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4.2 - Simulated XMM images of cosmic filaments
Filaments are a common feature of modern cosmological models and their existence is now a well observed feature of the galaxies mapped in redshift surveys. However, stars represent only a small fraction of the baryonic matter predicted by nucleosynthesis. Where is the rest? Simulations predict that most baryons are in moderately hot gas associated with filaments and groups (Cen and Ostriker 1999). If this is the case, then we have yet to observe a large fraction of the baryons in the universe.
In order to investigate their observability in the X-ray band, we have
performed detailed N-body and hydrodynamic simulations. We selected
two representative cosmological models: one dominated by a
cosmological constant (and favored by current observations), and the
other a flat, = 1 cosmology with a
tilted spectrum of density
perturbations. For both cosmologies, we focused our computational
efforts on a filament about 30 h-1 Mpc in length and
used an adaptive mesh refinement (AMR) method to solve the equations of
hydrodynamics. This new numerical technique (described in Bryan
1999) was used in order to achieve sufficient resolution
(50 h-1 kpc) in high-density regions and good
shock-capturing behaviour in low density regions.
The results (for both CDM and
tilted CDM models) are shown in Fig. 9-10 which demonstrate what
an idealized X-ray image of this region would show. While the extremely
diffuse gas of the filament is very difficult to observe, it's clear that
the filamentary structure itself is well outlined by a chain of gas clumps
with various masses and luminosities. In fact it is in this network of
groups that a large fraction of the filament's baryons reside, and
this work shows that a detection of that component is feasible. In
Fig. 11-12 we include all instrumental effects and the background
distribution of extragalactic objects (QSO). The groups embedded in
the filament are visible. While it is more difficult to delineate the
cluster outskirts, a cross-correlation with the optical survey, the
weak-lensing results and the Sunyaev-Zel'dovich map will substantially
enhance the strength of the signal. In this way, we may at last
complete our inventory of the baryonic matter content of the universe.
4.3 Towards new physics in the XMM group sample