Arif Babul

THE FORMATION AND EVOLUTION OF

GALAXY GROUPS AND CLUSTERS

...if matter was evenly disposed throughout an infinite space, it could never convene into one mass; but some of it would convene into one mass and some into another, so as to make an infinite number of great masses scattered at great distances from one to another...

-Isaac Newton in a letter to Bishop Bentley 1692

The nearly perfect isotropy of the cosmic microwave background (CMB) tells us that the early universe was, to a high degree, smooth and homogeneous. Explaining the transition from these smooth beginnings to today's highly structured universe is one of the Grand Challenges of modern theoretical astrophysics. Of all the pieces of this cosmic puzzle, perhaps none is as intriguing as the formation and evolution of galaxy groups and clusters, among the most recently formed and the most massive gravitationally bound objects in the Universe. As reservoirs of more than a third of the warm-hot diffuse gas and more than 90% of the galaxies in the local Universe, understanding the physical processes that occur in group and cluster environments, and govern their evolution, is key to gaining crucial insights into the evolution of cosmic baryons and galaxies, at least over the last 10 billion years.

Although the focus of intense study for four decades, the observational study of galaxy groups and clusters has been profoundly revolutionized by the recent commissioning of powerful ground-based optical and millimetre (Sunyaev-Zel'dovich Effect) as well as space-based optical and X-ray telescopes. The resulting unprecedented resolving power and wavelength coverage are opening new windows into the history of interactions between the various components that constitute these systems. Contrary to the prevailing views of only a decade ago, groups and clusters are proving to be extremely fascinating systems, exhibiting a variety of rich and complex phenomena that bear witness to intricate web of gravitational, radiative and hydrodynamic processes that underlie their formation and evolution. Clearly, the study of galaxy groups and clusters has entered an exciting era of discovery, and with these dramatic advances comes a growing sense of urgency to address questions that only a few years ago seemed confined to the territory of speculation:

• What is the relative distribution of dark matter and baryons in these systems? Does the balance vary with the total mass of the system? How does the baryonic fraction compare to the estimates of the universal baryon fractions derived from studies of the cosmic microwave background anisotropy?

• What fraction of the baryons in these systems is cold? When did this component emerge, before or after the formation of the group or cluster?

• What is the origin of the thermal, chemical and spatial structure characterizing the hot diffuse gas permeating the groups and clusters? What role have stars and active galactic nuclei played in shaping these characteristics?

• What is the impact of gas-galaxy interactions on the evolution of each of the two components? Do galaxies interact more frequently in groups and clusters? What is the nature and the rate of these interactions?

And yet, a self-consistent, physically intuitive theoretical framework, within the context of which the observations can be understood, remains sketchy at best. Many of the well-established trends indicated by the data cannot be easily reconciled with the predictions of the canonical model for the formation and evolution of galaxy groups and clusters.

Among the earliest indications of the failure of the canonical model were presented in my paper: Miralda-Escude, J., Babul, A., “Gravitational Lensing in Clusters of Galaxies: New Clues Regarding the Dynamics of Intracluster Gas”, 1995, ApJ, 449, 18. This work reported on an innovative approach to combine gravitational lensing and X-ray observations in order to derive the relative distributions of dark matter and hot, X-ray emitting gas in these systems. This initial groundbreaking study was subsequently followed by a series of publications describing the detailed analyses of the optical, X-ray and lensing properties of three specific clusters. Our combined results suggested that the hot diffuse X-ray emitting intracluster gas had greater than expected thermal energy content. These results challenged conventional wisdom and although initially controversial, they have since been independently confirmed by a number of groups.

Intrigued by the above results, and spurred on by a growing number of other, some seemingly unrelated, discrepancies between theory and observations, I have embarked upon a concerted, integrated, long-term program to develop a self-consistent model for the formation and evolution of galaxy groups and clusters that correctly captures all the key physical processes governing the evolution of the dark matter, the hot diffuse gas, and the galaxy components. The focus of the program is on establishing clear, physically motivated, intuitive understanding of these processes. The specific objectives of the program are:

• To gauge, through a close interplay between theory and observations, the impact of galaxy-galaxy and galaxy-gas interactions on the evolution of the hot diffuse gas and the galaxy components, respectively.

• To gain insights into the role and importance of stellar and active galactic nuclei processes on the observed global and structural properties of these systems.

• To identify signatures of ongoing formation of groups and clusters and determine how and where the energy released during the accretion is deposited. (In currently favoured “hierarchical” models for structure formation, systems, such as groups and clusters of galaxies, are built-up through a sequence of gravitationally driven accretion and mergers of smaller ``building blocks''.)

• To establish the relationship between the observable but minor baryonic component and the more ``fundamental properties'' of the systems, such as the total mass. Such an understanding is a prerequisite if the groups and clusters are to be used as precision cosmological probes that can compete with and complement results derived from the studies of the cosmic microwave background anisotropy and SNe Ia distance measurements.

This program is unique in that it is being pursued through a series of closely-linked investigations taking full advantage of three key approaches: analytic modeling, semi-analytic studies involving Monte Carlo descriptions of the stochastic nature of hierarchical clustering, and detailed numerical simulations that capitalize on the latest advances in massively parallel computing software and hardware to generate realistic ultra-high resolution numerical simulations of groups and clusters of galaxies that incorporate the key physical processes.

The first phase of this investigation has focused on studying the evolution of the hot diffuse X-ray emitting gas using analytic methods. My research team (consisting primarily of present and former graduate students) and I have carried an extensive investigation of the physics of the intracluster gas, the outcome of which is the development of an intuitive physically-insightful model for the evolution of the groups and clusters. The model is described in a series of two papers: Balogh, M.L., Babul, A., Patton. D.R., “Pre-heated isentropic gas in groups of galaxies”, 1999, MNRAS, 307, 463, and Babul, A., Balogh, M.L., Lewis, G.F., Poole, G.B., “Physical Implications of the X-ray Properties of Galaxy Groups and Clusters”, 2002, MNRAS, 330, 329. These papers argue that the impact of energetic phenomena, such as winds and outflows generated by supernovae and active galactic nuclei occurring during the epoch of galaxy formation, on the thermal structure of the hot diffuse gas is not negligible as previously thought. Using the resulting model, we were the first to show that all of the observed global optical/X-ray, X-ray/X-ray and more recently, X-ray/Sunyaev-Zel’dovich Effect, correlations can be accounted for. The two papers have received numerous citations. Based on citations, both are ranked in the top 10% of papers published during 1999 and 2002, respective. The latter is, in fact, ranked in the top 1%.

The adoption of an analytic approach at the onset was deliberate. Having analyzed cluster data for many years now, I was convinced that the deficiency of the canonical model lay in “missing physics” and particularly in processes that are not easily incorporated in numerical simulations. The key question was: Which process or processes? The great strength of analytic modeling is that it can be used to investigate a broad range of physical processes, rapidly, inexpensively and in a manner that lends itself to yielding deeper insights into the problem.

Presently, we are refining our analytic model to study the combined impact of heating (as discussed above) and cooling. We know that the latter is taking place because we observe X-ray emissions from the hot diffuse gas. The early results are surprisingly promising. The model not only able to explain the global trends, as already described, but also suggests an explanation for the observed scatter in these relations. To date, this scatter has been attributed to measurement errors; however, we are preparing to make the claim that the scatter has a physical basis. This has important implications for studies seeking to use galaxy clusters as precision cosmological probes.

In parallel with the above investigations, my collaborators and I have been actively working towards updating and testing a new state-of-the-art numerical simulation code designed specifically to study highly non-linear systems, such as groups and clusters, and optimized to run efficiently on the massively parallel computing platforms available to us here at the University of Victoria. As useful as the analytic model is and has been, there is no question that groups and clusters are highly complex systems, and that the evolution of each of the constituent components of these systems is governed by a set of highly non-linear processes, all of which are strongly coupled each other. A complete description of the full range of these processes is beyond the scope of analytic or semi-analytic modeling. As we further expand our investigations to study the joint evolution of the hot diffuse gas and the cluster galaxies, ultra-high resolution simulation studies offer the most promising avenue for making further progress.