Richard Schaeffer

The phase-diagram of cosmological baryons

We investigate the behaviour of cosmological baryons at low redshifts

The fundamental plane of galaxy clusters

z la 5

Nonlinear matter clustering properties of a cold dark matter universe

after reionization, through analytic means. In particular, we study the density-temperature phase-diagram that describes the history of the gas. We show how the location of the matter in this

Collision-induced Galaxy Formation

(rho,T)

Galaxies - fractal dimensions, counts in cells, and correlations

diagram expresses the various constraints implied by usual hierarchical scenarios. This yields robust model-independent results that agree with numerical simulations. The IGM is seen to be formed via two phases: a “cool” photo-ionized component and a “warm” component governed by shock-heating. We also briefly describe how the remainder of the matter is distributed over galaxies, groups and clusters. We recover the fraction of matter and the spatial clustering computed by numerical simulations. We also check that the soft X-ray background due to the “warm” IGM component is consistent with observations. We find in the present universe a baryon fraction of 7% in hot gas, 24% in the warm IGM, 38% in the cool IGM, 9% within star-like objects and, as a still un-observed component, 22% of dark baryons associated with collapsed structures, with a relative uncertainty no larger than 30% on these numbers.

The redshift evolution of bias and baryonic matter distribution

In the three—dimensional space defined by the logarithms of central velocity dispersion σ, effective radius R e and mean effective surface brightness I e, elliptical galaxies are confRned in a narrow plane (Dressler et al. 1987; Djorgovski & Davis 1987). Here we discuss the observational evidence for the existence of an analogous relation for galaxy clusters (Schaeffer et al., 1993).

Hot gas in evolving galaxy clusters and groups - Implications for the microwave and X-ray backgrounds

The matter distribution of strongly nonlinear scales in a CDM model is studied. To that effect, the count probabilities are determined in an unbiased catalog generated with a P 3 M simulation code and the results are compared with the predictions of the scale-invariant model (Balian and Schaeffer, 1988, ApJ, 335) and those of the thermodynamical model (Saslaw and Hamilton, 1984, ApJ, 276). It is found that the probability of finding a given amount of matter, i.e., N particules in a specified volume, is a power law with a lower and an upper cutoff. These cuts may be described for all volume sizes by two universal functions that are determined, in the proper limit of continuous distribution

The phase diagram of the intergalactic medium and the entropy floor of groups and clusters: are clusters born warm?

We present a semianalytical model in which galaxy collisions and strong tidal interactions, both in the field and during the collapse phase of groups and clusters, help determine galaxy morphology. From a semianalytical analysis based on simulation results of tidal collisions (Aguilar & White), we propose simple rules for energy exchanges during collisions that allow one to discriminate between different Hubble types: efficient collisions result in the disruption of disks and substantial star formation, leading to the formation of elliptical galaxies; inefficient collisions allow a large gas reservoir to survive and form disks. Assuming that galaxy formation proceeds in an Ω0 = 1 cold dark matter universe, the model both reproduces a number of observations and makes predictions, among which are the redshifts of formation of the different Hubble types in the field. When the model is normalized to the present-day abundance of X-ray clusters, the amount of energy exchange needed to produce elliptical galaxies in the field implies that they formed at z 2.5 while spiral galaxies formed at z 1.5. The model also offers a natural explanation for biasing between the various morphological types. We find that the present-day morphology-density relation in the field is well reproduced under the collision hypothesis. Finally, predictions of the evolution of the various galaxy populations with redshift are made, in the field as well as in clusters.

Cosmology and large scale structure

The luminosity function for clusters of galaxies is determined theoretically, assuming that the many-body galaxy correlation functions are scale-invariant. It is shown that the luminosity function is a power law with an upper and lower cutoff, and that the fractal dimensions for cluster-rich and empty regions of the resulting galaxy distribution are different. The specific values of these fractal dimensions are not in agreement with those determined by Jones et al. (1988). 24 references.

Cold, warm, or hot dark matter - Biased galaxy formation and pancakes

We study the distribution of baryonic and luminous matter within the framework of a hierarchical scenario. Using an analytical model for structure formation which has already been checked against observa- tions for galaxies, Lyman- clouds, clusters and reionization processes, we present its predictions for the bias of these objects. We describe its dependence on the luminosity (for galaxies or quasars) or the column density (for Lyman- absorbers) of the considered objects. We also study its redshift evolution, which can exhibit an intricate behaviour. These astrophysical objects do not trace the dark matter density eld, the Lyman- forest clouds being undercorrelated and the bright galaxies overcorrelated, while the intermediate class of Lyman-limit systems is seen to sample the matter eld quite well. We also present the distribution of baryonic matter over these various objects. We show that light does not trace baryonic mass, since bright galaxies which contain most of the stars only form a small fraction of the mass associated with virialized and cooled halos. We consider two cosmologies: a critical density universe and an open universe. In both cases, our results agree with observations and show that hierarchical scenarios provide a good model for structure formation and can describe a wide range of objects which spans at least the seven orders of magnitude in mass for which data exist. More detailed observations, in particular of the clustering evolution of galaxies, will constrain the astrophysical models involved.