Arif Babul |
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Summary of Major Research
Accomplishments |
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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: |
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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: |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |