Richard Ellis

Astronomy 105-24, Caltech

Pasadena CA 91125

Position: Professor of Astronomy; Director, Palomar Observatory
Office: (626)-395-2598
Cell: (626)-676-5530
Secretary: (626)-395-4970
Fax: (626)-568-9352
Email: rse@astro.caltech.edu

Overview of Research

I am working primarily in observational cosmology addressing issues related to the nature of the world model, the origin and evolution of galaxies, the growth of large scale structure and the nature and distribution of dark matter. I am enthusiastic about the use of new instruments and observational opportunities when they further the progress that can be made in these areas.

Research Programmes

My current interests at Caltech fall under 3 main headings:

Gravitational lensing: By coordinating observations made with different telescopes, I hope to convince skeptics that weak lensing can be a very powerful and quantitative probe of large scale structure. I am also defining the weak lensing case and survey parameters for SNAP , a major satellite mission, in conjunction with the Supernova Cosmology Group at LBL. Caltech recently organised a workshop entitled New Technologies in Weak Gravitational Lensing to focus national discussion on these and related questions. Current research activities under this heading include the study of cosmic shear on small angular scales using Keck's ESI imager. This instrument complements work done at the prime focus of the 4.2m William Herschel telescope on larger angular scales.Recently, with colleagues at Carnegie, Yale and in the UK and France, I was awarded 78 HST orbits to conduct a detailed panoramic imaging survey of the z=0.4 cluster Cl0024+16 . One goal of this large allocation is to trace the dark matter distribution out to very large radii.

Distant Supernovae: I have participated in the Supernova Cosmology Project since the late 1980's. The program recently reached fruition via the compilation of a Hubble diagram for 42 distant SNe . The resulting "accelerating Universe" is a bewildering result which demands a very high level of proof. I am examining the environmental properties of the host galaxies. The environmental studies are being undertaken using a snapshot program with HST's STIS imager. This determines, for example, whether each SN occurred in a disk or spheroidal population. Keck ESI echelle spectra are also being secured for the host galaxies to clarify the extinction variations. A more challenging test for possible evolution in the progenitor composition is being undertaken via high s/n Keck spectroscopy at several epochs for chosen SNe found via the collaboration.

Galaxy Formation and Evolution: The combination of HST images, ground-based IR imaging and Keck/Gemini spectroscopy is being used to better understand the origin of the wide diversity of galaxy morphologies. Key questions include the mass assembly history of galaxies, the reasons for the demise in the faint blue irregulars and the important question of whether field ellipticals are assembling from merged spirals or represent old remnants of systems which underwent monolithic collapse at high redshift. Using Keck, I have begun to address each of these questions. Detailed spectra of HDF ellipticals are being analysed to examine the origin of internal color variations recently found by my student, Felipe Menanteau. The assembly history of galaxies in general is being addressed by combining K-band photometry with dynamical data, in collaboration with Jarle Brinchmann and Nicole Vogt. A popular article summarising our recent progress in this area is available.