAS; updated: final version,
accepted for publicatio
Clustering of Galaxies in a Hierarchical Universe: III. Mock Redshift Surveys
This is the third paper in a series which combines N-body simulations and
semi-analytic modelling to provide a fully spatially resolved simulation of the
galaxy formation and clustering processes. Here we extract mock redshift
surveys from our simulations: a Cold Dark Matter model with either Omega_0=1
(tauCDM) or Omega_0=0.3 and Lambda=0.7 (LambdaCDM). We compare the mock
catalogues with the northern region (CfA2N) of the Center for Astrophysics
(CfA) Redshift Surveys. We study the properties of galaxy groups and clusters
identified using standard observational techniques and we study the relation of
these groups to real virialised systems. Most features of CfA2N groups are
reproduced quite well by both models with no obvious dependence on Omega_0.
Redshift space correlations and pairwise velocities are also similar in the two
cosmologies. The luminosity functions predicted by our galaxy formation models
depend sensitively on the treatment of star formation and feedback. For the
particular choices of Paper I they agree poorly with the CfA survey. To isolate
the effect of this discrepancy on our mock redshift surveys, we modify galaxy
luminosities in our simulations to reproduce the CfA luminosity function
exactly. This adjustment improves agreement with the observed abundance of
groups, which depends primarily on the galaxy luminosity density, but other
statistics, connected more closely with the underlying mass distribution,
remain unaffected. Regardless of the luminosity function adopted, modest
differences with observation remain. These can be attributed to the presence of
the ``Great Wall'' in the CfA2N. It is unclear whether the greater coherence of
the real structure is a result of cosmic variance, given the relatively small
region studied, or reflects a physical deficiency of the models.Comment: 47 pages, LaTex, 17 figures, MNRAS, in press; one figure adde
The Effects of Prescribed Fire on Ant-Mediated Seed Dispersal in Missouri
Many aspects of animal-mediated seed dispersal are vulnerable to disturbance, including partner identity and dispersal quantity and quality. This dissertation explores ant-mediated seed dispersal of the herb Sanguinaria canadensis in Missouri Ozark oak forests, where prescribed fire is a common land management tool. In Chapter 1, I test the definition of a keystone seed disperser using absolute and relative contributions of different ant species to the quantity and distance of seed dispersal, based on field observations of S. canadensis seed dispersal. I demonstrate that the ant Aphaenogaster rudis is better described as a numerically dominant rather than keystone seed disperser, while other species in the genera Formica and Camponotus provide higher quality dispersal in the form of longer dispersal distances. In Chapter 2, I explore the effects of prescribed burning and physical litter removal on the number of seeds dispersed, dispersal distances, and disperser identity, using field observations of S. canadensis seed dispersal in burned, litter removal, and control plots. The overall seed-dispersing ant community and different species therein responded differently to burning and litter removal between two field seasons. Burning increased dispersal distance in 2020, but reduced dispersal distance in 2021. Finally, in Chapter 3, I consider the effects of burning and litter removal on ground-foraging and litter-dwelling ant communities, using three rounds of pitfall traps in 2020 (April, July, and September), and one round of pitfall and Berlese litter extraction in 2021. In 2020, overall ant abundance was higher in burn compared to control plots in September, but diversity peaked in July and was negatively affected by burning when weighting species by their relative abundances. Although both burning and litter removal reduced leaf litter cover in 2021, litter removal increased ground-foraging ant abundance but decreased litter-dwelling ant abundance on an area basis, and only affected ant diversity when considering ant abundance in 2020 pitfall traps and 2021 Berlese samples. The results of this dissertation suggest Missouri Ozark ant communities and their seed dispersal services are either positively affected by or relatively resilient to prescribed burning
Quantifying Cosmic Superstructures
The Large Scale Structure (LSS) found in galaxy redshift surveys and in
computer simulations of cosmic structure formation shows a very complex network
of galaxy clusters, filaments, and sheets around large voids. Here, we
introduce a new algorithm, based on a Minimal Spanning Tree, to find basic
structural elements of this network and their properties. We demonstrate how
the algorithm works using simple test cases and then apply it to haloes from
the Millennium Run simulation (Springel et al. 2005). We show that about 70% of
the total halo mass is contained in a structure composed of more than 74,000
individual elements, the vast majority of which are filamentary, with lengths
of up to 15 Mpc/h preferred. Spatially more extended structures do exist, as do
examples of what appear to be sheet-like configurations of matter. What is
more, LSS appears to be composed of a fixed set of basic building blocks. The
LSS formed by mass selected subsamples of haloes shows a clear correlation
between the threshold mass and the mean extent of major branches, with
cluster-size haloes forming structures whose branches can extend to almost 200
Mpc/h - the backbone of LSS to which smaller branches consisting of smaller
haloes are attached.Comment: accepted for publication in Monthly Notices of the Royal Astronomical
Society; 13 pages, with 14 figures and 3 table
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
On the importance of high redshift intergalactic voids
We investigate the properties of one--dimensional flux ``voids'' (connected
regions in the flux distribution above the mean flux level) by comparing
hydrodynamical simulations of large cosmological volumes with a set of observed
high--resolution spectra at z ~ 2. After addressing the effects of box size and
resolution, we study how the void distribution changes when the most
significant cosmological and astrophysical parameters are varied. We find that
the void distribution in the flux is in excellent agreement with predictions of
the standard LCDM cosmology, which also fits other flux statistics remarkably
well. We then model the relation between flux voids and the corresponding
one--dimensional gas density field along the line--of--sight and make a
preliminary attempt to connect the one--dimensional properties of the gas
density field to the three--dimensional dark matter distribution at the same
redshift. This provides a framework that allows statistical interpretations of
the void population at high redshift using observed quasar spectra, and
eventually it will enable linking the void properties of the high--redshift
universe with those at lower redshifts, which are better known.Comment: Accepted for publication in MNRAS. Minor revisions. 11 pages and 10
figures. High resolution version available at
http://adlibitum.oat.ts.astro.it/viel/VOIDS/voids.p
The Influence of Large-Scale Structure on Halo Shapes and Alignments
Alignments of galaxy clusters (the Binggeli effect), as well as of galaxies
themselves have long been studied both observationally and theoretically. Here
we test the influence of large-scales structures and tidal fields on the shapes
and alignments of cluster-size and galaxy-size dark matter halos. We use a
high-resolution N-body simulation of a CDM universe, together with the
results of Colberg et al. (2005), who identified filaments connecting pairs of
clusters. We find that cluster pairs connected by a filament are strongly
aligned with the cluster-cluster axis, whereas unconnected ones are not. For
smaller, galaxy-size halos, there also is an alignment signal, but its strength
is independent of whether the halo is part of an obvious large-scale structure.
Additionally, we find no measureable dependence of galaxy halo shape on
membership of a filament. We also quantify the influence of tidal fields and
find that these do correlate strongly with alignments of halos. The alignments
of most halos are thus caused by tidal fields, with cluster-size halos being
strongly aligned through the added mechanism of infall of matter from
filaments.Comment: 8 pages, 6 figures, accepted for publication in MNRA
Peculiar Velocities of Galaxy Clusters
We investigate the peculiar velocities predicted for galaxy clusters by
theories in the cold dark matter family. A widely used hypothesis identifies
rich clusters with high peaks of a suitably smoothed version of the linear
density fluctuation field. Their peculiar velocities are then obtained by
extrapolating the similarly smoothed linear peculiar velocities at the
positions of these peaks. We test these ideas using large high resolution
N-body simulations carried out within the Virgo supercomputing consortium. We
find that at early times the barycentre of the material which ends up in a rich
cluster is generally very close to a high peak of the initial density field.
Furthermore the mean peculiar velocity of this material agrees well with the
linear value at the peak. The late-time growth of peculiar velocities is,
however, systematically underestimated by linear theory. At the time clusters
are identified we find their rms peculiar velocity to be about 40% larger than
predicted. Nonlinear effects are particularly important in superclusters. These
systematics must be borne in mind when using cluster peculiar velocities to
estimate the parameter combination .Comment: 8 pages, 4 figures; submitted to MNRA
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