26 June 2000

Arif Babul

Summary of Major Research Accomplishments

Over the course of my career, I have worked on a wide variety of topics ranging from flow of matter onto black holes and exotic early-universe features such as “superconducting cosmic strings”, to the distorting effects caused by the bending of light beams by gravity as predicted by Einstein’s theory of General Relativity.My most significant contributions, however, have been in the area of the Formation and Evolution of Galaxies and Galaxy Systems. Looking back over the years, the most memorable of these, the ones that I would classify as my “most significant” contributions are those that were the most fun to work on, that challenged my creativity, that often involved drawing together seemingly disparate physical ideas, and resulted in models or approaches that have contributed, even if only in a small way, to a better intuitive, physical understanding of the processes underlying the emergence of observed structure in the Universe. Of the works that fall into this category, the one stands out is:

Babul, A., Rees, M.J., “On Dwarf Ellipticals and the Faint Blue Counts”, 1992, MNRAS, 255, 346.

Approximately a decade ago, astronomers discovered a puzzling class of galaxies known as “faint blue galaxies”. Neither the large numbers, the spatial distribution nor the implied evolutionary behaviour of these galaxies could be easily understood within the generally accepted theory for the formation and evolution of galaxies. It was in this exciting milieu that the above paper was published. The paper represented a creative synthesis of the key features of the generally accepted hierarchical clustering model for the formation of structures in the Universe along with key physical ideas outlined in seminal papers such as Dekel and Silk (1986) and Rees (1986). The essence of the paper was the recognition that low-mass dwarf galaxies are extremely fragile systems whose formation and evolution depends sensitively on the internal feedback processes as well as the prevailing environmental conditions in the Universe 7 to 13 billion years ago. To date, this paper has received nearly 240 citations, nearly 10 of which are papers published this year (2002). The recent surge in citations (nearly 70 in the past 3 years) is particularly satisfying. Although originally aimed at explaining the dramatic evolution in the population of very faint galaxies, the physical ideas outlined in the paper are being used to explain a variety of phenomena ranging from discovery of gas-rich High Velocity Clouds to the paucity of dwarf satellites around the Milky Way. A follow-up paper:

Babul, A., Ferguson, H.C., “Faint Blue Galaxies and the Epoch of Dwarf-Galaxy Formation”, 1996, ApJ, 458, 100.

in which Harry Ferguson and I carried out a detailed study of the “Babul-Rees” model, including comparisons between model predictions and existing data, influenced several large-scale observational programs, the most notable of which is the first Hubble Deep Field (HDF) program. A recent analysis[1] of astronomy and astrophysics papers published in refereed journals worldwide over the period January 1981 to December 1998 ranked one of this paper as one of the top 10 high-impact papers published in 1996. Only four other Canadian astronomers/astrophysicists had publications listed amongst the annual top 10 over the period 1981-1998.

Another paper that also ranks among my most important contributions is:

Miralda-Escude, J., Babul, A., “Gravitational Lensing in Clusters of Galaxies: New Clues Regarding the Dynamics of Intracluster Gas”, 1995, ApJ, 449, 18.

This paper has received nearly 100 citations since publication. I was working on a project involving gravitational lensing at the time. Inspired by discussions with Alastair Edge and members of the X-ray group at the Institute of Astronomy at the University of Cambridge, I struck upon the idea of using the combination of X-ray and gravitational lensing observations to probe the thermal and dynamical properties of the intracluster medium in massive clusters. To my knowledge, the above paper was the first to outline such an approach. In addition, the paper also presented actual results of the first such “joint analyses” exercise. This was made possible through collaboration with Jordi Miralda-Escude, who had access to proprietary gravitational lensing data for the clusters. Over time, the approach advocated in the paper has come to be recognized as a powerful method for studying the internal dynamics of massive clusters. One direct result was my being invited to join the team of Nick Kaiser, Gordon Squires and collaborators to extend their on-going weak lensing study of clusters with a joint analyses combining strong lensing, measure of galaxy velocity dispersion and distribution, and X-rays with their weak lensing results.

The results of the joint analyses described above challenged many of the accepted ideas (at the time) regarding the prevailing conditions in galaxy clusters. More generally, the approach has resulted in a wealth of new information about the relative distributions of galaxies, hot X-ray emitting gas, and dark matter in cluster environments. These results provided the impetus from my recent work on modeling the distribution of the hot, diffuse gas in groups and clusters, including the “entropy floor” model. The latter in particular not only resolves the discrepancies highlighted by the joint analyses but also provides an explanation for the various correlations between the X-ray properties of both galaxy groups and clusters that were unaccounted for within the framework of the standard “shock-heating via gravitational infall” model. The model has the virtue of being analytic and hence, is both physically insightful and allows us to compute various observable properties (and track their evolution) with relative ease. The first paper in the series (Balogh, M.L., Babul, A., Patton, D.R., 1999, MNRAS, 307, 463) has been extensively cited.

Babul, A., Balogh, M.L., Lewis, G.F., Poole, G.B., “Physical Implications of the X-ray Properties of Galaxy Groups and Clusters”, MNRAS, 2002, 330,329.

The full details of the “entropy model” are presented in the above paper. This paper explores the various X-ray and optical correlations predicted by the model, and demonstrates the excellent agreement between observational results and our model predictions.

McCarthy, I.G., Holder, G.P., Babul, A., Balogh, M.L., “The SZ Effect Signature of an Entropy Floor in Distant, Massive, Galaxy Clusters”, 2003, ApJ. 591, 526.

Most recently, a graduate student and I have explored the use of the Sunyaev-Zeldovich Effect, the scattering of cosmic microwave background radiation by the hot gas in the clusters, to “measure” the entropy floor in massive clusters. This works is summarized in a set of two papers. The second of the two papers (listed above): breaks new ground and represents the first time that SZ data is used to “measure” the level of the entropy floor in cluster centers.

Of course, engaging in exciting research would be extremely difficulty without suitable infrastructure in place. To this end, I have invested considerable time and energy towards reinvigorating the theoretical astrophysics program here at the University of Victoria and contributing to the further strengthening of the program on the national stage. As an example, I championed an effort to establish the Canadian Computational Cosmology Collaboration (C4), of which I am the present (and founding) director. C4 is a unique venture that aims to draw together the outstanding but geographically isolated Canadian computational cosmologists and develop a cohesive, national, world-class research program focusing on the formation and evolution of galaxies, one of the grand challenges of the physical sciences. It represents a unique opportunity to jump-start a vibrant, internationally competitive, Canadian-based research program in this exciting and rapidly developing area of research. The establishment of C4, which I view as one of my significant research achievements, required my direct involvement (either as PI or as a significant co-I) in securing grants in excess of $4.4 Million, a large fraction of which has been directed towards establishing a high performance computing facility equipped with one of the most powerful supercomputers in Canada and assembling small team of highly qualified postdoctoral fellows with expertise in computational cosmology.