7 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
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