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Richard S. Ellis

Visiting Associate in Astronomy
Contact information for Richard S. Ellis
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B.Sc., University College (London), 1971; D. Phil., University of Oxford, 1974; D.Sc.h.c., University of Durham. Visiting Associate, Caltech, 1997-98; 2015-2016; 2017-2023. Professor, 1999-2002; Steele Professor, 2002-2015. Deputy Director, Palomar Observatory, 1999-2000; Director, 2000-02; Director, Caltech Optical Observatories, 2002-05.
Research Areas: Astronomy

My main interests are galaxy formation and evolution, observational cosmology and large-scale structure. I enjoy working with graduate students and have supervised 27 to date; all but 2 are still active in academic research, 9 are full professors.

In cosmology, I am trying to understand the dispersion and evolutionary trends I found in distant Type Ia supernovae following a 4-year Keck campaign. This is critical if we want to use supernovae to track dark energy. I am now undertaking a nearby supernova survey using Hubble to find out whether the composition of the progenitor star and perhaps even the nature of the explosion varies from galaxy to galaxy and with cosmic time.

Gravitational lensing also offers opportunities in cosmology. I am analyzing a large sample of galaxy clusters to determine the relative distributions of dark and baryonic matter. Using numerical simulations we can test both the nature of the dark matter and how it interacts gravitationally with baryons. Since structure formation is a competition between gravity and dark energy, dark matter maps are also a powerful probe of dark energy. With colleagues at JPL, I'm undertaking a number of ground- and space-based projects to further this area.

Hubble Space Telescope Lensing Credit: NASA/STScI

Lensing also enables progress in the study of distant galaxies. I am using clusters as natural "magnifying glasses" to zoom in on the detailed internal properties of selected distant galaxies. With the Keck adaptive optics system, we can secure dynamical data with exceptionally good resolution for early galaxies. We are also using lensing in Spitzer and Hubble data to explore the earlier galaxies which may be responsible for cosmic reionization.

Closer to home, I am keen to understand the assembly history of important sub-components of galaxies, such as central bulges and bars. By measuring the dynamical masses and sizes of distant bulges and comparing with local examples, I am estimating the rate at which bulges grow.

I'd welcome new students who wish to work in any of the above topics or related areas. Although we usually work in a small group, we maintain strong ties with international collaborators, particularly in the UK, France, Canada and Japan. We get telescope time on all the world's telescopes and all of my students use Keck and Palomar on a regular basis.

[Image credits: R. Ellis; NASA/STScI]