Cosmology with Clusters
Dark Matter
All dark energy measurements require an accurate knowledge of the dark matter distribution. The use of the baryonic mass fraction to derive the total mass of dark matter only works in the most massive systems where the "interesting physics" going on within clusters does not dominate the X-ray emission. Constraining dark matter in lower-mass systems and decoupling it from the other physical processes going on in clusters requires high-spectral-resolution imaging detectors of these spatially extended systems.
Most of the baryons in groups and clusters of galaxies lie in the hot X-ray emitting gas ~ 2-100 × 106 K (velocity dispersions of 180-1200 km s-1, which is in virial equilibrium with the dark matter potential well and the ratio of gas to stellar mass is 2-10, Ettori and Fabian 1999). This gas is enriched in heavy elements (Mushotzky et al. 1978) and is thus the reservoir of stellar evolution in these systems.
Since most of the baryons are in the gaseous phase and clusters are dark matter dominated, the detailed physics of cooling and star formation are less important than in galaxies, which makes clusters much more amenable to simulations. Detailed measurements of their density and temperature profiles (which requires high-spectral resolution) allow an accurate determination of the dark matter profile and total mass. While gravity is clearly dominant in massive systems, much of the entropy of the gas in low-mass systems may be produced by non-gravitational processes.
As discussed in detail by Evrard (2004), we now have a good understanding of the formation of the dark matter structure for clusters of galaxies. If gravity has completely controlled the formation of structure, the gas should be in hydrostatic equilibrium with the vast majority of the pressure due to gas pressure. If correct, the density and temperature structure provide a detailed measurement of the dark matter distribution in the cluster. Recent theoretical work has also taken into account other important process such as cooling and turbulence. The fundamental form of the Navarro, Frenk, and White (1997, hereafter NFW) dark matter potential results in a prediction, both from analytic (Komatsu and Seljak 2001) and numerical modeling (Loken et al. 2002), that the cluster gas should have a declining temperature profile at a sufficiently large distance from the center (in units of R/Rvirial). The size of the temperature drop in the outer regions is predicted to be roughly a factor of 2 by R/Rvirial ~0.5. This result is consistent with the ASCA results of Markevitch (1998); however, there is considerable controversy about this analysis. While XMM-Newton should resolve this controversy, its high and variable background has so far precluded definititive results. Constellation-X will need to have an overall background significantly lower than previous X-ray missions in order to constrain dark matter profiles of clusters at the important length scales.
Recent numerical work seems to validate the NFW potential, and much has been made of the fact that low-mass and low-surface brightness galaxies do not seem to follow this form in their central regions. Recent Chandra and XMM-Newton observations (Allen et al. 2002; Arabadjis et al. 2002; Pratt and Arnaud 2002) have been able to determine extremely accurate mass profiles via spatially resolved X-ray spectroscopy and the assumption of hydrostatic equilibrium. Perhaps the best documented of these examples are the Chandra data for Abell 2029 (Lewis, Buote, and Stocke 2003), in which the profile is determined over a factor of 100 in length scale with essentially no deviation from the NFW prediction. This striking result is also seen in other Chandra results in the cores of clusters. The data show that the central regions of clusters tend to have rather steep density profiles in the innermost radii, indicating that whatever causes the deviation of the form of the potential in dwarf galaxies does not occur in clusters. This result strongly constrains interacting dark matter models (e.g. Voit and Bryan 2001, Bautz and Arabadjis 2004).
References
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