FIG. 3. Correlation of the x-ray bolometric luminosity and x-ray temperature. The very strong correlation, of the form kT˜L0.3, does not evolve with redshift but has an intrinsic dispersion of ˜3 in luminosity at a fixed temperature.

larger number of studied objects (18, 45), indicate that many of the groups are relaxed and that the dark matter distribution for many of the groups is extended over R>200 kpc. This is confirmed by the dwarf galaxy distribution in the Zadludoff and Mulchaey sample (41).

X-ray luminous groups are very common (˜10-2 h5/Mpc3 at L(x)˜1042) in a high surface brightness limited survey (4). Using the scaling relations from numerical work (7) and the independent confirmation of this law from the x-ray data itself, the typical mass inside ˜200 kpc is ˜2×1013 M0h50-2. Using the Burns et al. luminosity function (4), a conversion from luminosity to temperature using the L(x) vs. kT relation gives a trivial estimate of the minimum mass in these systems relative to the closure density of Ogroups˜0.06 Ah2 where A is the correction for mass inside 200 kpc to the total mass (A˜5). The preliminary all sky survey results, upon which the Burns et al. paper is based, certainly miss many massive systems (45), and thus these limits on the mass density of the universe contributed by x-ray emitting groups are conservative.

The mean baryonic fraction in groups is not yet accurately known, but for many of them it is ˜10%, with a wide range of M(gas)/M(star). Thus the x-ray luminous groups may contain most of the visible mass in the universe.


The lack of evolution in cluster properties (abundance, luminosity function, L vs. T) combined with the type II SN origin of the metals argues for the very strong influence of nongravitational processes in structure formation and/or a low value of O. The high baryon fraction in clusters alone is the strongest argument for a low O. The “low” M/L for clusters (˜150) and the observed number density of clusters also indicate a low O. However, the evidence for the importance of heat in the early universe indicates that most simulations have not included important physics for cluster and galaxy formation.

The quality of the x-ray data will improve markedly in the next few years with the next generation of instruments on AXAF, XMM, and ASTRO-E. I anticipate that the next National Academy of Sciences meeting on this subject will be equally exciting.

I thank my collaborators at Goddard Space Flight Center and on the ASCA team, especially Michael Loewenstein, Caleb Scharf, Keith Arnaud, John Mulchaey, Dave Davis, Yasuo Tanaka, and Una Hwang, whose hard work have contributed to the results presented in this paper.

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