XMM-Newton XMM-LSS  

Index - ... - Simulations (1/3) - Simulations (2/3) - Simulations (3/3) - ...

4. Simulations (2/3)

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.

tcdm_z0p5_10ks.ps.gz (gzipped Postscript, 22469 bytes)

Figure 9: XMM simulated image of a cosmic filament seen at z = 0.5 in a tilted CDM model. This corresponds to a simulation box of 18 x 23 x 33 h-3 Mpc3. The box has been extracted from a larger simulation and encompasses a filament located between two large clusters (outside the box). Many groups are visible within the filamentary structures. From the simulated density and temperature cubes and using reshifted thermal spectra convolved by the XMM response, we have computed the number of photons received by XMM for a 10ks exposure from each simulation cell (100 h-1 kpc, which is about 18" at z = 0.5) and the integrated photon flux along the line of sight. The entire image has an apparent size of 1.2 x 1.7 sq.dg which is much larger than the XMM field of view (30'). This is what we should see ideally, when the XMM pointings have been tiled and the fore/background pointlike sources and noise have been removed. The image is computed in the [0.4-4] keV band.

lcdmrp_z0p5_10ksb.ps.gz (gzipped Postscript, 21794 bytes)

Figure 10: Same as above, but for a CDM model.

lcdmrp_z0p5_10ks_4as_qso.ps.gz (gzipped Postscript, 77073 bytes)

Figure 11: A detailed simulation of a single XMM survey pointing (30' diameter) centered on the area encompassing the squares on the previous CDM picture (Fig. 10). All presently known XMM characteristics have been taken into account: spectral response, PSF, vignetting, particle and diffuse background. The logN-LogS law for extragalactic point sources (QSO) has also been included. The pixel size is 4". This is the photon image for a 10 ks exposure computed in the [0.4-4]keV band.

lcdmrp_z0p5_10ks_4as_qsof3.ps.gz (gzipped Postscript, 41046 bytes)

Figure 12: Same image as above, but filtered by a multi-resolution wavelet algorithm combined with a model for Poisson photon statistics (Starck & Pierre 1998). The significance of all objects is > 3 , and noise is removed at this level. Such techniques will be used for the detection of faint extended sources in the survey. The three filament groups are clearly visible as extended sources. They have masses ranging from 7 x 1013 to 2 x 1014 M.

4.3 Towards new physics in the XMM group sample

Index - ... - Simulations (1/3) - Simulations (2/3) - Simulations (3/3) - ...
Web Pages : Alain Detal, Oct 2001.